pH Titration of Phosphoric Acid & Spectrophotometer Determination of the Percentage of Phosphoric Acid, in Cola Drinks (Experiment 23 & 24)
Partner: Debnil Chowdhury
TA’s: Russell Dondero &
Performed Feb. 16,2000
The objective of this lab was to determine the concentration of phosphoric acid in cola through two different methods: a pH titration and a spectrophotometer method.
pH titration method
First, start by preparing a pH meter for use. This was done by using a pH 7.0 buffer solution and pH 4.0 buffer solution. Next the sample of cola to be used must be degassed. This is done by placing 100mL of the cola sample in a flask. Mark the liquid level with a label and cover the top of the flask with a watch glass. Simmer the sample for 20 minutes to expel the carbon dioxide from the sample. After you have stopped heating, and the sample has cooled down to room temperature, add distilled water, if necessary, to restore the liquid level to its preheating height. Before you can start the titration you must standardize the NaOH solution. Rinse a buret once with distilled water and three times with small amounts of the solution of NaOH.. Standardize the sodium hydroxide solution by titrating it against accurately weighed 0.06 g samples of KHP. Rinse a 10-mL pipet once with distilled water and three times with small amounts of the decarbonated cola drink. Use the pipet twice to transfer 20.00 mL of the decarbonated cola drink into a 50-niL beaker. Place a stirring bar in your sample. Place the flask containing the sample on a magnetic stirrer. Immerse the tips of the electrodes in the decarbonated cola. Adjust the speed of the stirrer so that good mixing is achieved without endangering the electrode tips by a flying stirring bar. Read and record the initial pH. Begin the titration by adding 2 mL increments of the standardized NaOH. Record the pH and buret readings after each addition of base. As each equivalence point is approached, decrease the size of the base increments until, finally, single drops are being added. Continue the titration until well past the second equivalence point. Prepare a plot of pH versus volume of base added. Use this plot to locate the first equivalence point. Combine the Molarity of the base with the volume used to reach the equivalence point to find the concentration of phosphoric acid in the drink. Find the pH at the halfway point for the first portion of the curve to find a value of K., for phosphoric acid.. Now calculate an independent value of Ka1 by combining the initial pH of the drink with the concentration of phosphoric acid. Compare this value of Ka1 with the literature value. Find the percent ionization of the phosphoric acid in the drink. Evaluate Ka2 by finding the pH halfway between the first and second equivalence points. Calculate the percent ionization of H2PO4- in your sample.
Calculate an independent value for the concentration of phosphoric acid in the drink, based on the volume of base used to reach the second end point. Spectrophotometer Determination
Clean and rinse two matched spectrophotometer cells. Fill each cell with water and dry the cell on the outside using a soft, absorbent tissue. Inspect each cell to ensure that no dirt, air bubbles, or scratches are in the light path. Insert each cell in turn into the spectrophotometer and measure the absorbance. The cell that has the lower absorbance should be noted and used as your reference. With that reference cell in the cell compartment, close the cover and set the light control so that the meter reads 100% T (or zero absorbance). Replace the reference with the sample cell, close the cover, and read and record its % T (or absorbance). The difference between the absorbance of the sample cell and the absorbance of the...
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