Weighing a solid

Purpose

To put a stated mass of a solid into a mixing vessel, and to be able to say afterwards what mass actually went in.

When to use it

Whenever a formula gives a mass. It ends at the mixing vessel: what happens after that belongs to mixing a solution to a final volume.

It does not cover choosing which form of a salt to weigh. A formula written for an anhydrous salt and worked with a hydrate is a different quantity of the same chemistry, and the hydrate arithmetic is done before you switch the balance on, not at it.

Before you start

You should already know your own balance’s drift, repeatability and off-centre behaviour, from the balance check, and why a 0.01 g balance weighing 0.20 g of phenidone is 5 per cent uncertain from the display alone — measurement and uncertainty has the arithmetic.

On the bench: the balance on a level, still surface; the weighing vessel; a dedicated spatula or spoon that belongs to this shelf and never to a kitchen drawer; the mixing vessel; the source container; a damp cloth; the notebook, open, before you start rather than afterwards.

The level, and the one step that can raise it. This procedure is Level A as written, for a solid whose sheet classifies it no higher than irritant or harmful if swallowed. Step 6 is the step that moves: where the safety data sheet for the solid in your hand puts it above that, step 6 alone is declared at the higher level, and it carries that level’s controls — gloves chosen on the glove page, eye protection to the standard the sheet asks for, and the particulate mask the sheet specifies if it specifies one. The classification rubric is the authority for declaring one step above a session’s baseline; the operation is what sets the level, and a weighing is the operation here.

Procedure

1. Read the formula and write the target mass in the notebook, with the form of the salt named in full — anhydrous, monohydrate, pentahydrate.
2. Check the source container’s label against that: same substance, same form, same hydrate. A jar of sodium carbonate decahydrate and a jar of the anhydrous salt are 2.70 apart in the mass a formula needs.
3. Stand the balance level and still, away from a window and a radiator. Switch on and let it settle for a minute.
4. Zero it. Place the empty weighing vessel and tare. Never weigh onto the pan itself.
5. Work at bench level, not at eye level, so that the plume from the powder is below your breathing zone.
6. Add the solid slowly with the spatula until the display is close to the target. Stop where you stop. You cannot take it back, and scraping some out again puts it into the air. This is the step that carries the level. For an ordinary salt it is Level A; where the sheet classifies this solid higher, step 6 alone runs at that level with the controls named in Before you start, and no other step of this procedure moves with it.
7. Weigh by difference instead for anything hygroscopic, static-prone or expensive: weigh the source container, tip some out, weigh it again, and take the difference. The substance never sits exposed on a pan.
8. Record the actual reading before you do anything else. 49.87 g is the number your solution is made of; 50 g is the number you were aiming at, and only one of them explains a result later.
9. Cap the source container and return it to its group on the shelf before the next step.
10. Transfer the solid into the mixing vessel, then rinse the weighing vessel into the same vessel with part of the water the formula already calls for. What stays on the glass is missing from the batch.
11. Clean any spilt powder wet, with the damp cloth. Brushing a dry spill re-suspends the whole of it.

How you know it worked

• The notebook carries the reading, not the target, and the form of the salt beside it.
• The mass you recorded is one your balance can carry: the smaller the mass, the larger the share of it the display alone is uncertain by.
• The weighing vessel is empty and rinsed into the batch, not standing on the bench with a film of powder in it.
• The source container is capped, back in its group, and still says what it said before.

If a step fails

Step 3, the display will not settle. A draught, an unlevel surface or a warming instrument. Move it, let it stand five minutes, re-zero. If it still will not settle, run the balance check rather than weighing through it.

Step 6, you overshoot the target. Do not scoop any back. Either use the mass you have and scale the whole formula to it, or start again with a clean vessel and route what you weighed out by general chemical waste, reading the disposal caveat first. Scaling is usually the better answer, and it is arithmetic rather than judgement.

Step 6, the target mass is too small for your balance. Weigh ten times as much into ten times the volume and measure out a portion of the solution, which moves the precision from the balance to the graduate — the route concentration and dilution gives. Do not weigh a mass the display cannot resolve and write it down as though it were measured.

Step 6, the powder creeps or leaps off the spatula. Static. Go to step 7 and weigh by difference; it is the reason that step exists.

Step 6, the solid is caked, damp or has lost its shape. A deliquescent salt has taken up water and a weighed mass is partly water, so you are under-dosing; an effloresced hydrate such as sodium carbonate decahydrate has given water up, so you are over-dosing. Neither is weighable to a stated strength. Take it to the stock check and decide whether it is retired.

Step 6, the sheet for this solid asks for controls you do not have. Then the step is not classified for you today. Buy the substance as a solution if it is sold as one, or defer the batch. Princeton’s guidance reaches the same conclusion from the other direction: where liquid chemistry exists, buying it means the question does not arise.

Step 8, you did not write the reading down and the vessel is already tipped. The batch’s strength is now unknown. Carry on, and label the result with the words mass not recorded by the labelling procedure. It will do for work you are willing to lose and not for a comparison between two of your own batches.

Step 11, the spill is more than a smear. Stop and use the spill procedure instead, which is written for a quantity this one is not.

What to record

One line, in the notebook, before the solid leaves the weighing vessel: the date; the substance and its form; the target mass; the mass actually read; the balance used; and whether it was weighed directly or by difference. If you applied a deviation from the calibration log, write the correction and the corrected figure, not the corrected figure alone.