Chemguide: Support for CIE A level Chemistry

Using volumetric apparatus

Introduction

If you have done a lot of practical work, this will all be obvious to you. If you haven't, then you can easily waste several marks, and this page is to try to help you to avoid that.

Volumetric glassware and its uses

Volumetric flasks

A volumetric flask is used for making up fixed volumes of solutions of accurately known concentrations. Volumetric flasks come in various sizes, but the ones you are most likely to use in the lab at this level are 250 cm³ and 100 cm³.

A video link further down the page shows how such a flask is used.

If you are asked a question about how you make up an accurate solution, you must use the term "volumetric flask". You cannot use the markings on a conical flask or beaker. Examiner's Reports constantly talk about students trying to make up accurate solutions using these. You can't do it! These markings are only approximations. If you want to make up an accurate solution, you have to use a volumetric flask.

If you added 250 cm³ of water to a solid and then stirred it, the resulting volume is quite often not exactly 250 cm³. Interactions between the substance you are dissolving and the water molecules can cause the volume of the solution to change.

Pipettes

Pipettes are used to measure out fixed volumes of liquids very accurately. The most common sizes are 25 cm³ and 10 cm³. Although pipettes come in other sizes, you are probably unlikely to come across a bigger one in an A level lab, although you might occasionally use a smaller one such as 5 cm³. Small graduated pipettes are also available - rather like a dropper with markings on.

The proper use of a pipette is detailed in a video link further down the page.

Burettes

Burettes are used to measure out variable volumes of liquid very accurately - including, of course, in titrations. The most common size in an A level lab is 50 cm³.

If you need to measure out some bigger volume of liquid, there is no reason why you couldn't do this by refilling the burette. For example, there was a CIE question which asked you how you would add exactly 100 cm³ of one solution to another one. The Examiner's Report suggested using a burette, adding two separate lots of 50 cm³. But if you are going to do this, be precise - say that you are going to use a 50 cm³ burette twice.

Again, the use of a burette is detailed in the videos below.

Measuring cylinders

Measuring cylinders are not accurate enough if you need to add an exact volume, and you won't get a mark if you suggest using one where a burette or pipette would be more appropriate.

In titration work, a simple example of the use of a measuring cylinder is where you need to add an excess of dilute sulphuric acid during a potassium manganate(VII) titration. You might, for example, be told to add 10 cm³ of dilute sulfuric acid to the liquid in the flask before you run in the potassium manganate(VII) solution from the burette.

A measuring cylinder is fine for this, because all that matters is that the acid is in excess. As long as that is true, the exact amount of acid added doesn't matter.

Useful videos showing volumetric techniques

There are a lot of similar videos on YouTube, and after you have looked at the ones below, you will probably be offered other ones as well. You will find minor differences between some of them. Don't let that worry you - the ones below all show good technique.

Making up accurate solutions

Preparing a standard solution of sodium carbonate

(Waving a burette around as shown in the introductory titles is not good technique - but the rest is!)

Other videos show small differences in how the sodium carbonate is weighed out. In this video it is weighed on a clock glass, and all of it is then washed into the beaker. Another possibility is to weigh it in a weighing bottle.

You can also just tip the sodium carbonate into the beaker without trying to transfer all of it. You can reweigh the container (clock glass or weighing bottle) with whatever is left in it, and then work out how much has been added.

The video I have used seems to me to be the simplest to understand.

Using a pipette

How to use a pipette

This is an excellent video description of the way to use a pipette for accurate work. It mentions the term "aliquot", which is used in this sense to mean a measured sample taken from a liquid.

Using a burette and doing a simple titration

An excellent video description showing how to do this very accurately in a simple acid-base titration using phenolphthalein as indicator, running an alkali (the "titrant") into an acid (the "analyte"). Don't worry too much about these possibly unfamiliar terms. I don't remember ever coming across them at A level.

Washing apparatus out

It is easy to get confused by what you wash with what, and it is important that you understand what you are doing rather than just trying to learn it.

Washing a volumetric flask

This is only washed with water. Normally you would use tap water to start with followed by pure water. The purest water is distilled water, but these days most labs probably use deionised water. Deionised water has had any ions removed, but not non-ionic impurities. Since these don't matter in most titrations, that is usually OK.

What does matter is that you should use the term either "distilled water" or "deionised water", and not simply "water".

Washing a pipette or burette

These should be washed with pure water (probably with a tap water rinse first) followed by some of the solution you are going to put in them. If you left any water in the burette or pipette, then the solution you put in them would be diluted by that water, and so isn't the concentration you think it is.

Washing the conical flask that you are going to do the titration reaction in

This is where people sometimes get confused. The conical flask is only washed with water - probably tap water to start with, and certainly followed by pure water. You do not rinse it with the solution you are going to pipette into it.

Why not? Suppose you pipetted 25 cm³ of solution into a flask which already had some of the solution left in it from the rinse. What volume of solution would you now have? It is impossible to know.

So does it matter if there is some water left in the flask? No!

Let's assume that you know the concentration of the solution you are pipetting because that is the one you have made up accurately. If you add 25 cm³ from the pipette, then you could calculate exactly how many moles of your substance there were in the flask. Adding more water to the flask makes no difference to that.

The same thing applies to any water you add during the titration - for washing around the inside of the flask, for example. That makes no difference to the amounts of substances (measured in moles) that you have added from either the burette or pipette.

So - rinse the burette and pipette with water and the solutions that are going to be put into them. Rinse the conical flask only with water.

Making sure that your titration result is accurate and reliable

Accuracy

It is quite common to do a rough titration to start with, especially if you haven't had a lot of practice. You just add the liquid from the burette 1 cm³ at a time until the indicator changes colour. For example, you might find that it hadn't changed colour after 24 cm³, but had changed colour after 25 cm³.

You can now find an accurate result by quickly running in 24 cm³ (which you know is safe), and then adding the rest very slowly - a drop at a time (or even a partial drop at a time) as you near the endpoint.

So, if you are asked how to make your titration accurate, your answer is to add the liquid from the burette drop-wise as you get near the endpoint.

Reliability

To check the reliability of your result, you repeat the titration until you have concordant titres. These are results which are within 0.10 cm³ of each other. (The titre is the volume of liquid added to reach the end-point.)

When you average out your titration results, you should only use your concordant titres.

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