SOAP
a PLAYCHEM experiment
TITRATION of HARD WATER with SOAP
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In the chemistry laboratory, one frequently makes quantitative measurements on materials using fancy glassware, expensive  instruments and exotic reagents. Yet often you can also get acceptable values in a non-laboratory setting   using simple techniques, every day materials and a little care. This experiment illustrates this.

The hardness of water is frequently measured by titrating aliquots of hard water with standardized EDTA (sodium ethylenediaminetetraacetate) as the titrant and Eriochromswartz-T as the indicator. Using a buret and pipets to measure volumes, this procedure is accurate to 1 ppm Ca or better.
In this experiment, you will also analyze hard water by titration but instead of fancy equipment and reagents, you will measure volume by counting drops, the titrant will be a soap solution and the endpoint will be the "formation of substancial suds".

The EXPERIMENT

soap.jpg (13140 bytes) The three test tubes on the right contain equal amounts of water of diferent hardness
An equal number of drops of soap were added to each.
After shaking, the tube on the left showed the most foaming whereas the tube on the right shows essentially no foaming and the aqueous layer became milky due the precipitation of soap as the calcium salt.
In this experiment you will continue to add drops of soap to each tube until the foaming upon shaking equals that in the first tube. The number of drops of soap used is proportional to the hardness.

You will use distilled water as the reference to which foaming in other tubes will be compared. You will also measure the amount of soap needed to cause foaming in two water samples of different known hardness. Finally you will measure the hardness of an UNKNOWN water sample.

REMEMBER:

  • Use the same volume of Reference, Standards and UNKNOWN. The greater the volume, the more drops of soap are needed.
  • Impurities such as alkaline earth cations or acid will cause errors. Use clean sample tubes.
 

ABOUT SOAP

A classical formula for Soap is .................... C17H35COONa (Sodium Stearate)

Soap was originally made from animal fat and wood ashes. Animal fat is a triglyceride containing stearate groups and wood ash contains sodium and potassium carbonate. Boiling a mixture of lard and wood ash, adding salt and cooling precipitates out the soap. Glycerine is recovered as a byproduct hence the name "glyceride".

Back to the formula for Sodium Stearate. The left end of this is hydrophobic and dissolves in oils and grease. The right end is ionic and soluble in water (hydrophilic). The function of soap is to have the hydrophobic end dissolve in the grease or oil which holds dirt in clothing or on skin, and then the right end with its attraction for water emulsifies it so it can be washed away.

Hard water or acidity causes the precipitation of insoluble salts or the free acid.

C17H35COONa + Ca+2 -----> (C17H35COO)2Ca + 2 Na+

C17H35COONa + H+ -----------> C17H35COOH + Na+

It is these reactions which give rise to the "bathtub ring"

R.W. Kluiber  1/17/2000