pH INDICATORS 



43 



The curve with colour filters (Ilford 601-608 series) is closely similar to the grating 

 spectrum. 



It will be seen that if a suitable wave band is selected to suit the indicator (550 m//. 

 in the case of phenol red) it is as simple and rapid, and less liable to personal errors, to 



400 



480 560 



Wave length 



640 



400 



480 



Wave 



560 



length 



640 



Fig. 12 

 Absorption spectra of Phenol Red 



measure the pH by instrument as by eye, but it is first necessary to plot the extinction 

 coefficient of the dye at different pH values. Once done there is no longer danger of 

 matching against faded buffer standards. The instrument is first adjusted to 100 

 per cent, transmission at the selected wave band with the solution without indicator. 

 The indicator is added and the reading again taken, and from the calibration curve 

 the pH can be obtained directly. 



