Mixing a working solution from a stock

Purpose

To turn a bottle of stock into a stated volume of working solution at a stated strength, and to be able to say afterwards what you actually made rather than what you set out to make.

When to use it

Whenever a page, a data sheet or a formula asks you to dilute something you already have.

It does not cover making a stock from a solid. That starts at the balance and it is taught in the commissioning lab, whose Stage 2 is the procedure; this one begins at a bottle whose concentration is already known.

Before you start

You should already be able to state the difference between making up to volume and adding to a volume, and read the notation a source is using, both from concentration and dilution. You should know what your own graduate actually delivers, from the balance and thermometer check or the commissioning lab.

On the bench: the stock, labelled; the target volume and strength written down before you touch anything; the smallest graduate that will hold the volume of stock you need; the vessel you will make up in; a funnel; the bottle you will fill, already labelled; the notebook.

Procedure

1. Read the stock’s label and take its true concentration from it — the figure it was measured to be, not the round number it was aimed at — together with the date it was mixed.
2. Write down the target: the final volume you want and the strength you want, in the same units as the stock.
3. Calculate the volume of stock. V₁ = C₂V₂ ÷ C₁.
4. Check it by a different route before you pour. Work out how much solute the calculated volume of stock carries, then divide that by the final volume and confirm you get the target strength. A check that repeats the same arithmetic is not a check.
5. Read which operation the source is asking for. A 1+n instruction means one volume of stock and n volumes of water; a stated strength means make up to the final volume. Where the choice is yours, make up to volume — it assumes nothing about whether volumes add.
6. Measure the stock in the smallest graduate that will hold it, at eye level, reading the bottom of the meniscus, and correct for the volume that vessel actually delivers.
7. Deliver the stock into the mixing vessel, then rinse the measuring graduate into the same vessel with a little of the water the dilution already calls for. What stays on the wall of the graduate is missing from the batch.
8. Add the water. Making up to volume: fill to a little short of the mark, mix, then top up with the vessel on a level surface and your eye level with the mark. Additive: add the measured volume of water.
9. Cap and invert several times, or stir with the family’s own rod.
10. Calculate the strength you actually made, from the stock’s true concentration, the volume of stock you actually measured and the final volume you actually reached.
11. Fill the labelled bottle. The label carries the strength from step 10, not the one from step 2 — see labelling a container.
12. Return no surplus to the stock bottle. What is left over is used now or it becomes waste.

How you know it worked

• The two arithmetic routes in steps 3 and 4 agree.
• Every volume you measured sat inside a vessel that can measure it: ILFORD’s own floor for its liquid concentrate is 10 ml, below which a measuring cylinder is not the right instrument.
• The final level is at the mark, and you know whether you reached it by topping up or by adding a measured volume.
• The label on the bottle states the strength you calculated in step 10 and the date.
• The stock bottle contains only what it contained before, less what you took out.

If a step fails

Step 1, the stock carries no true strength or no date. It is not a stock, it is an unidentified liquid, and this course treats an unlabelled bottle as waste rather than as a puzzle. Retire it and note what happened, so that the labelling habit gets the credit or the blame.

Step 3, the calculated volume of stock is under about 10 ml. A measuring cylinder cannot deliver it usefully. Use a graduated pipette, or make it in two steps — but note that errors multiply through a serial dilution, so use the fewest steps in which every measured volume sits inside a vessel that can measure it.

Step 4, the two routes disagree. Do not pour. Redo both from the written figures rather than from memory; the fault is a unit far more often than it is arithmetic.

Step 6, the graduate you want is dirty or belongs to another family. Wash it properly — detergent, brush, rinse — or use another. A graduate that has held fixer does not measure developer again until it has been washed, and if you are unsure it keeps the family it last held.

Step 8, you overshoot the mark. Pour nothing back out. Record the actual final volume, redo step 10 with it, and label that. An overshoot you recorded is a solution of known strength; an overshoot you ignored is a mystery in a bottle.

Step 10, you did not write the figures down as you went. Measure what is still measurable — the volume now in the bottle — and label the batch with what you actually know, including the words strength not recorded. It will do for work where the exact figure does not matter, and it will not do for a comparison between two of your own batches, which is most of what this course asks of you.

What to record

One line per batch: the date; which stock, with its true strength and its own mixing date; the volume of stock taken and the vessel it was measured in; the final volume reached; the strength achieved; and any delivered-volume correction you applied. That line is what lets you explain a result three months later instead of guessing at one.