ACIDITY 293 



with a solution of known acidity. The method is much used, 

 and the indicator dyes have been very carefully calibrated. In 

 view of the fact that certain ions are responsible for acidity, 

 then any solution having these ions should have a measurable 

 potential. This is true, and the electrical method is a very accu- 

 rate one for determining acidity when conditions are favorable. 



Hydrogen is the element responsible for acidity. All acids 

 contain it. An acid may be defined as a substance which in 

 solution yields hydrogen ions. The older classical definition of 

 an acid terms it a compound containing hydrogen which is 

 replaceable by a metal. While the first definition is the one now 

 generally accepted, certain features of the older viewpoint still 

 persist, such as the custom of expressing concentrations of 

 solutions in terms of normality. The two definitions differ in 

 that one lays emphasis on the activity of hydrogen ions, while 

 the other includes all hydrogen replaceable by the metal of a 

 base, whether the hydrogen is ionized or not. Sometimes one 

 or the other is referred to as the true acidity, but in doing this 

 we simply lay emphasis on that physical, chemical, or physio- 

 logical property in which we are primarily interested. Both 

 viewpoints are correct. The chemist thinks of acidity in terms 

 of all the replaceable hydrogen (normality) because this method 

 is valuable to him. The physiologist centers his interest on the 

 ionized hydrogen because it is the hydrogen ions which determine 

 acidity for the organism. Let us take up the older viewpoint of 

 acidity first by considering what is meant by normality. 



Normality. — Hydrogen makes for acidity; the hydroxyl group 

 (OH) makes for alkalinity; the two neutralize each other. When 

 an acid is mixed with a base in proper proportions and in solution, 

 a neutral salt and water result (HCl + NaOH = NaCl + H2O). 

 A molecule of an acid containing one atom of replaceable hydro- 

 gen will just neutralize one molecule of a base, that is to say, 

 combine with it so as to produce a salt. This relationship is 

 known as normality. A normal solution of acid is one containing 

 1 (the molecular weight) gram of hydrogen per liter replaceable 

 by the metal of a base. A normal solution of hydrochloric acid 

 contains 1 gram of hydrogen and 35.5 (the molecular weight) 

 grams of chlorine in a liter, the molecular weight of hydrochloric 

 acid being 36.5. A normal solution of sulphuric acid, H2SO4, 

 must also contain 1 gram of hydrogen in a liter. The molecular 



