THE MEASUREMENT OF PH AND TITRATABLE ACIDITY 85 



Table 8. Drop-ratio Color Standards for pH Determinations 



* If a little more accuracy is desired one may use a 1-ml pipet graduated in tenths and 

 use the specified number of tenths of a milliliter instead of drops in preparing these 

 tubes. In that case each tube should be brought up to a total volume of 10 instead of 

 5 ml. 



place 8 drops, and so on to the last tube, which should contain 1 drop. 

 In the back row of tubes place 1 drop in the left hand tube, 2 in the next, 

 etc., up to 9 in the last. Make up approximately 20iV stock solutions of 

 NaOH and HCl [i.e., 0.2 per cent NaOH and 1 ml of concentrated HCl 

 (sp gr 1.19) diluted to 240 ml]. Then, except in the case of those indi- 

 cators for which different directions are given in Table 9, add 1 drop of the 

 stock acid solution to each tube in the front row and 1 drop of the stock 

 alkali solution to each tube in the back row; add enough distilled water to 

 each tube to bring its total contents to 5 ml, thoroughly mix the contents 

 of each tube, and return to its place in the rack. It will be seen from 

 Table 9 that two of the indicators, namely thymol blue and bromphenol 

 blue, require more of the alkali or the acid, respectively, than the other 

 standards in order to ensure the appearance of full alkaline or acid color. 

 In the case of thymol blue (alkaline range) and cresol red, the production 

 of the required acid color (yellow^) requires not a strong acid but a weaker 

 one such as mono-potassium phosphate or, in the case of thymol blue, 

 distilled water alone. 



The arrangement of tube pairs indicated in Table 8 produces progres- 

 sively different colors corresponding to steps of 10 per cent in the trans- 

 formation of the indicator from its acid to its alkaline color. That is, each 

 pair of tubes, when aligned between the eye and a source of white light, 

 will show a color mixture corresponding to a definite pH. This pH can 

 be computed by means of Eq. (5), which can be rewritten as 



^^, , , drops of alkalinized indicator .> x 



pH = pK' + log -^ ^ — . ,.^ ■■ . ,. — 7 — (5a) 



^ drops of acidified indicator 



