Lab: Testing for Residual Thiosulfate and Residual Silver

Purpose

To find out, for your own materials, your own fixer and your own sink, at what point a wash becomes adequate — by running two published spot tests on eighteen samples washed for six different lengths of time, against an unwashed control at one end and a deliberately under-fixed sheet at the other.

The session answers two questions that look like one and are not. Has the thiosulfate gone? is a question about washing. Has the silver gone? is a question about fixing. A sample can fail either independently, and the pattern of failures is the diagnosis.

Learning objectives

Prepare two test solutions at drop scale from published formulas and say what each is measuring; carry an unexposed control sheet through a processing batch and explain why the control has to be unexposed; read a stain against a reference rather than against a memory; separate a washing failure from a fixing failure by the pattern of results across a sample set; state the wash time your own materials need under the Part XII washing criterion and say what that figure is and is not; and keep two chemically incompatible reagents apart on a bench by layout rather than by remembering.

Prerequisites

The physics of washing, which owns the arithmetic behind the wash arms, and permanence and image deterioration, which states the criterion this session measures against and explains what a failure will eventually look like.

From Part XI, fixer capacity, exhaustion and residual silver, which is where the under-fixed sample comes from and which owns two-bath fixing. From Part II, laboratory safety and PPE and silver nitrate handling, whose procedure governs every minute the silver nitrate bottle is open. From Part III, complex formation, which numbered the stepwise complexes one of the two tests forms.

Safety classification

Level B on the course rubric, and three of the four Level B criteria apply.

Silver nitrate is named in the rubric’s Level B criteria by name. The encyclopaedia entry records Danger, with H314 in 99.9 per cent of the 802 ECHA reports that carry hazard codes, H318 in a third, H272 as an oxidiser and H400 and H410 for aquatic toxicity; the harmonised CLP entry carries H272, H314, H400 and H410. This session opens the bottle once, weighs 1 g and makes 100 mL of a 1 per cent solution.

The failure mode is a splash and a stain rather than a spoiled negative. Kodak’s own 1949 caution is the one to take seriously because it describes the ordinary outcome rather than the worst one: silver nitrate solution “stains the skin black; avoid direct contact with the solution”. The stain looks trivial and takes days to fade, and what it actually records is skin contact with a solution the harmonised classification marks H314, so it is the visible proof that a control failed.

Two reagents on one bench are chemically incompatible with each other. Sodium sulfide and any acid give hydrogen sulfide, which is why the incompatibility page lists the pair and why Kodak’s toning sheet forbids discarding sulfide with a stop or fixing bath. Silver nitrate and sulfide give silver sulfide instantly, so a trace of one destroys the other as a reagent. The control is bench layout, not care.

!One step in this session is not Level B, and it is not performed in the session

The sodium sulfide entry classifies the substance at Level C, on its acute toxicity by ingestion and skin contact, and its PPE line requires “respiratory protection appropriate to a deliquescent solid if it is ever weighed outside engineered extraction”. That assessment is not overridden here.

What follows from it is a design rather than an exception. The course adopts the pattern it already uses for the one dye it permits: the powder is weighed once, or never. Kodak’s 1928 primer and Wall’s 1912 dictionary both say the salt is best kept as a strong stock solution because the crystals deliquesce, and Wall adds that the stock should be made up as soon as the chemical is bought. So ILFORD’s 2 g in 125 mL is made once, as a separate operation, with the controls the encyclopaedia entry requires, and thereafter this reagent exists only as a solution dosed in drops. Everything inside the three hours of this session is dilute solution.

If you do not have those controls, run the session without the residual-silver arm. You will still measure washing, which is what the wash arms are for; you will not measure fixing, and the data sheet should say so rather than leaving a blank. That omission is a decision of the course’s safety rules and not an absence of evidence.

What is not a hazard here, and why. Nothing is heated: every solution is at room temperature and nothing in the session evolves a vapour on its own. There is no dust once the two stocks exist, because both reagents arrive at the bench as liquids and are dispensed from droppers. The processed samples themselves carry silver and thiosulfate but at drop-scale quantities in hardened gelatin, and a dry, processed sheet is handled as paper — the same distinction the silver nitrate procedure draws. There is no mains equipment, no ultraviolet source and no glassware under pressure or vacuum. The hydrogen sulfide risk, which is the one that would otherwise set the level, exists only if the sulfide meets an acid; the bench layout below is what makes that impossible rather than unlikely, and no step of the procedure brings them within reach of each other.

Hazards

What	Hazard	Where it arises	Control
Silver nitrate, solid	H272 oxidiser, H314 severe skin burns and eye damage, H400/H410 aquatic	Weighing 1 g, once	Goggles, gloves, weigh over a tray, the SOP
Silver nitrate, 1 % w/v	Skin and eye irritation; stains skin black	The three-minute immersion	Gloves, tongs, no fingers in the dish
Sodium sulfide, solid	H301 toxic if swallowed, H311 toxic in contact with skin, H314, H400; deliquescent	Only when the stock is first made, which is outside this session	Level C controls per the encyclopaedia entry; not performed here
Sodium sulfide, 0.16 % w/v working	Irritant; liberates hydrogen sulfide with any acid	Dispensing drops	Goggles, gloves, and separation from every acid on the bench
Hydrogen sulfide, if the two ever met	HSE sets 5 ppm over eight hours and 10 ppm over fifteen minutes; NIOSH records that the sense of smell is rapidly lost at higher concentrations	Only from a mistake	Layout, dedicated droppers, and separate waste containers
Spent fixer and wash water	Silver-bearing; aquatic toxicity	Every wash arm	Collected under the silver-bearing waste procedure

The odour warning deserves its own sentence because it is the reason the control is layout rather than vigilance: hydrogen sulfide announces itself at low concentration and then stops announcing itself as the concentration rises. A smell that goes away is not reassurance.

Required PPE

Chemical splash goggles that seal against the face, for the whole session — not safety glasses. Both reagents carry severe-eye-damage statements from almost every notifier, and both are dispensed by hand at eye level over a dish.

Nitrile gloves, changed the moment either test solution touches them. HSE’s COSHH essentials sheet P1 takes single-use nitrile of 0.2 mm as splash protection for manual film and plate development where the safety data sheet gives no more specific advice, and the glove table records that the permeation guide the course read carries no entry for silver nitrate at all — so these gloves are splash protection and not an immersion barrier.

An apron or overall, and long sleeves. Eyewash provision within reach before the first bottle is opened, which is what Level B assumes and what the first-aid reference explains.

Tongs or tweezers dedicated to each side of the bench, and never carried across. This is protective equipment in this session even though it is not worn.

Ventilation

The control here is a room with a through draught and a window that opens, at the general-ventilation level HSE’s COSHH essentials sheet P1 sets for manual film and plate development. Nothing in the procedure produces a vapour, so no local extraction is needed for the work as written; what the ventilation is for is the mistake — a spill that puts sulfide and an acid in the same puddle — and the requirement is therefore that the room can be cleared quickly rather than that air is being moved continuously.

Do not run this session in a bathroom with the door shut, and do not run it in the same room as a store of unexposed paper or film. Kodak’s 1928 primer records that a very small quantity of hydrogen sulfide converts enough silver halide to sulfide to produce severe fog, and that no photographic material should be stored where sulfide work is done. The gas that would hurt you also ruins the paper.

Materials

• Fibre-base paper, one 8 × 10 sheet cut into eight strips, plus one whole sheet for the reference. Use the paper you actually print on: the reference tint ILFORD describes is paper-specific.
• Resin-coated paper, one 8 × 10 sheet cut into eight strips.
• Film, about a metre of 35 mm scrap or leader, cut into eight pieces.
• Blotting paper or lint-free absorbent tissue, for the sulfide test’s blot.
• Labels and a pencil that writes on wet paper. Eighteen samples with the same white face is a design that fails on labelling before it fails on chemistry.

Chemicals

Chemical	Quantity for the session	Form
Silver nitrate	1 g	Solid, weighed once, made to 100 mL as a 1 % w/v solution
Sodium sulfide	2 g as an existing stock solution, of which the session uses about 2 mL	ILFORD’s stock is 2 g in 125 mL; made once, outside this session, under the controls its entry requires
Sodium thiosulfate or ammonium thiosulfate	1 L of working fixer, non-hardening	Plain hypo or a bought rapid fixer at its paper dilution
Sodium sulfite	10 g	For 1 L of the one per cent washing aid, used by one arm
Citric acid	Per the stop bath formula	1 L of working stop bath
Paper developer	1 L working	D-72 at its usual dilution, or the developer you print with

Note what is not on this list. No potassium permanganate and no sodium hydroxide, because the third published test in the corpus is described below and not performed; and no acetic acid, because nothing in this session needs an acid on the same bench as a sulfide.

Equipment

Balance reading to 0.01 g; a 100 mL and a 25 mL graduated cylinder; two small dishes or wide jars for the silver nitrate immersion and its rinse; two dedicated glass droppers, one per reagent, of visibly different appearance; six labelled wash vessels; a timer that can hold six intervals or six cheap timers; tongs for each side of the bench; a light box or a north-facing window for reading stains; and a silver-bearing waste container already labelled and open before the session starts.

The bench, laid out so the two reagents cannot meet

1. 1Silver zone — 1 % silver nitrate, its rinse dish, its own dropper and tongs, on its own tray; the acid stop bath never comes here and neither does the sulfide
2. 2Wet zone — processing trays and the six labelled wash vessels; every sample starts and returns here
3. 3Sulfide zone — the 1+9 working solution, its own dropper and tongs, blotting paper, on its own tray; no acid of any kind within reach
4. 4Two waste containers, at opposite ends — silver-bearing on the left, sulfide on the right, both labelled before the session starts; nothing is ever decanted from one into the other

A layout, not an apparatus: the point is that no reachable path exists between the sulfide and any acid, so the incompatibility is prevented by where things are rather than by remembering where they are.

Estimated cost

Band ££, and almost all of it is the first purchase of two reagents rather than the session.

The planner carries silver nitrate as the item that decides the cost of the practical half of this course, records that the two United Kingdom listings it found differ by nearly a factor of five per gram, and prices it accordingly; this session uses 1 g, which is a smaller quantity than any other page in the course that opens the bottle. The planner could not price sodium sulfide, and the 2 g in ILFORD’s stock will outlast several years of testing. Everything else — paper, film, developer, stop, fixer, sulfite — is stock you already hold, and the session consumes about two sheets of paper and a metre of film.

Estimated consumables cost

The band above is dominated by two first purchases that last for years. What this session actually consumes is small, and most of it is stock the darkroom already holds.

Consumed	This session	Sourced price	Cost this session
Silver nitrate, as 1 % HT-2	1 g	The planner prices it, and records two UK listings differing by nearly a factor of five per gram	See the planner; this is the smallest opening of the bottle in the course
Sodium sulfide, as ILFORD’s stock	about 2 g	None. The planner could not price it	—
Paper	about two sheets	None. photographic-paper carries a cost band and no dated figure	—
Film	about one metre	None. sheet-film carries a cost band and no dated figure	—
Developer, stop, fixer, sulfite	working volumes from stock	Held already; see the formulary entries this session cites	—

The two reagents are the whole of the difference between this page and a free one, and neither is consumed at a rate that will trouble a second year of testing.

Waste streams

Four, and the first rule is that two of them never meet.

• Silver-bearing: the spent 1 % silver nitrate, its rinse water, the spent fixer, and all the wash water from every arm, which carries both thiosulfate and dissolved silver. Into the labelled silver container under the silver-bearing waste procedure.
• Sulfide: the few millilitres of diluted test solution and the blotters that took up its excess. Into a separate labelled container, never the silver one and never a container that has held an acid, following the general chemical waste procedure.
• Acid: the spent stop bath, kept apart from the sulfide container for the same reason.
• Solids: tested strips, gloves and wipes, bagged. A dry processed strip carries silver in hardened gelatin and a sulfide or silver-nitrate spot on its face; it is not household paper.

Every one of these is subject to the disposal caveat, and the jurisdictional sentence there is not a formality: what a household may do with a photochemical stream is a question of local rules and local collection, and ILFORD’s own advice to domestic users in the United Kingdom is a household waste and recycling centre rather than a drain.

Preparation

1. Read the two lessons before this one. The wash arms below are the published sequences those pages explain, and the procedure will not repeat the reasons.
2. Label eighteen sample positions before you cut anything, and label the six wash vessels. Write the labels on the vessels, not on the samples: a wet white strip takes pencil badly and every one of them looks identical.
3. Make the 1 per cent silver nitrate: 1.00 g into 100 mL, in a brown or opaque bottle, dated and labelled per the labelling procedure, with the SOP open. Make it on the day; a silver nitrate solution darkens faster than the solid.
4. Dilute the sodium sulfide stock 1+9: 2 mL of ILFORD’s stock into 18 mL of water, in the sulfide zone, with its own dropper. That is the working solution and it is all you need.
5. Set up the bench to the layout above, with both waste containers open and labelled and the spill trays under the two outer zones before any bottle is opened.
6. Mix the processing baths: developer, stop, and 1 litre of fresh, non-hardening fixer. The fixer must be fresh, because the wash arms are the variable and an exhausted bath would put a second variable into every result.
7. Set aside 200 mL of that fresh fixer in a second vessel for the two-bath reference sheet.

Procedure

The session, in five stages

1. 1Process eighteen samples plus two controlsUnexposed, through developer, stop and fixer, in one batch, in the dark. Every sample sees identical chemistry up to the moment it enters its wash vessel.
2. 2Wash six ways at onceUnwashed, 2, 10, 30 and 60 minutes, and the optimum-permanence sequence. Six vessels, six timers, one start.
3. 3Blot and sortOut of the water, blotted, into labelled positions. The test reads a wet strip, so do not dry them.
4. 4Test A, residual thiosulfateHalf of each strip immersed three minutes in 1 % silver nitrate, rinsed, compared wet against its own untreated half.
5. 5Test B, residual silverA drop of diluted sodium sulfide on a white area, blotted, compared against the reference spot on the two-bath sheet.

Stage 1 — make the samples

1. In the dark, with the safelight on, take one sheet of fibre-base and one of resin-coated paper through developer, stop and fixer at your normal times, without exposing them. They come out white, which is the point: an unexposed processed sheet has the material’s full complement of retained chemistry and no image to read the stain against. This is Kodak’s own method — “process with the batch of prints an unexposed white sheet of photographic paper” of the same weight.
2. Take the film through the same fixer for its normal time. Film may be handled in room light once it has been fixed.
3. The reference sheet. Take a second fibre-base sheet through developer and stop, then fix it in two baths — half the time in the working fixer, half in the 200 mL of fresh fixer you set aside — and give it the full optimum-permanence wash sequence. This is ILFORD’s “print that is known to be well fixed and thoroughly washed”, and every sulfide reading in the session is made against it.
4. The under-fixed control. Take one more fibre-base strip through developer and stop, then fix it for 15 seconds only, and wash it with the 60-minute arm. It is thoroughly washed and badly fixed, which is exactly the combination the two tests have to be able to tell apart.
5. Cut the sheets into strips, keeping each material’s strips together, and hold them all in a tray of water for no more than a minute while you get the wash vessels ready.

Stage 2 — the six wash arms

Start all six at once. Each vessel takes one strip of each material.

Arm	Film	Fibre-base paper	Resin-coated paper
0. Control	Straight from the fixer, blotted, no wash	Same	Same
1. Two minutes	Running water	Running water	Running water
2. Ten minutes	Running water	Running water	Running water
3. Thirty minutes	Running water	Running water	Running water
4. Sixty minutes	Running water	Running water	Running water
5. Optimum permanence	ILFORD’s three changes, inverting 5, 10 and 20 times	5 min wash, 10 min in the 1 % sulfite aid, 5 min wash	30 s vigorous running water

Arm 5 is not a time; it is a sequence, and it is the one the manufacturers publish for materials intended to last. It is in the matrix so that the session compares a designed sequence against a number of minutes rather than comparing minutes against minutes.

6. At the end of each arm, blot each strip between clean blotters and lay it in its labelled position, wet. Do not dry them: the residual-thiosulfate test is read wet.

Stage 3 — Test A, residual thiosulfate

7. Cut each strip in two across its width. One half is the test half, the other is the untreated half and it is the control for that specific sample. Every comparison in this test is between two halves of one piece of material, which removes paper batch, developer, fixer and wash from the comparison in one stroke.
8. Immerse the test halves in the 1 per cent silver nitrate for about three minutes, using tongs A. Several can go in together provided they do not overlap.
9. Rinse in the second dish, briefly.
10. Lay each test half beside its own untreated half, both wet, and compare in subdued daylight or artificial light. Kodak’s endpoint is exact and it is a comparison, not a measurement: “if the hypo has been completely removed no colour difference should be observed. A yellow-brown tint indicates the presence of hypo.”

Stage 4 — Test B, residual silver

11. Put one drop of the diluted sodium sulfide solution on a white area of each remaining untreated half, using dropper B, in the sulfide zone, and on the reference sheet first so that you have the reference in front of you.
12. After a few seconds, remove the excess with clean blotting paper. ILFORD is specific about this, and the reason is that a pool left to dry gives a dark ring that is not a reading.
13. Compare each spot with the reference spot. ILFORD’s endpoint: the reference is “a barely visible cream tint”, and “any subsequent prints that show a yellowing of the test spot when tested are not properly fixed”.
14. Record every reading against the reference, in words, at the bench. A remembered tint is not data.

The unwashed control cannot be read by this test, and ILFORD says why: “prints must be well washed before using the test, it is not effective on prints direct from the fixer bath.” A strip still carrying the fixing bath carries free thiosulfate as well as silver, and thiosulfate will hold the silver the sulfide is trying to find. Record that row as not testable rather than as a result, and note that the reason is a property of the test rather than of the sample. It is the cleanest illustration in the session of a rule worth carrying out of it: a test has preconditions, and a reading taken outside them is not a weak reading but no reading at all.

Expected observations

The unwashed control will fail Test A dramatically, on every material. That is what the control is for, and it is worth looking at carefully before anything else, because it is the only sample in the session whose result you already know — which makes it the check that the test solution works.

The film will pass early and the resin-coated paper almost as early. Both have only their gelatin to clear, and neither has a base that holds solution.

The fibre-base strips are the experiment. Expect a visible tint at two minutes, a much fainter one at ten, and the interesting comparison between thirty minutes, sixty minutes and the twenty-minute optimum-permanence sequence. Reilly’s warning is the thing to watch for: with prints “the rate of washing slows down tremendously at the lower levels of thiosulfate concentration”, so the difference between thirty and sixty minutes may be smaller than the difference between two and ten. That is not a failed experiment; it is the shape of the curve.

The under-fixed strip should pass Test A and fail Test B. It was washed for an hour, so it has little thiosulfate; it was fixed for fifteen seconds, so it has a great deal of silver.

Every properly fixed sample should give the reference tint or paler on Test B, regardless of its wash arm — because washing is not what puts silver in the paper.

=Three results that are not what they look like

A yellow-brown tint on every sample, control and all. Kodak’s own footnote names the alternative explanation: the same effect “can be caused if hydrogen sulphide or wood extracts are present in the water supply”. Repeat one arm with distilled water for the final rinse before concluding anything about the wash.

A tint that appears on drying. The comparison is specified wet for a reason: both halves change as they dry, and they change at different rates once one of them carries silver. Read wet, or read both after the same drying time, and say in the notebook which you did.

A darker spot on the reference sheet than on a sample. That means the reference sheet is not what it claims to be. Make it again with genuinely fresh second-bath fixer, and do not adjust the other readings to fit it.

What is happening chemically

Test A puts a silver ion where the thiosulfate is. Silver nitrate is fully dissociated in water, so the strip meets free silver ion, which finds whatever thiosulfate the wash left behind. The first step is the one Part III numbered:

Ag⁺ + S₂O₃²⁻ → [Ag(S₂O₃)]⁻

Test A, step one: the mono complex, in a large excess of silver rather than of thiosulfate

Note the conditions, because they are the reverse of fixing. In the fixing bath thiosulfate is in vast excess and the second complex dominates; in this dish silver is in vast excess, so the reaction stops at the first step and the product is the compound Kodak’s 1924 primer called “almost insoluble in water”. The stain is what that compound becomes: the conservation literature identifies the yellow-brown product as silver sulfide, and the permanence page gives Reilly’s account of the same conversion happening slowly, in storage, to a print nobody tested. The course could not source a balanced equation for the intermediate steps between the complex and the coloured product and therefore prints none. What it can say is that the test and the failure it predicts are the same chemistry at different speeds, which is the strongest argument for the test there is.

Test B puts a sulfide ion where the silver is.

Na₂S + 2 Ag⁺ → Ag₂S + 2 Na⁺

Test B: any silver the fixer failed to remove becomes silver sulfide, and silver sulfide is brown

This is the same reaction as the redevelopment step of a classical sepia toner, run at a thousandth of the scale and for the opposite purpose: a toner wants to convert the image, and this test wants to find silver where there is no image. Spiller was doing exactly this in 1868, and Reilly quotes him — he detected retained silver in an albumen print “by the production of a brown stain upon moistening the white surface with sulphide of ammonium”.

Why the two tests cannot substitute for each other. Test A is blind to silver: a strip with no thiosulfate and a great deal of retained silver gives no tint, because there is nothing for the added silver ion to complex with. Test B is blind to thiosulfate: sulfide finds silver, and free thiosulfate in the paper is not silver. Run one and you have half a diagnosis.

⚗The third published test, printed as evidence and not performed

Kodak Limited’s 1949 handbook prints a second hypo test, and the course gives it in full because it is a published formula and because its limitations are instructive.

KODAK FORMULA HT-1a — hypo test solution for checking thoroughness of washing. Potassium permanganate 1.2 g, sodium hydroxide 2.4 g, distilled water to make 1000 c.c. To use it, add 1 c.c. of that solution to 250 c.c. of pure water in a clear glass and drain six quarter-plate films or plates from the wash into it; if hypo is present the violet colour turns orange in about thirty seconds, and with more hypo it goes on to yellow.

Three reasons this session does not run it. First and decisively, it tests the water, not the material: it reads what is coming out at that moment, and the question this lab asks is what is left in. Second, Kodak itself qualifies it for paper — “the test is not a completely reliable indication” — and recommends the hypo eliminator alongside it. Third, it is not specific: Kodak requires a comparison test with the same water, because “oxidisable organic matter, if present in the water, will react with the permanganate in a similar manner to hypo”.

Neither of its two reagents has an encyclopaedia entry in this course, so a reader who wants to run it works from the suppliers’ own safety data sheets, and should note before starting that a permanganate is an oxidiser and 2.4 g of sodium hydroxide per litre is a caustic solution.

📓On comparison scales, and why this session has none

The literature on these tests refers to reading a stain against a printed comparison scale. No comparison scale, printed or described, appears in any source this course holds, and the course does not invent one, because a fabricated scale would turn a comparison into a fake measurement.

What replaces it is what both published tests already specify, and both are better than a scale because they are internal to your own materials: Kodak compares the treated half of a strip against the untreated half of the same strip, and ILFORD compares a spot against a reference spot on your own paper, fixed and washed to a known standard. Neither needs a printed patch, and neither can be invalidated by a printer’s ink.

Data to record

The lab notebook worksheet carries the session-level fields. Add a matrix sheet with one row per sample and these columns.

• Sample identity: material, wash arm, and which sheet it was cut from.
• The processing that was identical: developer and dilution, times and temperature, stop, fixer and its dilution, the fixing time, and the fixer’s age and prior use. One line, once, at the top.
• Wash arm as executed: the actual elapsed time, the water temperature, and for arm 5 the times of all three steps.
• Test A reading: none, faint, distinct, or strong, against the sample’s own untreated half — and the words you used, not a number you invented.
• Test B reading: paler than the reference, equal to the reference, or yellower — and how much.
• Conditions of reading: the light you read under, and whether the samples were wet or dried.
• What you did not run, and why. A sheet with the residual-silver column deliberately empty and a sentence saying so is a record; a blank column is not.

Analysis

Find the shortest arm that passes Test A on each material. That figure, for your paper, your fixer and your water, is the answer the session exists to produce. Write it down as what it is: measured under the criterion defined in Part XII, which is a comparison against a control carried through the same session. It is not an ISO figure, it is not a residual-thiosulfate concentration, and it does not certify anything.

Compare arm 5 against arm 3 and arm 4. If the twenty-minute optimum-permanence sequence matches or beats sixty minutes of running water, you have measured, on your own bench, the claim the washing lesson makes arithmetically — that a designed sequence beats a longer one, on less water.

Build the two-by-two. Every sample falls into one of four boxes, and each box is a different instruction.

	Test B passes (little residual silver)	Test B fails
Test A passes (little thiosulfate)	Properly fixed and properly washed	Fixing failed. Longer washing will not help; the fixer was exhausted or the time too short, and Part XI’s criterion is what prevents it
Test A fails	Washing failed. Fixing was adequate; extend the wash or add a wash aid	Both failed — and check the fixer first, because an exhausted bath makes washing harder as well

The lower-right box is worth a sentence, because it is the one people misdiagnose. An overworked or over-long fixation loads the material with complexes that are harder to shift, so a fixing fault produces a washing fault. Fix the fixing first and re-measure; do not lengthen the wash and declare victory.

Say what the session could not resolve. If two arms are indistinguishable to your eye, they are indistinguishable — record that rather than picking one. The tests detect failure, not degree.

Troubleshooting

Every sample fails Test A, including a sample washed for an hour. Suspect the water before the method: Kodak names hydrogen sulfide and wood extracts in the supply as producing the same tint. Run one strip again with a final rinse in distilled water.

No sample fails Test A, including the unwashed control. Then the test solution is not working. The likely cause is contamination of the silver nitrate — a trace of sulfide, a chloride from tap water, or a dropper that has been used for something else. Make it again with distilled water and a clean bottle.

The sulfide spot spreads and dries as a ring. The excess was not blotted. ILFORD’s step is not optional and the ring is not a reading; test again on a fresh area.

The reference spot is darker than the samples. The reference sheet was not well fixed. Remake it.

Strips curl and touch each other in the silver nitrate dish. Overlapping strips give an uneven tint that reads as a gradient in washing that does not exist. Fewer at a time, or a larger dish.

Black stains on fingers. A glove failed or was not worn. The stain is the visible record of a skin contact that also delivered a corrosive solution; change gloves, wash, and note it, because the next one might be on an eye.

Alternative route

The full session needs a darkroom, because an unexposed sheet of paper can only be processed to white in the dark. If you have no darkroom, the film arm alone can be done in room light and it teaches most of the method.

Take a metre of 35 mm leader — the same material Part XI’s clearing-time method uses — fix it in room light until well past clearing, cut it into six pieces, and run the six wash arms and Test A exactly as above, comparing each treated half against its own untreated half. You lose the paper comparison, which is the most interesting part, and you lose Test B, which needs a white paper surface to read a tint on. What you keep is the whole of the residual-thiosulfate method and a real measurement of how fast your own wash clears a gelatin layer.

Record the reduced scope on the data sheet as a decision rather than leaving the paper rows blank.

Clean-up

1. Empty the two outer zones first, into their own containers, before anything is carried across the bench. This is the moment the incompatibility is most likely to happen, because the layout has done its work and attention has moved on.
2. Rinse the silver nitrate dish and its rinse dish into the silver container, not into the sink.
3. Rinse the sulfide dropper and vessel into the sulfide container, then wash them separately, last, with plenty of water.
4. Collect every strip. Bag the tested ones.
5. Wipe both spill trays and bin the wipes with the solids.
6. Wash hands before touching anything on the dry side, following the closing procedure.

Storage

The 1 per cent silver nitrate goes into a brown or opaque bottle, dated, and lives in the dark with the rest of the silver stock; it darkens with time and light, and a darkened solution is discarded rather than used, because you cannot tell how much silver it has lost.

The sodium sulfide stock stays sealed, dry and dated, away from every acid and out of the room where sensitive materials are stored — the encyclopaedia entry’s rules, which do not relax because the bottle is small.

The reference sheet is the most valuable object the session produces. Dry it, label it with the paper, the fixer, the date and the sequence used, and keep it flat and dark. It is the standard every future residual-silver reading on that paper is made against, and remaking it costs a sheet, a session and two fresh baths.

Keep the tested strips too, in a labelled envelope, and look at them again in a year. A comparison you cannot repeat is not evidence; a set of strips with known histories is a slow experiment already running.

Disposal considerations

Everything in this session that touched a sample is silver-bearing, including the wash water, and the chemistry is the reason: the wash is where the dissolved complexes go. That is the same stream Part II set up containers for and the same stream the second half of this part recovers silver from.

The sulfide stream is separate and it stays separate. Kodak’s instruction is unambiguous — sulfide solutions are not discarded with stop baths or fixing baths, because the combination generates hydrogen sulfide — and the practical form of it is one dedicated container, labelled, never used for anything else and never topped up from another.

Beyond that, the disposal caveat governs, and it is the honest answer rather than an evasion: the classification of a photochemical waste, and the route it must take, are set by local regulation and by the collection service that exists where you live. ILFORD’s advice for domestic users in the United Kingdom is that used chemistry goes to a household waste and recycling centre. Check your local regulations, and label the container so that whoever receives it knows what is in it.

Questions

Check your understanding

Question 1. A fibre-base print gives no tint at all in the silver nitrate test but a distinctly yellow spot in the sulfide test, clearly darker than the reference. What went wrong, and what is the corrective action?

• The wash was too short; extend it and add a wash aid
• The fixing was inadequate; longer washing will not remove what is there, and the print must be refixed in fresh fixer and rewashed
• The test solutions were swapped; repeat with the correct reagents
• The paper is simply of poor quality and nothing can be done

Show the answer and why

Answer: The fixing was inadequate; longer washing will not remove what is there, and the print must be refixed in fresh fixer and rewashed

The two tests answer different questions. Silver nitrate finds thiosulfate, and finding none means the wash did its job. Sulfide finds silver, and a yellow spot means the fixer left silver behind — either because it was exhausted, or because the time was too short, or because the material was moved on before the insoluble first compound had been converted to the soluble one. Kodak's 1924 primer is the reason this cannot be washed out: that first compound is almost insoluble, and it is invisible. ILFORD gives the remedy directly — soak in water for five minutes, then repeat the recommended fixing and washing sequence in fresh fixer.

Question 2. Why does Kodak's residual-thiosulfate method compare the treated half of a strip against the untreated half of the same strip, rather than against a printed reference patch?

• Because printing inks fade and a patch would drift over time
• Because the comparison then removes paper batch, developer, fixer, wash and the tint of the paper itself from the result, leaving only the effect of the silver nitrate
• Because a printed patch would be too expensive to supply with the formula
• Because the test is qualitative and any comparison would do

Show the answer and why

Answer: Because the comparison then removes paper batch, developer, fixer, wash and the tint of the paper itself from the result, leaving only the effect of the silver nitrate

A within-sample control is the strongest control available, and it is free. The two halves are the same paper from the same box, developed in the same tray, fixed in the same bath and washed in the same vessel, so every variable except the test itself is identical between them. No printed patch can match that, because no patch knows what your paper looks like wet. It is also why this course does not fabricate a comparison scale: none appears in any source it holds, and the published methods do not need one.

Question 3. You have 1 g of silver nitrate and you want 100 mL of the 1 per cent solution the test calls for. You also want to know how much silver ion the dish contains. Give the concentration in g/L and the amount of silver in the dish, using a relative molecular mass of 169.87 for silver nitrate and 107.87 for silver.

• 10 g/L; about 1.0 g of silver in the dish
• 10 g/L; about 0.64 g of silver in the dish
• 1 g/L; about 0.64 g of silver in the dish
• 100 g/L; about 0.11 g of silver in the dish

Show the answer and why

Answer: 10 g/L; about 0.64 g of silver in the dish

One per cent w/v means 1 g in 100 mL, which is 10 g in a litre, so 10 g/L. The silver fraction of silver nitrate is 107.87 ÷ 169.87 = 0.635, so 1 g of the salt carries 0.635 g of silver. Two things follow that matter at the bench. The dish holds two-thirds of a gram of silver, which is why the spent solution goes into the silver-bearing container rather than the sink. And the silver is in enormous excess over any thiosulfate a washed strip could carry, which is why the reaction stops at the first complex rather than going on to the soluble second one.

Question 4. The unwashed control gives a strong tint, the two-minute samples give a weaker one, the ten-minute samples weaker still — and the thirty-minute and sixty-minute fibre-base samples look the same as each other. What should you write down?

• That thirty minutes is sufficient, since sixty gives no improvement
• That the test is exhausted and the results above ten minutes are invalid
• That the two arms are indistinguishable by this method, which is a limit of the test rather than a measurement of equality, and that the shortest arm that passes is what the session establishes
• That the sixty-minute sample must have been contaminated

Show the answer and why

Answer: That the two arms are indistinguishable by this method, which is a limit of the test rather than a measurement of equality, and that the shortest arm that passes is what the session establishes

A visual comparison has a resolution, and beyond it the honest statement is "I cannot tell these apart" rather than "these are the same". Reilly predicts exactly this shape: with prints the rate of washing slows down tremendously at low thiosulfate concentrations, so the difference between thirty and sixty minutes is genuinely small as well as hard to see. The session's output is the shortest arm that passes, reported as a measurement under the course's own stated criterion, together with a note of which arms could not be separated.

Question 5. Why is the sodium sulfide solution kept at the opposite end of the bench from the silver nitrate and from the stop bath, rather than simply being handled carefully?

• Because sulfide fumes will fog photographic paper anywhere in the room
• Because sulfide with an acid gives hydrogen sulfide, and sulfide with silver nitrate destroys the silver nitrate as a reagent — and a control that depends on remembering fails at the moment attention lapses
• Because the two solutions have different disposal routes and mixing them complicates the paperwork
• Because silver nitrate is an oxidiser and sulfide is a reducing agent, so the mixture could ignite

Show the answer and why

Answer: Because sulfide with an acid gives hydrogen sulfide, and sulfide with silver nitrate destroys the silver nitrate as a reagent — and a control that depends on remembering fails at the moment attention lapses

Two independent reasons, one a hazard and one a spoiled measurement. Sodium sulfide with any acid liberates hydrogen sulfide — the Environment Agency writes the reaction out and Kodak forbids discarding sulfide with a stop or fixing bath for exactly this reason — and hydrogen sulfide is a gas whose smell disappears as its concentration rises, so the nose is not the alarm. Separately, a trace of sulfide in the silver nitrate precipitates silver sulfide and the test stops working. Geometry prevents both; care prevents them only while you are paying attention, which is not the whole session.

Question 6. Kodak's footnote to the residual-hypo test says the same yellow-brown tint "can be caused if hydrogen sulphide or wood extracts are present in the water supply". What kind of experimental error is that, and what control removes it?

• A random error, removed by repeating the test several times and averaging
• A systematic error affecting all samples alike, removed by rinsing one arm in distilled water and comparing the two results
• An error in the test solution, removed by making it fresh
• A reading error, removed by using a brighter light

Show the answer and why

Answer: A systematic error affecting all samples alike, removed by rinsing one arm in distilled water and comparing the two results

It is an interference: something other than the analyte produces the signal the test reads. Because it comes from the water supply it affects every sample in the same direction, so averaging cannot touch it and no amount of repetition will reveal it — it is systematic, not random. The control is to change the suspected source for one sample and see whether the signal changes, which here means a final rinse in distilled water. That is the same logic as Kodak's own instruction for the permanganate test, where a blank must be run with the same mains water because organic matter in it reduces permanganate exactly as hypo does.

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Further experiments

Extend the matrix downwards in time. The interesting region for resin-coated paper and film is under two minutes, and the arms in this session are too coarse to find it. Run 15 s, 30 s, 60 s and 120 s on RC paper alone and find the point where the tint appears.

Test the arithmetic directly. The washing lesson predicts that n complete changes of water beat continuous flow of the same total volume by a large factor. Wash one set of film strips in three complete changes with vigorous inversion, and another in the same total volume delivered as a trickle over the same elapsed time, and test both. Measure the held volume of your own materials first, by weighing a sheet dry and again after soaking and draining, so that you can state the ρ your prediction used.

Test the wash aid’s capacity. ILFORD publishes forty 8 × 10 sheets per litre and Reilly publishes twenty for the 1 per cent sulfite bath. Run a single litre of the sulfite aid through sheet after sheet, testing one strip from every fifth sheet, and find where the sequence stops working on your paper.

Test the hardening claim. ILFORD states that a hardening fixer “reduces washing efficiency”. Fix one set of fibre-base strips in a non-hardening fixer and another in an F-5 type acid hardening bath, wash both identically, and test. This is the one experiment here that changes the fixer rather than the wash, and it needs the two baths to be at the same silver loading to be fair.

Keep the strips and repeat in a year. Store one set of tested strips in an envelope in a stable interior room and another taped inside a cheap wooden frame. Look at both in twelve months. The sample that fails first will have told you something about storage that no single session can.