//. COLORIMETRIC TECHNIQUES 



A. CAPILLARY TUBE TECHNIQUE 



During the course of their classical investigations dealing with 

 the composition of glomerular urine, Richards and his group at the 

 University of Pennsylvania developed a simple and clever technique 

 of capillary tube colorimetry which enabled them to carry out 

 analyses on less than 1 fx\. liquid with an accuracy comparable to 

 that of macro procedures. The chief problem, as stated by Richards 

 et al. (1933), was "to introduce the minute amount of fluid to be 

 analyzed into a capillary tube without evaporation or contamination, 

 to dilute it quantitatively with water if necessary, to introduce into 

 the same capillary in quantitatively accurate proportions and with- 

 out mixing the one or more reagents required for production of color, 

 to effect mixture of the fluids in the capillary tube at a given moment, 

 and to compare the resulting color with those developed in standard 

 solutions treated simultaneously in identical or equivalent fashion." 



The recent introduction of microcuvettes for the colorimetry of 

 small volumes of liquid in photoelectric apparatus (page 216) will 

 very largely displace capillary tube colorimetry because of the ob- 

 vious advantages of greater objectivity and accuracy of the analyses, 

 and the greater ease of manipulation in most cases. However, the 

 capillary tube technique and methods are included here because 

 there are instances in which the equipment for the cuvette methods 

 is not available, or the volumes to be handled are still too small to 

 permit the use of cuvettes, even of the micro variety. Furthermore, 

 some of the capillary tube methods might be adapted to cuvette 

 colorimetry when the equipment for the latter is available, and in 

 that case the assembly of the methodology of the former would also 

 be useful. 



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