What is Titration?
Suddenly you have started hearing about titration in class and in your textbooks. You may even have carried out a titration, or at least watched a demo in your college lab.
And it’s very different to mixing things in test tubes and beakers, or setting up a tripod to heat substances over a Bunsen burner.
The procedure is completely new to you. The level of precision is much higher. The notes you’re asked to make are very different too.
So, what exactly is titration?
Titration is a way of measuring the concentration of something, usually the concentration of a substance in a solution. For example, you can use it to find the concentration of a solution of an acid.
Titration is a type of chemical analysis. We consider it to be analysis because we use it to make a measurement. You may hear or read titration described as an analytical method or technique.
Titration can also be described as a volumetric method of analysis, because it involves measuring the volume of a reagent required to react.
In titration we use specialised lab apparatus and reagents.
There are many uses for titration – it can be used to find the concentration of many different substances. At A-Level we usually consider acid-base titrations and redox titrations. We’ll consider these in more detail later in this article.
The Principle of Titration
As we have mentioned, titration is a way of finding out the concentration of a something. If you have a solution of a substance and you need to find its concentration that’s where titration comes in. Let’s consider a solution of hydrochloric acid as an example.
The first step is to measure out a known volume of the sample that you want to find the concentration of into a flask. In our example we would accurately measure a known volume of the hydrochloric acid solution into the flask.
The next step is to exactly react it with something that it reacts with. In our example we could react the acid sample with a solution of sodium hydroxide (of known concentration) until the base exactly neutralises all the acid in the flask.
The point of the reaction where we have completely exactly reacted all our sample substance is called the end-point. When the end-point is reached we record the volume of reagent we have added to the flask. In our example, we would record the volume of the sodium hydroxide solution added when the end-point has been reached.
We often use an indicator solution to enable us to see the end-point. An indicator changes colour when the end-point is reached. In our example, we could use phenolphthalein indicator; this changes from colourless to deep pink when all the acid has been neutralised by the base.
Now we can work out the number of moles of reactant we have added, that exactly reacted with the sample. We know the volume added and the concentration, so we can calculate the moles of reactant added. In our example we know the concentration of the sodium hydroxide, and we know the volume that was required to neutralise the acid in the sample, so we are able to calculate the number of moles of sodium hydroxide used.
Finally, we can calculate the concentration of our sample. From the balanced equation for the reaction done in the titration we can see the ratio of the reaction between the sample substance and the reactant we used – the stoichiometry of the reaction. In our example we can see that the hydrochloric acid and sodium hydroxide react in a 1:1 ratio – that means one mole of acid reacts with one mole of hydroxide. We can then calculate the number of moles of sample substance in the volume we initially measured into the flask, and hence calculate the concentration of the sample.
What Reagents and Apparatus Are Used?
We mentioned that we use a solution of something that reacts with our sample – a solution that we know the exact concentration of. We call this the standard solution, or sometimes the titrant.
We usually need an indicator solution. When we carry out the titration this will change colour at the end-point. (Note that an indicator solution isn’t always necessary because some titration reactions have a clear colour change).
We also need some glassware and apparatus to carry out the titration.
We use a pipette to measure out a known volume of the sample solution. This is a much more accurate measurement than if we used a measuring cylinder. A pipette is a glass tube with a fixed volume – pipettes are available in a range of volumes (e.g. 10mL, 20mL, 25mL pipettes are common).
A burette is used to add the standard to the sample during the titration. A burette is a vertical glass tube with a tap at the lower end. The tube is marked with ‘graduations’ showing the volume of standard solution in the burette. Using the graduations to read the volume at the start and end of the titration gives the volume of standard solution that was required in the titration.
A retort stand and clamp are used to suspend the burette vertically. This allows the standard solution to flow out of the burette when the burette tap is opened.
We measure out the sample into a conical flask. The shape of this flask makes it perfect for swirling the sample and standard solutions together during the titration.
We often position a white tile (or white piece of paper) under the flask during the titration to make it easier to see the colour change at the end-point. This is completely optional but is particularly useful if you are trying to see the colour change on an old, dark wood bench or over a dull, rusty retort stand.
It’s really important that we use clean glassware throughout the titration. We want to be sure that there isn’t any contamination that could interfere with the reaction and lead to an inaccurate analysis.
You may wonder why we don’t use a measuring cylinder to measure any of the volumes. It’s because a measuring cylinder gives only a rough measurement and is useful for approximate measurements of volumes. But it is not accurate enough to be used for measurements used during titration or any other analysis. Pipettes and burettes are designed to be much more accurate and are suitable for use in volumetric analysis.
How To Carry Out a Titration
Now we know what titration is, and what we need, we are now going to describe the procedure. We’ll also relate it to our earlier example of titrating a hydrochloric acid sample with a sodium hydroxide standard.
The first step is to use the pipette to measure a volume of the sample solution into the conical flask. A few drops of indicator are added (if needed).
Next, set the burette up on the retort stand, and filled with the standard solution. Open the tap for long enough to allow standard to run out – this is to remove any air within lower part of the burette.
Read the initial volume of the standard from the burette and record that volume.
Take the conical flask and hold it under the burette. Slowly add the standard to the flask, 1 or 2 mL at a time to start with, by opening and closing the tap. Swirl the flask throughout the process, paying attention to any colour change.
As the colour change seems to last longer in the flask, slow down the addition of standard so you are adding it one drop at a time. The titration is nearing the end-point. Swirl the flask after each drop added, and only add the next drop once the colour changes back.
When you get to the point where the colour doesn’t change back you have reached the end-point. Stop adding standard at this point, and read the volume from the burette. Record this as the final volume.
Listen to me describe how to titrate
Calculate the volume of standard added by simply subtracting the initial volume from the final volume. The volume of standard used is often referred to as the “titre”.
The procedure is repeated at least twice more until there are two titrations with very similar titre values. Work out the average titre discarding any values that are obvious outliers.
Calculating the Concentration
Now we have carried out the titration we need to calculate the number of moles of the substance that was in the standard, the substance we added from the burette.
We know what the substance is, the concentration of the standard and now we also know the average titre required to react exactly with the sample in the flask. Now we just multiply the average titre volume by the concentration (in mol dm-3) – remember to take into account that the titre is in mL, not dm3.
We need to know the balanced equation for the reaction between the sample and the standard. The balanced equation tells us the ratio in which they react, the stoichiometry of the reaction.
Taking the stoichiometry into account we can work out the number of moles of sample substance in the flask. And because we know the volume we pipetted into the flask, we can calculate how many moles of substance per litre are in the sample – our sample concentration that we needed to find.
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