278 Dr Hartridge, Colourimeter Design 



The Colourimeter in Practice. 



Experiment has shown that if two solutions of the same colour 

 contain different pigments in solution, then the thicknesses re- 

 quired for a match vary not only with the observer and with the 

 quality of the light, but also with the same observer from time to 

 time. It is for this reason that the technique has been introduced 

 of using the same pigment for the standard as that required to be 

 estimated. Thus creatinin is no longer estimated by comparing the 

 colour which develops when picric acid and soda are added with the 

 colour of a solution of potassium dichromate ; but a standard solution 

 of creatinin is used, picric acid being added to it at the same time as 

 it is added to the solution to be standardised. If, then, the thick- 

 ness of the standard is 20 mm. and that of the unknown 17 mm., 

 it is assumed that the strengths of the solutions are in the inverse 

 ratio of those numbers. Such is not the case however, because 

 the sodium picrate itself absorbs rays from the same part of the 

 spectrum as does the sodium picramate, and therefore, although 

 the light may encounter the same number of coloured radicals in 

 both limbs of the instrument, yet the sodium picrate absorption is 

 greater on one side than the other, because the fluids are not of 

 the same thickness. It is principally for this reason that I have 

 adopted an instrument in which double troughs are used, on both 

 sides of the instrument ; the lower pair on both sides being filled 

 with sodium picrate solution in the case taken above as example, the 

 upper pairs containing the picric acid plus creatinin. In this way 

 the number of picrate radicals is kept approximately constant, since 

 the total thickness of sodium picrate solution is the same on both 

 sides of the instrument. The balance is not perfect however, because 

 a certain amount of picric acid is used up in forming the sodium 

 picramate, and this amount cannot be ascertained without assum- 

 ing that the estimation to be done has already been accurately 

 performed. The problem is, in fact, represented by a simultaneous 

 equation involving two unknowns. I find that the matter can be 

 solved in the following manner. Having diluted both the standard 

 and the unknown solutions with equal amounts of standard picric 

 acid and soda solutions, and having allowed the colour to develop in 

 the ordinary manner, an estimate is made of the relative strengths of 

 the solutions in the colourimeter. Having found that, say, a 20 mm. 

 thickness of the standard has the same tint as 13"4 mm. of the 

 unknown solution, a fresh sample of the unknown is taken and 

 13'4 c.c. of it diluted with water to bring the total to 20 c.c. The 

 solution of the unknown has thus been brought to approximately 

 the same concentration as the standard. (Where the approximate 

 strength is known a preliminary dilution before making the initial 

 estimation is beneficial.) The correctly diluted solution of the un- 



