CHE 230
Lab #6
Calcium in Antacid Tablet by Gravimetric Analysis
Objectives:
 Use gravimetric analysis to determine the calcium content of a commercial antacid tablet.
 Compare the calcium content of two different brands of antacid tablets using pooled class data.
 Understand the role of equilibrium in gravimetric analyses.
Tools/Skills:
 Desiccator
 Vacuum filtration


Drying to a constant mass
Quantitative transfer
Procedure:
Write out a step-by-step procedure based on the reference cited below, with the following exceptions:
a) The samples and crucibles will already have been dried for you.
b) You will only analyze one replicate.
c) You will be analyzing a commercial antacid tablet rather than an unknown (and the starting
mass may have to be adjusted accordingly).
Reference:
Henrickson, C.H.; Robinson, P.R. Gravimetric Determination of Calcium as CaC2O4*H2O. J. Chem.
Ed. Vol. 56(5): 341-2. 1979. http://pubs.acs.org/doi/abs/10.1021/ed056p341
Pre-Lab:
If you perform gravimetric analysis on one antacid tablet, theoretically how much precipitate should
form? Is this a reasonable amount? If not, adjust your procedure to provide a reasonable amount of
precipitate.
Discussion:
1. Gravimetric analyses are prone to systematic error. Give a specific example from this lab
experiment which would result in a positive systematic error. Give a specific example from this
lab experiment which would result in a negative systematic error.
2. In this experiment, the pH of the solution is gradually raised by thermal decomposition of the
urea. Why not just add sodium hydroxide to raise the pH?
3. Write out the Ksp reaction for calcium oxalate. Explain a stress (equilibrium shift) that would
cause more calcium oxalate to dissolve. Explain a stress (equilibrium shift) that would cause
more calcium oxalate to precipitate.
4. After the formation of calcium oxalate crystals is complete (color change to yellow), is there any
dissolved calcium left in solution? If so, how would the dissolved calcium affect that analysis
results?
5. Using pooled class data, determine whether the two brands of antacids have statistically different
calcium contents.
6. Comment on the accuracy and precision of the class data.