The alkalis entering into the manufacture of soap such as caustic soda or sodium hydroxide, caustic potash or potassium hydrate, carbonate of soda or sodium carbonate, carbonate of potash or potassium carbonate usually contain impurities which do not enter into combination with the fats or fatty acids to form soap. It is out of the question to use chemically pure alkalis in soap making, hence it is often necessary to determine the alkalinity of an alkali. It may again be pointed out that in saponifying a neutral fat or oil only caustic soda or potash are efficient and the carbonate contained in these only combines to a more or less extent with any free fatty acids contained in the oils or fats. Caustic soda or potash or lyes made from these alkalis upon exposure to the air are gradually converted into sodium or potassium carbonate by the action of the carbon dioxide contained in the air. While the amount of carbonate thus formed is not very great and is greatest upon the surface, all lyes as well as caustic alkalis contain some carbonate. This carbonate introduces an error in the analysis of caustic alkalis when accuracy is required and thus in the analysis of caustic soda or potash it is necessary to remove the carbonate when the true alkalinity as sodium hydroxide or potassium hydroxide is desired. This may be done by titration in alcohol which has been neutralized.
In order to determine the alkalinity of any of the above mentioned alkalis, it is first necessary to obtain a representative sample of the substance to be analyzed. To do this take small samples from various portions of the package and combine them into a composite sample. Caustic potash and soda are hygroscopic and samples should be weighed at once or kept in a well stoppered bottle. Sodium or potassium carbonate can be weighed more easily as they do not rapidly absorb moisture from the air.
To weigh the caustic soda or potash place about five grams on a watch glass on a balance and weigh as rapidly as possible. Wash into a 500 cubic centimeter volumetric flask and bring to the mark with distilled water. Pipette off 50 cubic centimeters into a 200 cubic centimeter beaker, dilute slightly with distilled water, add a few drops of methyl orange indicator and titrate with normal acid. For the carbonates about 1 gram may be weighed, washed into a 400 cubic centimeter beaker, diluted with distilled water, methyl orange indicator added and titrated with normal acid. It is advisable to use methyl orange indicator in these titrations as phenolphthalein is affected by the carbon dioxide generated when an acid reacts with a carbonate and does not give the proper end point, unless the solution is boiled to expel the carbon dioxide. Litmus may also be used as the indicator, but here again it is necessary to boil as carbon dioxide also affects this substance. As an aid to the action of these common indicators the following table may be helpful:
| Indicator. | Color in Acid Solution. | Color in Alkaline Solution. | Action of CO2. |
| Methyl orange | Red | Yellow | Very slightly acid |
| Phenolphthalein | Colorless | Red | Acid |
| Litmus | Red | Blue | Acid |
It may be further stated that methyl orange at the neutral point is orange in color.
To calculate the percentage of effective alkali from the above titrations, it must be first pointed out that in the case of caustic potash or soda aliquot portions are taken. This is done to reduce the error necessarily involved by weighing, as the absorption of water is decided. Thus we had, say, exactly 5 grams which weighed 5.05 grams by the time it was balanced. This was dissolved in 500 cubic centimeters of water and 50 cubic centimeters or one tenth of the amount of the solution was taken, or in each 50 cubic centimeters there were 0.505 grams of the sample. We thus reduced the error of weighing by one tenth provided other conditions introduce no error. In the case of the carbonates the weight is taken directly.
One cubic centimeter of a normal acid solution is the equivalent of:
| Grams. | |
| Sodium Carbonate, Na2CO3 | 0.05305 |
| Sodium Hydroxide, NaOH | 0.04006 |
| Sodium Oxide, Na2O | 0.02905 |
| Carbonate K2CO3 | 0.06908 |
| Potassium Hydroxide, KOH | 0.05616 |
| Potassium Oxide, K2O | 0.04715 |
Hence to arrive at the alkalinity we multiply the number of cubic centimeters, read on the burette, by the factor opposite the terms in which we desire to express the alkalinity, divide the weight in grams thus obtained by the original weight taken, and multiply the result by 100, which gives the percentage of alkali in the proper terms. For example, say, we took the 0.505 grams of caustic potash as explained above and required 8.7 cubic centimeter normal acid to neutralize the solution, then
8.7 × .05616 = .4886 grams KOH in sample