DETERMINATION OF KHP PURITY

Standardization of NaOH Dry about 6 grams of primary standard potassium biphthalate (KHP, MW = 204.23) for at least 1 hour at 110oC (DO NOT overdry your sample). Make sure you record the purity of the standard. While your sample is drying add about 6 mL of a 50% NaOH solution to about 500 mL of DI water in a polyethylene storage bottle. Mix well, and add another 500 mL of DI water. This will give you a solution that is about 0.1 M NaOH.

If you have not done so already, remove the KHP from the drying oven and allow it to cool to room temperature (about 10 minutes) in your desiccator. Using an analytical balance, weigh out 5 samples of the standard into separate numbered conical flasks. Be sure to choose sample sizes that will require between 30 and 50 mL of titrant. Dissolve the samples in about 75 mL of DI water and add 3-4 drops of phenolphthalein indicator to each sample. Titrate each sample with the NaOH until the solution just turns pink. At this point rinse down the side of the flask, and check to see that the color remains. If it does not, add more titrant (usually 1 drop) until the solution again turns pink. Record the total volume of NaOH titrated.

Calculate the molarity of the NaOH solution using the volume of NaOH titrated, the mass and purity of the standard KHP, the molecular weight of KHP, and the balanced chemical equation for the reaction of KHP and NaOH.

Determination of KHP Purity Record your unknown number in your lab notebook. Dry your unknown for at least 1 hour at 110oC. Again, do not overdry the sample. Using an analytical balance, weigh 4 samples into separate numbered conical flasks. Choose sample sizes that will require between 30 and 50 mL of titrant. Dissolve each sample in about 75 mL of DI water, add 3-4 drops of phenolphthalein indicator to each flask, and titrate to the pink endpoint, again rinsing down the side of your flask to check if the color remains. Record the total volume of NaOH required to reach the endpoint.

Calculate the mass of KHP present using the molarity and volume of NaOH, the molecular weight of KHP, and the balanced chemical equation for the reaction of KHP and NaOH. Use this mass and the mass of your unknown to calculate the % purity of KHP in your unknown.

Report The report should include your name, unknown number, percent purity of the standard, and balanced chemical equation for the reaction of KHP with NaOH; masses of KHP, volumes of NaOH required for the titration of the standard, and the calculated molarities of NaOH; masses of unknown, volumes of NaOH required for the titration of the unknown, and the percent purity of the KHP values. Be sure to perform the statistical analyses of the molarities and percent purities of KHP.


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