xm, A,i Heise and Behrman: Water Analysis in Field 15 



when the mortar is washed with the water under examination, 

 a certain amount remains in the mortar, which affects the volume 

 subsequently employed for the next titration. Also the lack of 

 uniformity in the pellets may introduce a very appreciable error. 



In our own work additional sources of probable error have 

 been encountered with "tabloid" methods. Our pellets are 

 molded by hand and are, consequently, not as uniform as ma- 

 chine-made pellets. This is especially true of the potassium 

 palmitate pellets, which form a sticky pill mass that dries very 

 quickly and that is very difficult to mold uniformly. Again 

 kaolin is used in the sodium carbonate and silver nitrate pellets 

 and obscures the end points, thus decreasing the accuracy of 

 the determinations. 



In the "tabloid" determinations outlined above our methods 

 differ from Leighton's in that, in the determination of chlorides 

 and of total alkalinity, Leighton treated a known quantity of 

 water with an excess of reagent to obtain an end point, while 

 in all cases we titrate a known amount of reagent with the water 

 to secure an end point. The former method gives values that 

 lie between certain limits, as the excess of reagent is added in 

 the form of parts of a pellet, and consequently the exact amount 

 of reagent required for the titration is not determined. By 

 making the excess small, the deviation from the true value is 

 correspondingly decreased. 



By our method, however, the exact titrating volume required 

 is determined quickly and fairly accurately. The approach to 

 the end point is thus reversed. This probably introduces an 

 error in the determination of chlorides, which, however, is cer- 

 tainly much less than that involved in Leighton's method. It 

 should be also remembered that the standardization of the pellets 

 is made in the same manner as the field determination, thus 

 decreasing the probable error. In the case of the determination 

 of alkalinity, however, where methyl orange or butter yellow 

 is employed as indicator, the reversed approach to the end point 

 (that is, from acid to alkali) is theoretically the more correct 

 of the two procedures and should, therefore, further increase 

 the accuracy of the method as outlined above. 



BACTERIOLOGICAL EXAMINATION 



The bacteriological examination consists of two parts. One 

 of these is a colony count made from two plate cultures. The 

 other is a presumptive test for the presence of organisms of 

 the B. coli group, which is made with one or more culture tubes. 



The culture medium used in both cases is litmus lactose agar 



