CALCIUM 223 



13. Prepare simultaneously reagent blanks containing sulfuric 

 acid, potassium iodide, and alcohol in the same concentrations as in 

 the standards and unknowns. Use these blanks to set galvanometer 

 at 100 just before reading the standards and unknowns. 



14. Read 10 ml. portions at 25 ±5° in the Evelyn colorimeter 

 with filters indicated in the table. The yellow color may be read at 

 any time within 1 hr. after addition of the potassium iodide. Data 

 are given in the table for use of blue color with starch if filters for 

 yellow color are unavailable. In the latter case, add water to about 

 80% of vol. after the potassium iodide has been added, add the 

 starch slowly to the soln. at 25 ±1°, mixing by rotation, add water 

 to vol., and read color promptly at 25 ±1°. The blue-color method 

 is not reliable for samples of serum smaller than 0.05 ml. 



Result. Since blue color readings are not reproducible from day 

 to day, a new calibration curve must be obtained for each day's 

 work. Galvanometer readings plotted semilogarithmically are linear 

 functions of oxalate concentration. For calibration of yellow-color 

 readings, the percentage transmission readings of 90, 80, 70, 60, 50, 

 40, 30, 25, 20, 15, and 10 correspond to the respective values of 

 iodine, in milliequivalents per liter in the color solns., of 0.0022, 

 0.0048, 0.0077, 0.0111, 0.0151, 0.0200, 0.0267, 0.0308, 0.0362, 0.0433, 

 and 0.0544. To redetermine the calibration curve, treat 0.5 to 0.7 ml. 

 of 0.133 mM potassium iodate with 2.4 ml. of 0.085 M phosphoric 

 acid and 1.2 ml. of 5% potassium iodide, dilute to 40 ml. with 

 filtered 95% alcohol, and then to 100 ml. with water. Read with 

 filter No. 586-5. 



Calculate the concentration of the calcium (in milliequivalents 

 per liter) in the unknown from: 



j-C-C^s^, _ ^j^^^-j^ 



where Si, So, and u refer to standards and unknown sample, C = 

 (C204^")si + {C204^~)s2, and D = V/v where V is the volume (in 

 ml.) at final dilution of color soln. and v is the volume (in ml.) of 

 original sample used. The C and D values are given in the table and 

 the (lo) values are obtained from calibration curve data correspond- 

 ing to the readings observed. 



