CHE 143 Lab 6 Base and Vinegar

CHE 143 Lab 6 Base and Vinegar - SUSB-011 Standardization...

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SUSB-011 67 Standardization of Base and Titration of Vinegar SUSB-011 Prepared by M. J. Akhtar and R. C. Kerber, SUNY at Stony Brook (Rev 05/04 – RFS) Purpose of this Exercise: To determine the concentration of acetic acid in a vinegar sample, by means of titration with sodium hydroxide solution which you have previously standardized. Concepts: fermentation, aerobic, weighing by difference, titration, primary standard, equivalence point (end point), indicator, back titration Background Information: Vinegar is the end product of aerobic fermentation of the sugars initially present in fruit juices. For example, in freshly-pressed apple juice, the sugar present is first converted to alcohol by the action of yeast enzymes: C 6 H 12 O 6 2 C 2 H 5 OH + 2 CO 2 . sugar alcohol In the presence of air, the alcohol in turn is oxidized by bacterial enzymes to acetic acid: C 2 H 5 OH + O 2 CH 3 COOH + H 2 O. alcohol acetic acid The result of aerobic fermentation of natural fruit juices is formation of a roughly 5% solution of acetic acid in water, which is called vinegar. Depending on the fruit juice used, other flavor components may also be present in low concentrations, leading to slight flavor differences among wine vinegar, cider vinegar, etc. Other flavoring agents may also be intentionally added. In any event, vinegar is an important component of cookery, in which it is used to impart a sour component to the flavor of a food. Sourness is the mouth's reaction to acidity, and it is the acetic acid which is chiefly responsible for the effect of vinegar. Recipes generally describe the use of a certain quantity of vinegar, assuming that all vinegars are about the same concentration. In fact they vary somewhat, and a really punctilious cook should probably titrate the vinegar in order to determine how much to use to secure an optimum effect. That is not common kitchen practice, however, as burets are rarely found in homes. Chemistry students, however, can determine the strength of a vinegar sample by titrating it with a solution of a standard base, such as sodium hydroxide. The concentration of the standard base, in turn, can be determined by using it to titrate carefully weighed samples of an acid sufficiently pure as to be considered a primary standard . The most commonly used primary standard for standardizing sodium hydroxide solutions is potassium hydrogen phthalate (pronounced "thal- late"), which we shall abbreviate as KHP. KHP is a weak acid which reacts cleanly with sodium hydroxide according to the equation: Na + OH - + HOOC(C 6 H 4 )COO - K + Na + - OOC(C 6 H 4 )COO - K + + H 2 O
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SUSB-011 68 According to this equation, one mole of KHP (molar mass 204.2 g/mol) will neutralize one mole of sodium hydroxide, and the concentration of the sodium hydroxide solution (in moles per liter, M.) can be accurately determined by measuring the volume required to neutralize an accurately known mass of KHP in a titration. At the equivalence point, mmoles KHP = mmoles NaOH
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CHE 143 Lab 6 Base and Vinegar - SUSB-011 Standardization...

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