Adjusting pH with a concentrated alkali

Purpose

To move a bath’s pH by a stated amount, with a record that says which named bath you made rather than claiming to have tested pH.

When to use it

Whenever an alkali is dissolved as a concentrate or added to move a pH: making an alkaline stock, trimming a developer to a target, or mixing a concentrate whose formula dissolves alkali at hundreds of grams to the litre.

It is not an experiment on pH, and the record must not say it is. Moving pH with a different alkali moves the buffer capacity, the ionic strength and the sulphite speciation in the same gesture, and the design lesson sizes all three. What you have made is the difference between two named baths.

Before you start

You should already know which alkali brings a conjugate pair with it and which brings none, from the alkalis lesson, because a hydroxide bath’s pH falls while it works and a carbonate bath’s barely moves.

Level B. Sodium hydroxide is signal word Danger with severe skin burns and eye damage in nearly every report that classifies it. The controls: chemical splash goggles, not spectacles, whenever the concentrate is open; nitrile gloves at the sheet’s thickness or 0.2 mm; an apron; an eyewash and a spill kit within reach at the bench rather than in another room; a through draught running.

Read the alkali spill procedure before the session, not during it. And where a solution can be bought ready made, buying it removes the operation entirely, which the rubric prefers.

Make a dilute stock rather than dosing from the jar. Weighing pellets into a bath a few at a time means opening a corrosive solid repeatedly over an open vessel; a stock of known strength, made once, turns every later adjustment into a volume measurement.

On the bench: cold water in the vessel first; the calibrated meter, by its own procedure, bracketed at two buffers that straddle your target; a thermometer; the notebook.

Procedure

Making the stock

1. Calibrate the meter first, at two buffers that bracket the pH you are aiming at, and write down the temperature of the calibration.
2. Put the goggles, gloves and apron on before the jar is opened.
3. Put cold water into the vessel first — the volume the formula names, less the alkali’s own — and stand the vessel in a tray.
4. Add the alkali to the water, never the water to the alkali, a little at a time, stirring, and let the heat go before adding more.
5. Cap the jar and put it away before the next step.
6. Make the solution up to its final volume once it is back at room temperature, not while it is warm.
7. Label it at once with substance, strength as per cent w/v, make-up volume, date and initials.

Adjusting a bath

8. Write down the bath’s identity, its version, its volume and its pH before you add anything.
9. Bring the bath to the temperature you will report the pH at, and record that temperature.
10. Add the stock in small, measured increments from a graduate or syringe, stirring after each, and record the running total.
11. Read the pH between increments rather than continuously, letting the reading settle each time.
12. Stop at the target and record the final volume added as well as the final reading.
13. Give the result a new version identifier by the versioning procedure, and write it on the label before the bottle is filled. A bath whose pH you moved is not the formula it came from.
14. Rinse the meter’s electrode and store it as its own maker directs.
15. Route the rinses and any spoiled batch to the alkaline stream by general chemical waste, never into the acid container. Check your local regulations; they govern what happens to it, and the disposal caveat says why the course cannot.

How you know it worked

• The meter was calibrated today, at two buffers that bracket the reading you took.
• The alkali went into water, cold, in a vessel standing in a tray, with the goggles on first.
• The made-up volume was reached after the solution had cooled.
• The stock’s label carries substance, strength, make-up volume, date and initials.
• The adjustment is recorded as a volume of a known stock, not as a number of pellets.
• The adjusted bath carries a new version identifier.

If a step fails

Step 1, the target is above the range your electrode is specified over. Read it anyway and record it as a repeatable fingerprint of your own bath rather than as a verdict on its true pH: above about pH 9 a glass membrane responds to sodium as well as to hydrogen and reads low. Part III teaches the bounding procedure; do not argue with a published figure using a reading taken outside the electrode’s range.

Step 3, water was added to the alkali. Stop, stand back, and let it be. That is the direction the rule runs against, and the heat and spatter are the reason.

Step 4, the vessel is hot to the touch. Wait. A dissolution that outruns the heat you can carry away is one that will spit when the next portion goes in.

Step 6, the volume was made up while it was still warm. The strength is wrong by the thermal expansion, and the honest answer is to record the temperature you made it up at rather than to correct it by arithmetic you have not sourced.

Step 10, you overshoot the target pH. Do not correct it with an acid: that adds a salt and moves the ionic strength in a way the record will not capture. Start again, and use a more dilute stock so that the increments are smaller.

Step 11, the reading drifts and will not settle. Re-run the meter calibration rather than waiting it out. A drifting electrode at high pH is the case where waiting produces a confident wrong number.

Any of it reaches skin or an eye. Irrigate immediately and keep irrigating; an alkali burn goes on penetrating after the contact has ended, which is why the eyewash is at the bench.

What to record

The stock: substance, lot, strength, make-up volume, the temperature it was made up at, and the date. The adjustment: the bath’s identity and version before, its volume and temperature, the meter’s calibration buffers and their temperature, each increment and the running total, the final reading, and the new version identifier. Then one sentence naming what the change actually is — the difference between two named baths — so that nobody later reads the file as an experiment on pH.