Practical investigations
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I am doing an experiment on the different methods of determining the concentration of ascorbic acid. One of the methods will later be used to determine the concentration during the decomposition occurring on heating. The methods I have chosen are DCPIP titrations and iodine titrations. I am very confused as how to convert the concentration of ascorbic acid which is given as a percentage, i.e 1%, into a concentration which can be compared with the concentration of iodine and DCPIP. Please help as I need to work out whether my results are reliable and therefore work out if the concentration of ascorbic acid I work out in mol dm-3 is correct.
Igloo writes ...
Concentrations of solutions in food science are usually quoted in g dm-3 or in percentage terms. A 1% solution of ascorbic acid means that 1 gram of the pure solid is present in every 100g of solution, so this equates to a concentration of about 10g dm-3.
However, the experiments are usually carried out with reference to a solution of known concentration, so you needn’t worry about the usual type of titration calculations. It is much easier than that! It’s all to do with “proportion”. Let me give you a worked example.
Suppose that you are supplied with a 1% solution of ascorbic acid, and a solution of DCPIP (or iodine) of unknown concentration.
Let’s assume that 25.0 cm3 of the 1% ascorbic acid solution required 20.0 cm3 of DCPIP solution, but that when 25.0 cm3 of your unknown ascorbic acid solution was titrated with the same DCPIP, the titration reading was only 15.0 cm3.
You can see that LESS DCPIP solution was needed for the unknown ascorbic acid than for the “standard” 1% solution, so it must be LESS concentrated. In fact, more precisely, it must be (15.0/20.0) times as concentrated, i.e. 0.75%
Notice that you do not need to know the concentration of the DCPIP or the equation for the reaction taking place. It is simply a matter of straightforward maths (proportion).
If you want to convert this concentration of 0.75% into g dm-3, all you need to do is to multiply by 10 (as explained earlier), and you obviously arrive at 7.5g dm-3. If you want to convert this into a mol dm-3 value then you will need to work out the molar mass of ascorbic acid and divide this into 7.5g
Always carry out a risk assessment and check with your teacher before carrying out any practical work.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 12 February 2007
