SEA-FISHERIES LABORATORY. 135 



filtrate. It is to be regretted that the filtering could 

 not have been done through a Gooch filter, as then the 

 precipitated silver haloid could have been collected and 

 weighed, and so would have formed an additional check 

 on the values obtained by titrating the filtrate. 



This filtrate was poured into a porcelain dish, a 

 drop of iron alum solution added, and titrated drop by 

 drop with the sulpho-cyanide solution till a red colour 

 appeared. To this was then added enough silver 

 nitrate solution to just destroy the colour, and this 

 process of alternately titrating with the sulpbo- cyanide 

 and the silver solution was frequently repeated in a 

 zigzag manner till a great number of determinations 

 of the end point were obtained, the mean of which was 

 considered accurate. 



(c.) — Using Potassium Chromate as Indicator. 



As in the previous case a sample of about lOcc. of 

 the water was first titrated to find about how much 

 silver solution was required. 



In the accurate determinations lOcc. of the sea water 

 were mixed with some distilled water in a beaker, and 

 then about 0*5cc. less silver solution than was required 

 for complete precipitation was added. The mixture 

 was well mixed and a drop of the chromate solution 

 added, and then the silver solution was run in drop by 

 drop till a permanent change of colour was obtained. 

 The end point was obtained repeatedly by zigzag 

 titrations with the salt and silver nitrate solutions. 

 A mean of the values obtained was taken as correct. 



Considerable errors occur in both methods, but more 

 especially in this, owing to the difficulty of obtaining a 

 standard of colour which represents the end of the 

 reaction. 



