HYDKOGEX lOX COXCEXTRATION 1o 



ill determining the end point in titration of acid and 

 alkalies. It must be remembered that the end points 

 which they indicate are not identical but occur at different 

 pK values, about pii 4 for Congo red, about 7 for litmus 

 and about 9 for phenolphthalein. Other indicators change 

 colour gradually over a range of two to three units of jjK 

 value, the actual colour observed being determined by the 

 ^H of the solution. By matching the colour of a suitable 

 indicator in a solution of unknown j^H against the range 

 of colours which it gives in solutions of known ^^H value, 

 the hydrogen ion concentration of the unknown solution 

 can be estimated. Indicators covering piL values from 

 1 to 13 are available, and each indicator is put up in an 

 appropriate series of standard buffer solutions of known 

 pK values in sealed tubes against which the comparisons 

 are made. An even more convenient method is the use 

 of the Lovibond comparator in Avhich the standard tubes 

 containing the indicator are replaced by a disc carrying 

 tinted glasses of colours which match the standards, at 

 intervals of ^^H of 0-2 unit. Besides convenience and speed 

 in use these glass colour standards possess the advantage 

 of not fading, a fault to which the buffer solution indicator 

 standards are subject, particularly if they are exposed 

 to light. 



A chart showing the colour changes and 2:)H range of 

 the more useful indicators is given in Fig. 3. 



Buffer Solutions. — In a great deal of biological work, 

 particularly when dealing with enz}- me systems or reactions 

 of a similar nature, it is important and often even essential 

 to maintain the ^^H of the solution constant in spite of 

 the fact that an acid may be produced or used up during 

 the course of the reaction. If such a reaction were carried 

 out in simple aqueous solution the ^^H value would 

 obviously change j^rogressively with time, but by making 

 use of buffer solutions these changes can be eliminated, 

 the buffer solution offering a reserve of acid or alkali 

 capable of taking up or supplying alkali or acid as occasion 



