Exp 3 - Lab Manual: Quantitative Analysis of CO32-/OH- a...

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Lab Manual: Quantitative Analysis of CO 3 2- /OH - a Mixture and Citric Acid in a Soft Drink 1 Quantitative Analysis of CO 3 2- /OH - a Mixture and Citric Acid in a Soft Drink (I) Determination of Hydroxide and Carbonate Ions in a Mixture (II) Determination of Citric Acid in a Soft Drink U INTRODUCTION & THEORY Part I – Hydroxide and Carbonate Mixture Carbon dioxide has a limited solubility in water but dissolves from the air according to 22 () ( ) CO g CO aq Some of the dissolved carbon dioxide forms carbonic acid, H B 2 B CO B 3 B , according to the reaction, 2 3 CO aq H O H CO aq   which as a weak acid, 23 2 3 3 H CO aq H O HCO H O   1 6.35 a pK 2 32 3 3 HCO H O CO H O   2 10.33 a pK The conjugate base, 3 HCO , can act as either a weak acid or a weak base, depending on the solution conditions. In a strong basic environment such as a NaOH solution, carbonic acid will be completely deprotonated and exist mainly as the carbonate ion. Therefore, the determination of the carbonate concentration in a sodium hydroxide solution is an important quality test for such solutions. The net reaction of dissolved 2 CO in a basic solution is the loss of hydroxide ions and the formation of carbonate ions: 2 2 ()2 CO aq OH aq CO aq H O   Carbonate ions react with acids according to the following equation. Carbonate ion can, like the hydroxide ion, be titrated directly with acid. 2 3 2 2 ()2 () CO aq H aq H CO aq H O CO aq  The purpose of this experiment is to determine the concentration of carbonate ions in a sodium hydroxide solution without separating the two ions. Your strategy is to first determine the total amount of base present. This includes both the hydroxide and carbonate ions, and is performed by titrating a sample with standardized monoprotic acid. The choice of indicators for this titration is critical to the calculations. If a basic indicator (such as phenolphthalein) is used, the carbonate will only titrate to the bicarbonate species while the hydroxide will titrate to water. If x equals the moles of carbonate and y, the moles of sodium hydroxide in the unknown, then the moles of standardized acid added would equal x + y. If, however, an acidic indicator (such as methyl orange, pK B a B = 3.46, or bromocresol green) is used, the carbonate will titrate to the carbonic acid species.
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This note was uploaded on 06/02/2011 for the course CHEM 100A taught by Professor Dai during the Winter '06 term at UCSD.

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Exp 3 - Lab Manual: Quantitative Analysis of CO32-/OH- a...

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