44 



BIOLOGICAL CHEMISTRY 



fresh standard solution is prepared with a higher or lower 

 concentration of hydrogen ions depending on whether the 

 colour with the first standard solution showed less or more 

 acid than the unknown solution. For the colours and ranges 

 of change of colour see figure 8. 



[H] x 10 10" id 10 10" id id id id id id id 



P H " r - LOO/H'/ I 2 3 4 5 6 7 8 9 10 II 12 



METHYL VIOLET 



GREEN 

 BLUE 



THYMOL BLUE 



BftOM PHENOL 

 BLUE 



DIMETHYL-AMIHO- 

 AZO- BENZOL 



RED 



BROM-CRESOL 

 PURPLE 



BROM -THYMOL 

 BLU 



HEUTRAL RED 



-NAPHTHOL 

 PHTH ALIEN 



PHENOLPHTHAUEN 



COLOURL SS// 



THYMOL PHTHALIEN 



', S|L<*f 



_L 



FIG. 8. Diagram showing range of hydrogen ion concentrations at 



which various indicators change colour. 

 The shaded areas are left without boundaries to indicate that the transition is not abrupt. 



This colorimetric method is extremely easy and convenient. 

 It requires very little apparatus and can be used for field work 

 away from large laboratories. It suffers from the disadvant- 

 age that the presence of neutral salts interferes with the 

 accuracy of the measurements.* 



* S. P. L. Sorensen and S. Palitzsch, Biochem. Zeitschr., 1910, vol. 

 24, p. 387. 



