224 CUVETTE COLORIMETRY 



When 0.02 ml. samples of serum are used it has been found neces- 

 sary to correct the yellow color readings for the effects of traces of 

 residual serum. The correction varies with the reading and is to be 

 subtracted from the latter: for readings of 15, 20, 30, 40, 50, and 60 

 subtract corrections 0^-^, 0-, 0^, l'^, V, and P, respectively. 



example: Analysis of 0.02 ml. samples of a serum gave yellow color readings 

 of 36\ 36^ for the standards, 23^ and 55". The former were corrected to 35^ by 

 subtracting 1°. Values for iodine in milliequivalents per liter from the calibra- 

 tion curve were 0.0230 for the serum, and 0.0325 and 0.0130 for the standards. 

 Then the calcium concentration in the original serum (in milliequivalents 

 per liter) was: 



[( °°^°° + 0.0325 + 0.0130 ^ _ „^23g-| ^ ^ ^ ^^^ 



In the case of blue-color readings a semilogarithmic plot is made with 

 milliequivalents of oxalate per liter from to 0.0171 as abscissa and galvanom- 

 eter readings from 10 to 100 ordinates. A straight line is drawn between 

 the points representing the readings of the two standards, Si and S2, at concen- 

 trations of 0.016 and 0.004 milliequivalent oxalate per liter. Oxalate values for 

 serum analyses, obtained by interpolation of their galvanometer readings on 

 this line, times D give directly milliequivalents calcium per liter in the sample. 



example: Analysis of 0.05 ml. samples of a serum gave blue color readings of 

 32^ and 32*, for the standards, 2V and 49^. A straight line was drawn through 

 the two latter values located at 0.016 and 0.004 milliequivalent oxalate per liter, 

 respectively. Interpolated values for the serum were 0.01005 and 0.01013 milli- 

 equivalents per liter. Then the average calcium concentration in the original 

 serum was: 



0.01008 X 500 = 5.04 milliequivalents per liter. 



CHLORIDE 



Colorimetric chloride methods have not been specifically adapted 

 to histochemical work. However, the procedure of Sendroy (1939b, 

 1942a), which was designed for use with the macro Evelyn photo- 

 electric colorimeter, could be adapted to the smaller quantities 

 sufficient for use with microcuvettes. Even with the macro apparatus, 

 10 lA. serum is adequate for analysis. The principle of the Sendroy 

 method is conversion of the chloride in acid solution to its silver 

 salt by shaking with solid silver iodate; the iodate liberated by the 

 chloride is made to act on potassium iodide, and the yellow color of 

 the iodine set free is measured using filter No. 420 with the Evelyn 

 instrument. For other methods see pages 200 and 281. 



