Stop bath preparation

Purpose

To make up a working stop bath of stated strength, labelled with its date, its starting pH and the count of work it may take.

When to use it

Before a printing session or a film run, and whenever the bath in use has reached its counted capacity.

It does not cover diluting glacial acetic acid — H314 and H226 in over 99 per cent of its aggregated ECHA reports, and Level B as a concentrated acid. This procedure is Level A: weigh a solid acid, or buy the liquid one already dilute at a stated strength, as environmental-health guidance for photographic chemicals recommends.

Before you start

You should already be able to say what a bath’s capacity is and what spends it, from why stopping works; choose your acid, from the formulations lesson; and make up to a mark, from concentration and dilution.

The product’s safety data sheet is read and recorded, by its own procedure, before its cap comes off.

On the bench: single-use nitrile gloves and eye protection; a balance; the jar or bottle; a vessel half again as large as the batch; a stirring rod kept for acids; a graduate; the labelled container; a pH meter calibrated today, or blue litmus paper; the notebook. Plastic or glass, nothing metal — citric acid attacks metals. Run lab opening first.

Procedure

1. Write the target down before anything is opened: which bath, the final volume, the strength. The course’s citric bath is 19.2 g of anhydrous citric acid to 1.000 L. An acetic bath within two per cent of SB-1 carries 0.30 mol/L, or 18.0 g of acetic acid per litre, reached here from a pre-diluted product. A concentrate takes its maker’s ratio; ILFORD publishes ILFOSTOP at 1+19.
2. Read the jar for the exact article, not the remembered one. Anhydrous citric acid and its monohydrate are nine per cent apart by mass — 19.2 g against 21.0 g for the same 0.100 mol.
3. Label the container before it is filled, by the labelling procedure. Gloves and eye protection on before the first cap comes off.
4. Open the window or start the extractor: more than five air changes an hour with a through draught, HSE’s figure for manual film development.
5. For a solid acid, weigh it by the weighing procedure, tipped gently from the scoop over a tray. Raise no dust, and keep it from any flame.
6. Dissolve it in about 800 mL of water at room temperature, stirring until nothing is left on the bottom. Citric acid dissolves at 59 g per 100 mL at 20 °C, so no warming is needed.
7. Make up to the final volume by that procedure, then go to step 11.
8. For a stated-strength liquid acid, weigh the solution rather than measure it: 18.0 g of acetic acid is 64.3 g of a 28 per cent w/w product. Weighing needs no density; a volume does.
9. Add the acid to the water, never water to the acid, down the wall of a vessel already holding about 800 mL. Rinse the weighing vessel in and make up to the final volume.
10. For a bought concentrate, dilute at the published ratio by the stock procedure: measure it, deliver it, rinse the cylinder into the mixing vessel, and make up to the final working volume.
11. Stir, and bring the bath to its working temperature: ILFORD publishes 18 to 24 °C for its own product and quotes its ten seconds at 20 °C.
12. Measure the starting pH and record it with the temperature beside it, by the meter’s procedure. Without a meter, touch blue litmus to the bath and record that it turned red — Kodak’s 1928 test, which answers yes or no.
13. Write the capacity on the label as a count of work, not a date: the maker’s table for a bought product, or for SB-1 Kodak’s 90 sheets of 8 × 10 inches per 160 fluid ounces, which is 19.8 per litre. For the course’s citric bath none is published — write capacity not established and tally anyway.
14. Write the working life beside it. Kodak gives SB-1 three days in a dish, one month in a tank and indefinite keeping in a stoppered bottle; ILFORD gives its working solution seven working days. Past those, evaporation may have over-concentrated the bath — a fault, not an expiry.
15. Start the tally, on the label or in the notebook, and mark it as each film or sheet goes through.
16. Route the rinsings and any surplus by the general waste procedure, under the disposal caveat. Never into a container holding fixer: acid meeting thiosulfate is the incompatibility most easily met in a darkroom. Where an acid bath is neutralised first, sodium bicarbonate solution goes in slowly because it foams, ventilated, in gloves, goggles and an apron.

How you know it worked

• The article on the jar matches the mass in the target — hydrate for hydrate, per cent for per cent.
• The bath is clear, nothing is settled on the bottom, and nothing metal touched it.
• The label carries what it is, its strength, the date, the starting pH with its temperature, the capacity as a count of work, and the working life.
• The tally exists and reads zero. A tally never started is not a tally at zero.

If a step fails

Step 2, the jar names no hydrate. The hydrate is part of the identity: send it to the unknown-container procedure rather than guess.

Step 5, the solid has caked. It no longer weighs what the formula assumes. Retire it by the stock check rather than crush it.

Step 6, the acid will not all dissolve. At this concentration it should. Check the mass and the volume against step 1, and that the water is not cold. Do not add more water: that changes the strength you are about to label.

Step 9, water went in onto neat acid. At these strengths the heat of dilution is small; the rule is kept regardless, because the habit protects you where it is not. Ventilate, and treat anything outside the vessel as a spill.

Step 10, the concentrate measures under about 10 mL. A cylinder cannot deliver it usefully. Make a larger volume, or take the stock procedure’s small-volume route.

Step 12, the meter will not settle or the litmus is ambiguous. Record the range you can honestly resolve, not a figure you cannot. Litmus neither clearly red nor clearly blue is a discard: an ambiguous answer to a yes-or-no test is a no.

Step 13, nothing is published for the bath you made. Write those words on the label and tally from the first sheet; Part X’s experiment is how the figure gets established.

Step 15, the tally was not kept. How much carryover the bath has absorbed is unknown, and an indicator dye will not recover it: the colour does not move until most of the reserve is gone. Discard it and start again with a tally.

What to record

One line per bath: the date; the formula or product and its strength; the mass or volume of acid actually measured, and in what; the final volume; the starting pH with its temperature, or the litmus result; the capacity, with its source or the words capacity not established; the working life. The label repeats the short version, and the tally is marked as the session runs. Recording a batch puts the container into the inventory, and the lab notebook worksheet carries the fields.