216 CUVETTE COLORIMETRY 



HYDROGEN ION CONCENTRATION 



Capillary tube colorimetry has been employed for the measure- 

 ment of the hydrogen ion concentration of less than 1 /xl. liquid by 

 Montgomery (1935), who used quartz capillary tubes having an 

 internal diameter of 4-5 mm. When comparisons of indicator colors 

 given with protein-free buffer solutions were made, the error of 

 measurement was less than 0.02 pH. However when applied to 

 biological fluids, the indicator color may not be an accurate indica- 

 tion of the pH value. Montgomery (1935) observed that the capil- 

 lary tube method gave values for blood plasma from frogs and 

 Necturus which were consistently lower by an average of 0.11 pH 

 than those obtained with a glass electrode, a deviation which he 

 ascribed to the protein error of the indicator. It may be possible in 

 some cases of this nature to apply a correction factor. For electro- 

 metric measurements see page 183. 



B. CUVETTE TECHNIQUE 



1. Apparatus 



General. Cuvettes for the colorimetric measurement of small 

 volumes of liquid have been designed for use with certain standard 

 colorimeters. Zeiss cuvettes having a capacity of 0.2 ml. are made 

 for the Pulfrich step photometer. The Evelyn photoelectric colorim- 

 eter has a micro attachment made to accommodate cells which re- 

 quire 0.15 ml. Adapters which enable 0.2 ml. cuvettes to be used with 

 the Coleman Junior spectrophotometer (model 6) are obtainable 

 from &. Ash (Lowry, Lopez, and Bessey, 1945). Quartz cuvettes 

 permitting the use of volumes of 0.05 ml., or less, with a special 

 adapter for the Beckman quartz spectrophotometer have been de- 

 scribed by Lowry and Bessey ( 1946) . With their adaptation meas- 

 urements can be carried out on 0.05 ml. volumes from about 225 to 

 1050 m^ with spectral widths of no more than 3 m/x. With 0.025 ml. 

 volumes a range of 235-935 m/x can be utilized with the 3 m^u, 

 spectral bands. The cuvettes and adapters may be obtained from 

 Pyrocell Manufacturing Co.* 



* Since this writing a capillary absorption cell has been described by Kirk 

 et al. (see Bibliography Appendix, Ref. 40; see also Ref. 42). 



