The presence of fatty or volatile acids is recognized by boiling a few cubic centimetres of the filtrate in a test tube. A strip of wet, blue litmus paper is held over the vapors escaping at the top of the test tube. Their presence will turn blue litmus paper red. The quantity of these fatty acids can be ascertained by boiling 10 c.c. of the filtrate for about half an hour, adding to the residue sufficient distilled water until the quantity amounts again to 10 c.c, and now determining the degree of acidity in this liquid by phenol-phthalein and sodium hydrate. This figure subtracted from the figure of the total acidity of the filtrate will give the quantity of the fatty acids.

Acetic Acid

Acetic acid if present in larger quantities can easily be detected by its characteristic smell; if present in smaller quantities it may be detected by neutralizing the watery residue of the ethereal extract with carbonate of soda, and then adding neutral chloride-of-iron solution, when a beautiful red color is developed.

Estimation Of Lactic Acid

The quantitative determination of lactic acid may be made in the following way: 10 c.c. of the filtrate are well shaken with a larger quantity of ether. The ether is then separated from the watery solution and the degree of acidity determined in this. By subtracting the figure thus obtained from the total acidity and multiplying by 0.09, we have the percentage of lactic acid. This method presupposes the absence of volatile acids; if they are present, they have to be first eliminated by boiling. The further steps in the process of determining the quantity of lactic acid will then be performed in the way described.