Model_report_Expt2-revised_2009 - Experiment 2: Acid / base...

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Experiment 2: Acid / base titration 62.0 0.5 mM @ 95% confidence level unknown c Nikolai Skrynnikov TA: Boone Prentice Section number: 1 25 Jan 2008 (data courtesy of Ike Fehrenbacher, 2004) 1
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Introduction This laboratory exercise relies on a titration technique to determine an unknown concentration of monoprotic acid in solution. In the process of titration, a basic solution is gradually added to the acidic solution until complete neutralization is obtained. The ‘end point’ of the titration is detected with the help of an indicator as color of the solution changes upon neutralization. By measuring the volume of the titrant required to reach the ‘end point’, it is possible to relate the concentration of the acid to the concentration of the base. In this manner, the unknown concentration can be expressed through the known concentration. The concentration determination is repeated several times in order to improve the precision of the measurements and to estimate the experimental error. Results and discussion The experiment involves two steps: ( i ) Standardization of sodium hydroxide (NaOH) solution using potassium hydrogen phtalate (KHP) solution, and ( ii ) titration of an unknown monoprotic acid solution using the standardized NaOH solution. The two steps, ( i ) and ( ii ), are essentially similar. Therefore, only the first step is briefly described below. The neutralization reaction proceeds as follows: NaOH + KHP Na + + K + + P 2- + H 2 O Once this reaction is complete, an excess of NaOH starts building up, triggering the response from the indicator: NaOH + HIn(colorless) Na + + In - (pink) + H 2 O. An interesting question is why NaOH first reacts with KHP and only then, after KHP is consumed, it starts reacting with HIn. At this point I do not know the answer, but hope that this question will be addressed later in class. Another question arises as to why step ( i ) is needed at all. Indeed, one could envisage a simpler measurement scheme where the solution of NaOH is prepared with known concentration and used to titrate an unknown acid. Bear in mind, however, that NaOH is a poor primary standard: it is highly hygroscopic, chemically unstable (reacts with CO
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This note was uploaded on 02/16/2012 for the course CHEM 125 taught by Professor Na during the Fall '11 term at Purdue University-West Lafayette.

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Model_report_Expt2-revised_2009 - Experiment 2: Acid / base...

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