The purpose of this experiment is to use Volumetric Analysis to determine the concentration of acetic acid in white vinegar.
The concentration is a measurement of relative amounts of solute and solvent. There are many diverse ways of expressing concentration but the most accepted and widely used is molar concentration and regularly referred to as molarity which is defined in mole, the amount of solute dissolved in one litre of solution.
In this experiment there are solutions used with accurately known concentrations called standard solutions. The reaction of chemicals in the atmosphere and decomposition of them are the causes for many chemicals to be tainted or impure. This also means that common strong acids and bases cannot be made directly into standard solutions.
The amount of a certain type of substance, in mole that can be calculated accurately from their mass are called primary standards due its purity. Primary standards should be always readily obtainable in pure form. It also should also have a known formula and be simple to put away without deteriorating or reacting with the atmosphere. It is also preferable that the substance have a high molar mass in order to decrease the effect of errors in weighing. A primary standard can be used to prepare a standard solution by making up an accurately known volume of solution by means of dissolving an accurately known mass in it. Volumetric flasks are used to obtain the precise volumes of solutions.
A way that is quite widespread for finding an unknown concentration is by using a procedure called volumetric analysis. The procedure grips together through reaction of a measured volume of a standard solution with a measured volume of the standard solution of unknown concentration. Essentially, the solutions are combined until they have just reacted entirely. This method is recognized as performing a titration. In general, the features of acid-base titrations has one of the solutions, usually the acid, dispensed slowly from a burette into a known volume, measured with a pipette, of the other solution; the equivalence point is the point where for the period of the titration when the solutions have been combined in the mole ratio revealed by the reaction equation. This is where the solutions have been neutralised at this stage of the acid-base reaction.
Often solutions of common acids and bases, in addition the products of their reactions are colourless, therefore an indicator is required to be added to the reaction to discover the equivalence point. Acid-base indicators are substances whose colour depends on the concentration of H3O+ ions in solutions. As indicators are relatively weak acids, with their acid form being one colour and their conjugate base form being another. The end point is when the indicator changes colour. Careful selection of indicator is necessary to ensure that the end point during the titration closely matches the equivalence point of the reaction.
In the volumetric analysis burettes and pipettes are used for measurement. The burette is used to deliver the variable volumes accurately. The volume of a liquid measured by a burette in a titration is called a titre. Pipettes are used to deliver fixed, accurate volumes of liquid. Aliquots are volumes measured by a pipette. There are at all times errors made in volumetric analysis or experimental work for that matter. Uncertainties associated with the equipment used for this volumetric analysis are:
|Volumetric Flask |250 mL |± 0.23 mL | |Pipette |20 mL |± 0.06 mL | |Pipette |25 mL |± 0.04 mL | |Burette | |± 0.1 mL (0.02 mL | | | |for each reading) | | | | | | | |...
Bibliography: Commons, Chris and others 2003, Heinemann Chemistry two 3rd Edition, Malcolm Parsons, Singapore
Microsoft Encarta Encyclopedia Standard, 2005, CD ROM, Microsoft Corporation, U.S.A
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< http://www.avogadro.co.uk/chemeqm/acidbase/titration/phcurves.htm> Viewed 17th September
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