Read sections 9B1 and 9B2 in your book for preparation for this experiment.

In this experiment you will be measuring the properties of a triprotic acid (H₃PO₄). There are several parts to this experiment, you do not need to do all activities in order. Coordinate with your colleagues to best make use of the equipment.

Look up the definition of end point and equivalence point of a titration in your textbook or elsewhere. Define both in your notebook. How much bias will be introduced in this experiment by measuring the end point with the indicators and the equivalence point with pH meters?

Write down the procedure for the standardization of NaOH with KHP.  In your procedure assume you will be starting with NaOH pellets.  See freshman lab manual, key word search on internet, Analytical chemistry for technicians by John Kenkel, (available in the library) or by checkout from Chemistry office, or Kennedy, 1990 (also available via check out from the Chemistry office).

Determine the procedure to make 250mL of 0.1M H₃PO₄ from concentrated 85% H₃PO₄ .  The density of 85% H₃PO₄  is 1.685g/mL.

1) Make standardized NaOH of approximately 0.1M concentration. Make sure you make enough NaOH to do the entire experiment. (see prelab for NaOH standardization procedure). Store your NaOH in a plastic bottle. Next, make 250mL of approximately 0.1M H₃PO₄, you will determine the exact concentration in part 4.

2) Prepare a natural indicator and measure the color change pH ranges as indicated below.

3) Become familiar with the general operation of the Vernier computer interface system.

4) Measure the titration curve of H₃PO₄ with the Vernier system with NaOH. You will have to calibrate the rate NaOH is flowing out of the burette. Use the titration curve information to determine the exact concentration of H₃PO₄. Repeat 2 more times. In your lab report, determine K_a1, K_a2, and (K_a3 if you can) for H₃PO₄.

5) Repeat the titration one more time, this time do it by hand with a pH meter. Take pH readings a minimum of every 0.5mL, more often as your pH starts to dramatically change. The more often you take pH readings the more accurate your pH curve will be. For your report graph each of these titration curves and report the average value for pK_a1, pK_a2, and pK_a3 obtained in parts 3 and 4.

6) Prepare samples of Coca-Cola for analysis.  You will need to do the titration 3X, each time requiring approximately 250mL of Coke.  To prepare the sample, boil the Coke for approximately 5 minutes, this will rid the sample of the majority of the CO2.  After boiling, cover your sample and take steps not to reintroduce more CO2 (i.e. do not shake excessively, etc.).

7) With your prepared sample of Coca-Cola, determine the concentration of H₃PO₄ in Coca-Cola. Repeat the titration a total of 3x. You may use the computer for these titrations.

8) Your instructor will provide an unknown phosphate species.  Determine the concentration of phosphate in the unknown along with the identity of the phosphate species (i.e. is it H₃PO₄, H₂PO₄^-, HPO₄^2-, or PO₄^3-).   Do not use the computers for this titration. Pick the appropriate indicators based upon the titration curves obtained in 4) and the pH color change ranges measured in 2) by you and your colleagues.  Repeat this titration 3X.

You have all used synthetic indicators many times. Examples of these include phenophalein, bromothymol blue, cresol red, etc. Less well known, but equally dramatic are the color changes associated with material readily extractable from natural products. In this part of the experiment, each pair will extract an acid-base indicator. After the material is extracted, you will measure the pH where the indicator changes color. Each of the materials used in this experiment has at least 2 distinctive colors of varying pHs. Finally after all the groups have extracted and measured their indicator, select indicators that would be appropriate to measure the titration curve in #8 above.

Extraction: Place the chopped solid starting material in a beaker with enough distilled water to cover the solid. Heat below the boiling point for about 40 minutes. Dilute as needed.

Measurement of color change ranges: Measure out with a graduated cylinder approximately 20mL of 1M HCl.  Take a small portion of the liquid from the natural indicator that you prepared above.  Add it to the HCl.  Measure the pH and record the color of your solution.  Slowly add 0.2M NaOH with a buret, carefully observing any color changes.  In this titration it is not important to measure the volume of NaOH added rather it is important to note the pH(s) where the color changes. Continue up through a pH of 12.

Red apple skin     blueberries     red cabbage     cherries

Radish skin         strawberries    tomatoes            etc.

This lab could have been called "Endpoint vs. Equivalence Point" because we were measuring the titration curves sometimes with endpoints and sometimes with equivalence points. In several sets of titrations you did, sometimes you measured endpoint and sometimes you measured equivalence point. Indicate how you measured the titration curve for each set of titrations. The titrations sets are step 1, step 4, step 5, step 7 and step 8. What are the advantages and disadvantages using endpoints to measure titration curves? Advantages and disadvantages of using equivalence points to measure titration curves?

Justify the choices you made for natural indicators in part 8. Use your book to pick synthetic indicators that you would have used for measurement of the H₃PO₄ titration curve.

The pK_as measured in lab should have become progressively larger. What does this mean in terms of how dissociated each phosphate species is? Write the appropriate equilbria for the dissociation of a polyprotic acid.

Average K_a1, K_a2, and K_a3 values and pK_a1, pK_a2, pK_a3 values obtained in steps 4 and 5.

Titration curves obtained in parts 4, 5,  and 7 plotted on the computer along with first derivative (and if useful, second derivative) plots to find the equivalence point.

The concentration of H₃PO₄ in Coca-Cola in moles/liter and mg/liter along with the standard deviation and 95% confidence interval.

pH range your natural indicators changed colors in

Concentration, standard deviation, and 95% confidence interval of the phosphate containing species in the instructor provided unknown along with the identity (i.e. H₃PO₄, H₂PO₄^-, HPO₄^2-, or PO₄^3-