SECTION ONE 



Some observations 



on 



pH CHANGES INDUCED BY HEATING DISTILLED 



WATER AND LONDON TAP WATER 



London tap water (pH 8-2) was found to behave in the same 

 way as the solutions of acid dyes mentioned above. On heating 

 gradually to 60° C, the pH fell gradually to 6-8. At this point, 

 as the heating was continued the pH rose steadily until at 100° C 

 the pH registered was the same as that at 20° C. On cooling the 

 same changes took place only in reverse. 



The same experiments were carried out with distilled water. 

 Here it should be mentioned that the pH of distilled water, even 

 freshly distilled, is often between 5-0 and 6-0, and not 7-0, as 

 appears to have been generally believed in many biological 

 laboratories. Therefore, when neutral distilled water is essential 

 in a particular staining technique it is advisable, in cases of doubt, 

 to buffer the water to pH 7-0. 



In the experiments referred to above, a large number of samples 

 of freshly distilled water were tested. It was found that whatever 

 the pH of the sample at room temperature (20° C), when the water 

 was boiled and then allowed to cool again the pH reverted to that 

 originally registered at room temperature. It was found that as the 

 samples were heated over the range 20°-ioo° C, the pH of the 

 samples gradually rose by about 2-0. That is to say, if the pH at 

 20° C was 5 '4, then at 100° C it was found to be pH7-4. The 

 experiments were checked afterwards by the use of indicators. The 

 colour changes of indicators of the sulphonphthalein group, at 

 least, are due to dissociation of the hydroxy 1 group. In some 

 cases, bromocresol purple for instance, it was found that an 

 irreversible change in the molecular structure of the indicator 

 appeared to take place between 90° C and 100" C. On cooling 

 samples of the boiled water containing this indicator, no further 

 colour change took place, even after addition of acids and alkalis. 



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