PHOTOMETRIC END POINTS AND METHODS 265 



B. PHOTOMETRIC END POINTS 



For the objective determination of end points in micro titrations, 

 Zamecnik, Lavin, and Bergmann (1945) employed a photoelectric 

 apparatus. A Pfaltz and Bauer fluorophotometer (model A) was 

 used, and the titration vessel was a square, 6X6 mm. ungraduated 

 hemometer tube 3 cm. long. The tube was placed in an adapter 

 which fitted into the cuvette holder and the titration was carried out 

 with the vessel in the instrument (Fig. 92). Electromagnetic stirring 

 was employed as shown. The end point was indicated by the galva- 

 nometer reading which corresponded to the maximum rate of de- 

 crease in light transmission per microliter of standard titration 

 solution. 



The apparatus was applied to the Linderstr0m-Lang acetone 

 titration of amino groups using naphthyl red as the indicator. A 

 Wratten filter 77 was used in order to obtain light in the region of 

 540-550 uifji for the photometry. Actually, the acetone titration can 

 be performed satisfactorily by visually matching the color to the 

 arbitrary end point color chosen for the blank (page 304), but there 

 could be instances in which it would be of advantage to have an 

 objectively fixed end point. 



C. METHODS 



SODIUM AND POTASSIUM (COMBINED) 



A method has been described by Linderstr0m-Lang (1936) for 

 the estimation of sodium plus potassium in small samples of bio- 

 logical material containing less than 4 X 10"^ milliequivalent of the 

 alkalis with a precision of 1 X 10 ^^ milliequivalent. The determina- 

 tion involves ashing the sample with a reagent to convert the 

 sodium and potassium to chlorides, removal of other chlorides, and 

 electrometric titration of the residual chloride with silver nitrate. 

 During the removal of extraneous chlorides by heating, a slight loss 

 of sodium and potassium chloride occurs even when the temperature 

 is held as low as 330°, the sublimation temperature of ammonium 

 chloride. However, this loss is fairly reproducible and it is usually 



