Verifying volumetric glassware by weighing water

Purpose

To find out how many millilitres your graduate actually delivers when it is filled to a mark and poured, and to write that number on the vessel so it travels with the object.

When to use it

At commissioning, for every vessel you will measure with; again for any vessel that arrives afterwards; and again for any vessel whose markings you cannot read or cannot trust. It is the third of the three commissioning checks, and it is the only one of them that tests a vessel rather than an instrument with a display.

Run it at each mark you actually use. A graduate verified at 100 ml has been verified at 100 ml, and it says nothing about the 250 ml mark on the same vessel.

Before you start

You should already have the balance’s own findings from the balance check, because this procedure measures a vessel with a balance and inherits whatever the balance is doing. You should know the difference between a vessel calibrated to contain and one calibrated to deliver, from measurement and uncertainty, which is what makes the result of this check the smaller of the two numbers.

On the bench: the balance, and a beaker it can hold at the mass of water you are about to pour; the vessel under test; water at room temperature; a thermometer; the calibration record, which already carries a graduate sheet.

Procedure

1. Read what the vessel itself claims: nominal volume, a reference temperature, and any mark saying whether it is calibrated to contain or to deliver. Write down whatever is there, including nothing.
2. Record the water temperature.
3. Weigh the dry beaker and record its mass, or tare it, which is the same operation with the arithmetic already done.
4. Fill the vessel to the mark with water, standing on a level surface, bringing your eye level with the mark and reading the bottom of the meniscus.
5. Pour it into the beaker, draining for five seconds. Do not shake the last drop out, and do not touch the vessel’s rim to the beaker to coax it.
6. Weigh the beaker and record the mass of water delivered.
7. Repeat steps 3 to 6 twice more. Record all three masses.
8. Take the mean and convert: 1 litre of water weighs 1000 g, so 1 g of water is 1 ml. A mean of 96.4 g is a delivered volume of about 96.4 ml.
9. Write the finding on the vessel itself in permanent marker — delivers 96.4 ml at the 100 ml mark, drained 5 s — so that it is on the object rather than only in a notebook.
10. Write the same line, with the method, on the graduate sheet of the calibration record.
11. Repeat from step 3 for every other mark on that vessel you intend to use, and for every other vessel.

How you know it worked

• Three masses are written down, not one, and they agree closely enough that you would accept any of them.
• The record says delivers, names the drain time, and gives the water temperature.
• The number is written on the vessel as well as in the log.
• Every graduate you will measure with this year now carries a figure; the ones that do not have not been checked, and you can tell at a glance which is which.

If a step fails

Step 1, the vessel carries no nominal volume, no reference temperature and no contain-or-deliver mark. Then you do not know its tolerance from the vessel, which is the case this check was written for. Verify it and use the measured figure. This course quotes no tolerance for any class or size, having read no standard that defines them.

Step 4, you overshoot the mark. Empty it and start again. Drawing liquid back out with a pipette changes the film left on the wall and therefore changes the thing being measured.

Step 5, drops keep forming at the lip after five seconds. Stop at five seconds anyway, every time. The figure is a delivered volume poured the way you pour, so an inconsistent drain time is the error, not the drops.

Step 7, the three masses scatter by more than about one part in a hundred. Something in the method is moving. In order of likelihood: the drain time is not being kept, the meniscus is being read from a different height each time, or the beaker is not dry at the start of each trial. Redo all three with a timer and the vessel at eye level, and use the redone set.

Step 8, the delivered volume is several per cent below nominal. That is a finding rather than a failure, and it is the finding this check exists to make: a 3 per cent error in a final volume is a 3 per cent error in every concentration made in that vessel. Record it and use the true figure, or retire the vessel.

Step 8, your balance cannot hold the beaker and the water. Verify a smaller mark on the same vessel and say in the record that the larger mark is unverified. Do not scale a small result up to a large mark: the film left behind is not proportional to the volume poured.

What to record

On the graduate sheet of the calibration record: the date; the vessel and the mark tested; the water temperature; the three masses and their mean; the delivered volume; and the drain time used. Then the same delivered volume on the vessel itself.

Say what has been measured and what has not. This is the volume the vessel delivers, poured that way, which is smaller than the volume it contains by whatever film stayed on the wall. This course takes 1 ml of water as 1 g, on ILFORD’s published figure, and has read no density-of-water table, so it applies no temperature correction — which makes the check good enough to find a vessel several per cent out, and not good enough to be called a calibration. Write that sentence in the record the first time, so the next reader is not left to infer it.