Chemistry Laboratory #4:Preparation and Standardization of a Base & KHP Determination

*Adapted from "Exploring Chemical Analysis", D. Harris, Freeman, 1997.

January 23 & 30, 2001
 

Purpose: The objective of this lab is to standardize a 0.1 M solution of NaOH and use it to determine the purity of a sample of potassium hydrogen phthalate (KHP).

Description: In this experiment you will determine the weight percent of potassium hydrogen phthalate (abbreviated as KHP, which does NOT mean potassium hydrogen phosphide!) in a sample unknown. To do this, you will titrate a known mass of the material with sodium hydroxide solution, which you will prepare and standardize (see below). The analysis will be performed two ways: titration with an indicator and, titration with a pH meter. You will then compare the results to confirm the reproducibility of the result.

The concentration of the sodium hydroxide will be determined (in other words, the solution will be standardized) by titration of pure KHP, a weak acid. The pure KHP is a good example of a primary standard because it is relatively cheap and non-toxic, very pure, and stable. These properties make it ideal for standardizing base solutions. You will titrate the KHP with your prepared NaOH solution in sets of three samples until the relative standard deviation (standard deviation divided by the mean) is 0.5% or better. At that point, you can then analyze your unknown sample.

Procedure.

NaOH Preparation & Standardization.

1) Using the stock NaOH solution (caution!) prepare one liter of ~0.10 M solution by dilution. Store in a plastic bottle with a screw-top cap to minimize exposure to air.

2) Carefully weigh four samples of pure KHP of ~ 0.5 g KHP (to the nearest 0.0001 mg) and place them in well-labeled 125 mL Erlenmeyer flasks. Dissolve each in about 25 mL of deionized water. Add three drops of phenolphthalein indicator to each. .Titrate one sample crudely to find the approximate endpoint. Then, based on your result, titrate the other three more carefully, being particularly careful not to titrate past the endpoint. At the endpoint, the solution should have a faint pink hue which should fade after 15 - 30 seconds.

3) For the three solutions that you titrated carefully, calculate the mean molarity, the standard deviation and the relative standard deviation. The relative standard deviation should be less than 0.4%. If it is not, repeat the standardization with three new samples of pure KHP.

Unknown KHP Analysis.

4) Obtain an unknown from your leader and immediately write the sample number in your lab notebook. Measure four samples, each about 0.5 g. Transfer to Erlenmeyer flasks and dissolve in deionized water as was done with the pure KHP samples. Add phenolphthalein indicator. Titrate with the standardized NaOH solution. As before, the first titration can be crude - this provides a rough estimate of the purity. The final three titrations should be done more carefully. Calculate the mean percent of KHP in the unknown, the standard deviation and the relative standard deviation. The relative standard deviation should be less than 0.8%.

5) Using the same unknown sample as in step 4, perform a titration using a pH meter instead of an indicator. Measure ~0.5 g of the unknown and, based on the results obtained in step 4, calculate what the endpoint volume of your NaOH should be. Dissolve the unknown in deionized water.

6)Set up and calibrate the pH meter as instructed. Before adding any NaOH, measure the initial pH of the solution. Add NaOH titrant in approximately 0.05V_eq increments (that is, each addition should be about 5% of the endpoint volume), measuring the pH after each addition. Continue until you are about 90% complete. Then add titrant in about 0.1 mL increments until you have passed the endpoint by about 1 mL. Then add additional NaOH in about five 0.05V_eq increments.

7) Plot your date as pH vs. volume NaOH (V) added and determine the endpoint visually (the volume corresponding to the inflection point of the titration curve). Then plot the first derivative (D pH/ DV) vs V. Determine the endpoint by finding the maximum point on the derivative plot. Calculate the weight percent of the KHP in the unknown using this value and compare to the result obtained using your indicator.

Report. Submit the completed report sheet and the two graphs along with your analyses of the graphical results.
 
 

Name: ___________________________
 

NaOH Standardization
 

Trial #	mass KHP (g)	volume NaOH (mL)	molarity (M)

Average Molarity: _______________
 

Standard Deviation: ________________
 

Relative Standard Deviation (%): ________________
 

95% Confidence Interval: ________________
 
 

KHP Determination (unknown # _______)
 

Trial #	sample mass (g)	volume  NaOH (mL)	mass  KHP (g)	wt % KHP

Average Weight %: ____________
 

Standard Deviation: _____________
 

Relative Standard Deviation (%): _________
 

95% Confidence Interval: __________