Mixing a fixer

Purpose

To make up a fixing bath whose strength, date and fresh clearing time are all written down, in an order that never lets an acid meet undissolved thiosulfate.

When to use it

Every time a bath is made — a plain hypo bath, an acid hardening bath from solids, or a working solution from a bought concentrate.

Before you start

You should already be able to say what each ingredient is for and which failure it answers, from the formulations lesson, and to make a solution up to a final volume from solids, from the commissioning lab.

Two rulings apply before you weigh anything. Boric acid carries a reproductive-toxicity classification, so a bath containing it is Level B work and is not made under this procedure; the composition class without it is a published one. And no chrome alum: the chromium policy governs where chromium appears in this course, and it is not here. Buy the acid dilute at a stated strength — the glacial concentrate is Level B in its own right.

On the bench: the arithmetic already done and checked on paper; a balance; the solids in closed tubs; a 2 L beaker per bath; a stirring rod; about 600 mL per litre of water slightly warmer than the working temperature, because dissolving the salt cools it; the labelled bottles; a pH meter calibrated today by its own procedure; scrap film; the notebook. Run lab opening first, and wear gloves and eye protection from the first cap.

Procedure

1. Do the hydrate arithmetic and check it a second way before touching a solid. Where a published formula gives both the crystalline and the anhydrous mass, converting one into the other is a free check on your own working; the commonest error here is the right number of grams of the wrong hydrate.
2. Label every bottle now, before it is filled, by the labelling procedure.
3. Weigh each solid over a tray, one tub open at a time, without raising dust, and record every reading as you take it. This is the one step whose controls sit above the rest of the session: sodium sulfite is the most seriously classified solid here, so the weighing runs in still air with gloves and eye protection on and the window shut, and the window is opened afterwards.
4. Put about 600 mL of the warmed water per litre of bath into the beaker and take its temperature.
5. Dissolve the thiosulfate in it, in three or four portions, stirring each in before the next. Take the temperature again when it is in.
6. Stop here for a plain hypo bath and go to step 11.
7. Dissolve the sulfite completely. Nothing acid goes near the beaker until it is in solution, and “completely” is the operative word — hardener added to hypo that has not fully dissolved will throw down sulfur.
8. Add the acid slowly, with stirring, into the bath. Never pour bath into acid.
9. Let the bath come back to room temperature.
10. Dissolve the alum last, added steadily into a cool bath and stirred hard. If the bath is still warm, wait.
11. Make up to the final volume, stir, and let it stand ten minutes before judging its clarity.
12. For a bought concentrate the order reverses. Pour the concentrate into the mixing vessel first, then add the water gradually while stirring throughout: these concentrates do not readily mix with water, and a badly stirred bottle is a bath of two strengths.
13. Measure and record the pH with the temperature beside it.
14. Measure the fresh clearing time on a scrap of the film you actually use, by the clearing-time test. It is the most valuable number this procedure produces, and every later retirement decision is made against it.
15. Fill and cap the bottles, and write on each label what it is, the date mixed, the water used, the volume, the dilution and that fresh clearing time.
16. Route the rinsings. A newly mixed bath has dissolved nothing, so it is not silver-bearing yet: it goes to the general chemical waste stream by its own procedure and under the disposal caveat. That changes the moment the bath meets a film.

How you know it worked

• The arithmetic was checked by a second route before the first weighing, and both routes agreed.
• Each addition was fully in solution before the next one went in.
• The finished bath is clear after ten minutes standing, with no milkiness and no settled sulfur.
• The bottle carries the pH, the volume, the dilution, the date and the fresh clearing time.
• The bath cleared a scrap of your own film inside the maker’s published fixing band.

If a step fails

Step 1, the two arithmetic routes disagree. Do not weigh. The fault is a hydrate or a unit far more often than it is the multiplication.

Step 5, the salt will not all dissolve. The water cooled too far. Warm it gently back towards the starting temperature and keep stirring; do not add more water, which changes the strength.

Step 7 or 8, the bath goes milky or smells sharp. Acid has met thiosulfate without enough sulfite in the way — the pairing the incompatibility matrix lists. Stop, cover the vessel, leave the room with the window open, and come back in eye protection. The batch is lost; route it as acid waste and mix again, this time waiting out step 7.

Step 10, the alum went into a warm bath and the solution clouded. Let it stand and judge it at ten minutes. A bath that is still cloudy is remixed rather than filtered.

Step 12, the concentrate and the water separate visibly. It was under-stirred. Stir it properly now and re-measure the clearing time before using it; a bath of two strengths gives two answers.

Step 14, the fresh bath clears slowly. Check the dilution and the temperature first. A correctly mixed fresh bath that still clears slowly is a bath whose concentrate was old, and the label should say so.

What to record

One line per bath in the batch record: the date and time; the formula or product and its version; every weighed mass to the resolution of the balance; the water used; the final volume; the dilution; the measured pH with its temperature; and the fresh clearing time with the film it was measured on. The formula version record worksheet carries the fields, and the same line is copied onto the label.