Dr Hartridge, Colourimeter Design 279 



known is now treated, ab initio, with fresh picric acid solution and 

 soda, and is then estimated against the standard in the colourimeter. 

 It is now found that a 20 mm. thickness of the standard has the 

 same tint as one of, say, 19"85 of the unknown after dilution. The 

 strength of the unknown is thus ascertained, with considerable 

 accuracy, because the conditions of equilibrium under which the 

 sodium picramate develops and exists, and the quantities of picric 

 acid used up in the determination are approximately constant. 



It should be pointed out that the above technique presents no 

 difficulties, and takes little longer than the ordinary method. The 

 p)rinciple may with advantage be applied to all estimations made 

 Avith the colourimeter. 



The Accuracy of the Colourimeter. 



Since colour is due to absorption the colourimeter depends for 

 its utility on the fact that a change in the number of coloured 

 radicals encountered by light causes a change in the retinal stimu- 

 lus when that light falls on the eye. We may, therefore, arbitrarily 

 state that the accuracy of the determinations depends, firstly, on 

 the rate of change in the quality of the light which is passed 

 through the pigment, and, secondly, on the acuteness of the per- 

 ception of the eye for the change in quality of the light. The 

 greater the rate of change and the greater the acuteness of percep- 

 tion of that change, the greater will be the accuracy. Many bodies 

 which absorb light do so selectively, that is, they have a gref^ter 

 effect in one part of the spectrum than in another ; they therefore 

 show colour, that is, they are pigments. Under ordinary circum- 

 stances the greater the absorption the stronger the colour and the 

 less the intensity of the transmitted light. As the concentration 

 of a pigment is altered, and therefore the degree of absorption, the 

 strength of colour and the brightness of the transmitted light both 

 vary. The colourinietric determination, therefore, depends on the 

 simultaneous occurrence of both these changes. The important 

 questions that arise are : (1) on what do the magnitudes of these 

 changes depend ? (2) which is the more important ? and (3) how 

 can the changes be increased for a given alteration in concentra- 

 tion ? A study of absorption band formation gives a definite answer 

 to each of these questions as follows: (1) The changes for a given 

 alteration of concentration are greater the flatter and broader the 

 absorption band. If, therefore, there were two pigments of the same 

 concentration and the same colour, i>ne of which had a sharp well- 

 defined band, while that of the other was broad and flat, the latter 

 pigment would be found to give the more accurate readings in the 

 colourimeter. (2) Of the two changes, that of colour is usually the 

 more important, particularly with pigments showing single absorp- 

 tion bands. In pigments with multiple bands the intensity change 



