Practical investigations
Read our general notes on Risk Assessment
I have just started my coursework on analysis of vinegars and after reading on this site that you must standardise the alkali which will be used for titrating the vinegar, I was left confused. Could you please explain what “standardisation” means and how to go about it because I have been unable to find anything about it on the internet so far
Igloo writes ...
A standard solution is one for which you know the exact concentration, i.e. once known, it can be relied upon not to change before use.
If you are using an alkali to determine the concentration of acids in vinegars, it is obviously vital that you know the concentration of the alkali you are using beforehand. Unfortunately, alkalis, e.g. sodium hydroxide, react with acidic gases in the air, e.g. carbon dioxide, so solutions of alkalis have a steadily decreasing concentration of hydroxide ions, and therefore have unreliable concentrations.
The procedure adopted, therefore, is first to make up the alkali solution by dissolving the relevant mass of solid alkali in a known volume of solution contained in a graduated flask. Next, it is necessary to use an acid which can be made up to solutions of known and reliable concentrations. Potassium hydrogenphthalate is a good choice of standard here, so that when you prepare, for example, a 0.1 molar solution, by dissolving 0.1 mol of the solid in 1.00 dm3 of solution, you can be certain that its concentration will remain fixed, even if it is left for some time. Finally, a titration can be carried out between the solution of alkali and potassium hydrogenphthalate, and, from an appropriate calculation, the concentration of the alkali can be determined.
It is then vital that the alkali is used soon afterwards in the titrations with the vinegars, otherwise another standardization process will need to be undertaken. By “soon afterwards”, I am referring to the same practical session.
You will gain credit in your investigation if you have standardized your alkali in this way prior to using it with the vinegars, even if the entire procedure I have described seems somewhat cumbersome.
Always carry out a risk assessment before starting any practical work, and check with your teacher.
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.
back to Practical investigations
Rate this page or react
Share your views on this page, 12 ratings so far
, rated at 
updated: 02 March 2004
