ACIDITY 167 



Indicators 



The ability to determine when sufficient reagent has been added in 

 titration of an acid or base depends upon the sensitivity of certain dyes 

 to changes in acidity. Such compounds are called indicators. Many 

 dyes that are used as indicators change color in either slightly acidic 

 or basic media rather than at exact neutrality. This is a desirable 

 characteristic, as may be seen by a study of the salts formed through 

 interaction of the respective acids and bases. Salts of strong bases 

 and weak acids, e.g.;, sodium carbonate, undergo hydrolysis when dis- 

 solved in water, producing basic solutions, whereas those salts formed 

 by union of weak bases and strong acids, like ammonium sulfate, are 

 somewhat acidic for a similar reason. Therefore when titrating an acid 

 with a base, or vice versa, it is essential that the standard solution be 

 added until the same degree of acidity or alkalinity is produced that 

 would result by dissolving the corresponding salt in water. Choice of 

 indicators is made accordingly rather than with the idea of determining 

 the point of exact neutrality. Methyl orange, methyl red, bromthymol 

 blue, and phenolphthalein are examples of indicators in common use. 

 The first two are suitable for titration of weak bases, and the last one 

 for weak acids. 



HYDROGEN-ION CONCENTRATION 



"Active" acidity as contrasted to "total" acidity is due solely to that 

 portion of the total replaceable hydrogen that, under prevailing condi- 

 tions, exists in the ionic state. As a commonplace illustration one may 

 liken acidity of a solution to the wealth of an individual. Total acidity 

 corresponds to total wealth, which includes currency, real estate, personal 

 property, notes, bonds, and so on. Active acidity, on the other hand, 

 is comparable only to currency, and just as the response of a ticket sales- 

 man is conditioned by the currency in the hand of a prospective pur- 

 chaser, so the behavior of a cell is conditioned by the active hydrogen 

 in the aqueous medium surrounding it. It is true that other forms of 

 wealth are convertible into currency, and, likewise, acids tend to dis- 

 sociate further as some of their hydrogen ions are used up by chemical 

 reaction. 



The hydrogen-ion concentration has much more to do with enzyme 

 action and the maintenance of a normal colloidal structure in cells than 

 has total acidity. A fatigued muscle may contain as much as 0.4-0.5 

 per cent lactic acid for a time without undergoing injury, but a like 

 concentration of hydrochloric or sulfuric acid would result in death to 

 the tissue. Consider also the supply of carbon dioxide — potentially car- 

 bonic acid — carried by the blood stream. Introduction into the blood 



