Mixing a solution to a final volume

Purpose

To make a stated volume of solution at a stated strength from a weighed solid, and to leave the bench knowing the strength you achieved rather than the one you intended.

When to use it

Whenever a formula’s water line reads water to a volume. That typography is an instruction: the stated figure is the total bulk after the solids have dissolved, and period British formulae write it the same way.

It is not the procedure for diluting something you already have in a bottle — that is mixing from a stock — and it is not the procedure for a formula that genuinely says in a litre of water, which is the other operation and gives a different concentration.

Before you start

You should already be able to tell making up to volume from adding to a volume, and say why the gap between them grows with strength, from concentration and dilution. You should know what your own vessel delivers, from verifying the glassware.

On the bench: the weighed solid, in the mixing beaker, by weighing a solid; a beaker for dissolving; the graduate or flask you will make up in; a funnel that belongs to this family and no other; a stirring rod; distilled or deionised water; the labelled bottle, labelled before it is filled; the notebook and the chemical inventory.

Procedure

1. Write the target down before you touch anything: the final volume, and the strength, in the unit the formula uses.
2. Calculate the mass from the target. Per cent w/v is grams per 100 ml of solution, so 500 ml at 10 per cent w/v is 50.0 g.
3. Weigh it by weighing a solid, and carry the actual reading forward, not the target.
4. Add about three-quarters of the final volume of water to the beaker and stir until the solid has dissolved. Warm water dissolves it faster; the volume is made up cold.
5. Record the temperature of the water before the solid went in and again once it has dissolved, and how long it took. Do not predict which way the temperature will move — write down what it does.
6. Transfer to the graduate or flask through the funnel, then rinse the beaker twice with a little of the remaining water and add the rinsings. What stays in the beaker is missing from the solution.
7. Top up to a little short of the mark and mix.
8. Stand the vessel on a level surface, bring your eye level with the mark, and top up until the bottom of the meniscus sits on it.
9. Stopper and invert several times, or stir with the rod that belongs to this family.
10. Calculate the strength you actually made, from the mass recorded at step 3 and the volume actually reached: 49.87 g in 500 ml is 49.87 ÷ 5 = 9.97 per cent w/v.
11. Fill the bottle, whose label already carries the figure from step 10 and the date, by labelling a container.
12. Enter the batch in the inventory by recording a batch.

How you know it worked

• Nothing undissolved is left in the beaker or on the bottom of the vessel.
• The final level is on the mark, read with your eye level with it and the vessel on a level surface.
• The label carries the strength from step 10, which is arithmetic on two measurements, and not the round number from step 1.
• The notebook can answer “what was in that bottle?” without anyone having to remember.

If a step fails

Step 4, the solid will not dissolve. Warm the water further and give it time before you add more water: adding water is the one move that changes the answer, because the final volume is fixed by the mark and not by how much water it took. If it still will not dissolve, the solid is the wrong form, the wrong substance or spoilt — go to the stock check.

Step 5, the solution warms or cools noticeably. Let it come back to room temperature before step 8. A warm solution occupies more volume than the same solution cold, so a mark hit hot is a mark missed cold.

Step 6, there is no funnel for this family. Wash one properly — detergent, brush, rinse — or transfer without it and accept the loss you can see. A funnel that has held fixer does not carry developer until it has been washed; a narrow neck you cannot inspect is the worst thing on the bench to share.

Step 8, you overshoot the mark. Pour nothing back out. Record the volume you actually reached, redo step 10 with it, and label that. An overshoot you recorded is a solution of known strength.

Step 8, the vessel has no mark at the volume you want. Make up in the vessel that does have one, even if it means making a different volume, and scale the mass to it. Estimating a mark is the error the graduate check exists to stop you making.

Step 10, you cannot reconstruct the mass or the volume. The batch is of unknown strength. Label it strength not recorded and treat it as usable for work you are willing to lose. It will not serve a comparison between two of your own batches, which is most of what this course asks.

Step 12, the inventory is not to hand. Write the line in the notebook now and copy it across the same day. A batch that was mixed and never entered is a bottle nobody can plan around.

What to record

One line per batch: the date; the substance and its form; the target volume and strength; the mass actually weighed; the volume actually reached; the strength calculated from those two; the water used; and the temperatures and dissolving time from step 5. The same figures go on the label and into the inventory, and the three should agree the moment you finish.