The acid exclusion rule at a hexacyanoferrate or thiocyanate bench

Purpose

To keep every acid out of a room in which a hexacyanoferrate or a thiocyanate is open, and — more importantly — out of the containers those solutions end up in.

When to use it

Whenever either substance is open, in a tray, or standing in a waste container: the cyanotype bench, the residual-iron spot test of Part XXIV, the ferric-oxalate quality test in Part XXV, and any gold thiocyanate toner.

It is a standing rule and not a session step. A part-used waste bottle obeys it for months after the session that filled it.

Before you start

You should already have read the cyanide question, which sets out both halves of the argument: the cyanide in a hexacyanoferrate is bound in a stable low-spin complex, and the regulatory guidance excludes those complexes by name from the statement about liberating a very toxic gas — and the safety card still records that the solid decomposes on contact with acids to give toxic gases including hydrogen cyanide. The course keeps the rule absolute because the cost of being wrong is asymmetric, and because poisoning cases are on record from acidifying a ferricyanide reducer.

A thiocyanate is a different substance with the same rule and a blunter source: violent reaction with acids, and an old classification whose words are contact with acids liberates very toxic gas.

Level B for the bench these substances sit on: gloves, eye protection, and dust control at the balance if you weigh either. What this sheet adds is layout rather than equipment.

Procedure

Clearing the room

1. Name the acids you own before you open anything: acid stop bath, acid or rapid fixer, citric or acetic concentrate, vinegar, descaler, household bleach, and any acid waste container.
2. Take every one of them out of the room, or seal them and put them away where the session will not reach them. The waste corner counts as part of the room.
3. Check the sink and its trap. A trap standing full of a chlorine-bearing cleaner is an acid problem in the one place you cannot ventilate.
4. Confirm that nothing on the bench will be heated. Heat is the second of the two conditions in the warning, and this bench does not create it.

Working

5. Give the hexacyanoferrate or thiocyanate bench its own tray, its own tongs, its own pipette or funnel and its own square of blotting paper, and put that station physically apart from any acidic bath.
6. Where an acid bath is genuinely part of the same process — an iron-silver clearing sequence, a platinum developer’s acidity — put its trays on the other side of the bench and never let a tool cross.
7. Use the one exception knowingly: a deliberate, dilute, weak development or clearing bath of about 1 per cent citric acid or less, which is the published process and is thousands of times below the “very concentrated” the warning describes. Nothing else on this bench is an acid.
8. Never acidify a gold thiocyanate toner, at any dilution, for any reason. The processes it serves want no acid anyway: the fixer is plain and alkaline, there is no stop bath, and the clearing agent is a sulphite.
9. Rinse a tool into its own station’s waste, not into the sink and not into another station’s bottle.

The containers, which is where it actually goes wrong

10. Label every container before the session, by the labelling procedure, with the contents in words rather than an abbreviation.
11. Give the ferricyanide or thiocyanate stream its own container and its own funnel, and stand it apart from the acid container rather than beside it.
12. Read the label on a part-used container before you add anything to it. Nobody plans to pour an acid into a spent ferricyanide; they top up a bottle they did not read.
13. Where a label cannot be read, the container is unknown and the unlabelled container procedure governs. Do not sniff it and do not add to it.
14. Keep the two streams apart in storage as well as in the session, and do not consolidate them to save space. Check your local regulations, which govern where each may go and which the disposal caveat explains the course cannot answer for you.

How you know it worked

• No acid was in the room, sealed or otherwise, while either substance was open.
• The spot-test or toner station stood apart, with its own tray, tools and bottle.
• Nothing was heated.
• The only acid the process met was the published dilute one, at the strength the published process gives.
• Every container was labelled in words before the first pour, and no funnel served two streams.
• The two waste bottles are still two bottles.

If a step fails

Step 2, an acid is found on the bench after the session has started. Cap what is open, take the acid out, and only then carry on. If the two have already met — a cloud, a sharp smell, a sudden warmth — leave the room, close the door, ventilate from outside, and do not return until the air has changed.

Step 3, some of a bath has gone down a sink holding a household cleaner. Leave the room. That is the chlorine pair as well, and it is in the one place you cannot ventilate. Flush the drain only once the air has changed, and follow the acid spill procedure for what remains on the bench.

Step 5, a tool has crossed between stations. It is contaminated in the direction that matters. Retire it from the ferricyanide or thiocyanate station, rinse it into that station’s waste, and label its replacement.

Step 7, a session needs a stronger acid than the published bath. Then it is a different procedure and it is not this bench. Do it on another day, in a room where neither substance is present, and record the separation.

Step 12, an acid has been added to a part-used container of either. Do not cap it and do not move it. Leave the room, ventilate from outside, and treat the container as an unknown thereafter. A gas made under a cap waits for the person who unscrews it.

Step 13, a bottle carries no label at all. It leaves the room only as an unknown, and it never goes near either stream again.

What to record

Which acids were in the room and where they went; the layout of the separated station; the strength and volume of any deliberate weak-acid bath; every container by contents, volume and date; and any crossing of a tool or a stream, with what was retired as a result. The line worth writing last is which container you had to move in a hurry, because that is a shelf problem rather than a session one.