There’s a spill in a laboratory. This is obviously a huge safety concern, particularly if we don’t know what it is. Handily, if it is an acid or a base there are techniques which can be used to see how strong that chemical is. Then the spillage can be cleared up safely and quickly.
Carrying out an acid-base titration
When you are working with unknown concentrations of potentially dangerous compounds it is essential to work as safely as possible. Details on health and safety in a lab environment are covered in the acylation of salicylic acid resource.
Acid-base titrations are used to determine the concentration of a sample of acid or base and are carried out using a piece of equipment called a burette. It is a long, glass tube with a tap at the end which can be used to very carefully add drops of liquid to a test solution. It has a scale (normally in cubic centimetres) down the side to ensure accurate measurements.
To begin with we need to have a solution with a known concentration, which we call a standard solution. If the unknown sample is an acid, then the standard in the burette needs to be a base as you are aiming to see at what point the unknown sample is neutralised. The standard is then poured carefully into a burette - be aware that the burette is a tall piece of equipment, so any splashes are more likely to get in your eyes. Remember to clamp the burette very gently so as not the crack the glass. Once the burette is filled to the required level (not necessarily right to the top!) and the liquid is settled, you will be familiar with the curved surface of the liquid in the burette, caused by surface tension. The bottom of the curve is known as the meniscus and this point is what you should use to take your measurements. The shape of the meniscus is determined by the attraction of particles in the liquid to the walls of the container. If particles are more attracted to the container than to each other they will form a concave meniscus. However mercury in a glass container will form a convex meniscus, with a curve the other way around.
A known volume of the unknown concentration solution should be placed in a beaker under the burette. To this you should add a small amount of universal indicator, which will confirm if your unknown sample is acidic or basic. You then need to turn the tap on the burette to allow your standard solution to mix with your unknown sample. Mix the beaker's contents regularly and as the colour begins to change towards green, turn the tap to allow it to drip slowly until the colour of the universal indicator changes to green to indicate you have neutralised the solution. At this point you should turn the tap fully off. Don't worry if you can't get this perfectly neutralised the first time! It takes practice to perfect the technique.
By reading where the meniscus now lies off the burette scale you can work out how many millilitres you have used up. Since you know how much standard you have used and its concentration you can work out the concentration of the unknown sample. Remember you should always repeat whole process at least 3 times to ensure you have an accurate result, as there is the potential for both random and systematic errors to affect your results. A random error is always present in measurement and is a result of the precision and accuracy of your equipment, an example of a random error would be how accurately you are able to read the volume from the burette. Systematic errors are more likely to be caused by poor experimental technique, for example, not properly washing and drying your burette before use can mean the liquid in your burette is not an accurate representation of your unknown sample.
How do we calculate the pH of our unknown sample?
What can we use to detect pH change?
pH is the measure of how acid or basic a solution is. In the acid-base titration above you are trying to see how much of the solution of known concentration is need to neutralise the unknown solution. When the sample is neutral it has a pH of 7. This is the same pH as water. The pH is related to the concentration of hydrogen ions available per litre. Since the number of hydrogen ions in moles is huge and difficult to think about, the mathematical function negative log base 10 of c is used to scale the numbers down. ‘c’ just means the concentration of H+ ions. This mathematic expression reduces the numbers to a scale of 1 to 14 with 1 being highly acidic and 14 being highly basic. It is worth noting here that because the pH scale is logarithmic, each step on the pH scale is not of equal size. pH 7 actually represents a very small band of the scale, which is why it can sometimes be so difficult to precisely neutralise your unknown sample.
We can measure the pH in a number of ways. The oldest form of pH indicator is litmus paper and it is still in use today. The word ‘litmus’ come from the old Norse word for lichens which are types of moss. Dyes have been extracted from a number of different lichens since at least the 14th century and were big business in The Netherlands. These dyes can be used as a solution to put straight into your experiment. However, this contaminates your reactions with a small amount of lots of different compounds. To get around this the litmus solution can be infused into a paper strip. This allows you to dip the paper in and then remove it easily.
There are two types of litmus paper: red, which only turns blue when put in an alkaline solution and blue, which only turns red in acidic conditions. This can be very useful but sometimes we need to know more accurately what the pH is rather than whether it is just acid or base. For this we can use universal indicator, another carefully combined collection of dyes, which also comes as a solution or a paper strip. This time the colours vary over the whole pH range, starting at red for acid and ending with blue for basic. Universal indicator is the one you are likely to use when carrying out an acid base titration. You may also have access to a pH meter and a pH probe which gives a digital reading of pH. Probes typically use electrodes to measure a potential difference between your sample and a sample of a fixed pH. This gives a quick and accurate way of measuring pH, but the equipment is considerably more expensive than using a colour-based indicator like universal indicator.
In the Laboratory Confessions podcast researchers talk about their laboratory experiences in the context of A Level practical assessments. In this episode, we look at measuring pH and the use of indicators, the use of laboratory equipment, and the use of volumetric flasks and making up a standard solution.
Strong and weak acids and bases
Acids and bases are not all the same strength. They can be described as weak or strong and you might be asked what that means during your exams. Acids are compounds that split into two ions when dissolved in water; one or more positive hydrogen ions and negative counter ions. A strong acid, such as HCl, completely splits up when it’s in water. The splitting is called dissociation. A weak acid, such as citric acid, only partially dissociates. This means that some of the acid splits into its ions but some of it stays together. A weak acid exists in equilibrium so some of the whole acid is being formed at the same time some of the acid is splitting into its ions so there should always be the same amount of ions present. This applies to bases as well but in this case the ion of interest is OH-.
You can never know by looking at a solution how strong it might be so always work as safe as possible!
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