Things You'll Need:
- Two 250 ml Erlenmeyer flask with standard-taper glass joint Air condenser with standard-taper joint to fit the Erlenmeyer flasks Weighing scale - calibrated to the nearest 0.1 milligram Hotplate or other heating appliance 50 ml pipette Standardized hydrochloric acid, 0.5 normality Alcoholic potassium hydroxide, 0.5 normality
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Step 1
Weigh 2 grams of the stearic acid sample on the calibrated weighing scale. The weight of the stearic acid sample should fall within a 0.1 margin of error. Transfer the weighed sample of stearic acid into the 250ml Erlenmeyer flask.
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Step 2
Use a well-calibrated pipette to add 50 ml of the alcoholic potassium hydroxide solution.
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Step 3
Prepare a control experiment by adding 50 milliliters of the alcoholic potassium hydroxide into the second 250 ml Erlenmeyer flask. However, do not add a sample of stearic acid to the control. In other words: it is a blank.
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Step 4
Connect the air condenser and place the two Erlenmeyer flasks - one containing the stearic acid solution and the other a blank, on the hot plate. Set the hot plate to moderate temperature and let the two solutions to a boil for approximately four hours. After four hours, turn off the hot plate, and let the solutions to cool.
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Step 5
Wash the inside of the condensers with a little distilled water, after both solutions have completely cooled.
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Step 6
Add 1 ml of phenolphthalein indicator to each of the Erlenmeyer flasks. Titrate both samples against the standard hydrocholoric acid to end point, which is reached at the point where the pink color just disappears. This is a critical step that requires patience and accuracy.
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Step 7
Use the end point value to carry out the calculation for the saponification value using the formula as shown in the diagram Where: A = value of hydrochloric acid used to titrate the blank in milliliter
B = volume of hydrochloric used to titrate the blank sample in milliliter
C = normality of the hydrochloric acid
D = mass of sample in grams.
