46 Estimations occurring in Agricultural Analysis [70 



measuring flask, rinsing out the beaker well with cold distilled 

 water and adding the rinsings to the solution in the flask. 

 Fill up with distilled water to the mark, and shake up the 

 solution so as to mix thoroughly. Measure out 25 c.c. of this 

 solution with a pipette into an 8-oz. beaker, and add one 

 drop of the K 2 Cr0 4 solution. 



Fill up a burette with the standard silver nitrate solution, 

 add an Erdmann's float, taking care that there are no bubbles 

 of air either attached to the float or in the stopcock. Note 

 the position of the mark on the float. Stand the beaker 

 containing the NaCl on a white tile, and allow the silver solu- 

 tion to run into it, drop by drop, stirring continuously with a 

 glass rod. As each drop of AgN0 3 enters the liquid in the 

 beaker a bright-red precipitate of Ag 2 Cr0 4 is momentarily 

 formed, but this immediately becomes white as it is turned by 

 the salt into AgCl. When the white precipitate begins to 

 curdle and settle, add the AgN0 3 more slowly, stirring after 

 each drop until the red colour entirely disappears. As soon 

 as the red colour remains permanent, leaving the solution just 

 faintly coloured, close the stopcock of the burette and note 

 down the new position of the float. The difference between 

 the two positions gives the amount added. Make two or 

 three determinations in this manner, using the mean of your 

 readings as the true result. 



70. Calculation. — First multiply the factor by the number 

 of c.c. used. This will give the number of c.c. which would 

 have been used had the solution been decinormal — *>., had it 

 contained 17 grams of AgN0 3 or 10 -8 of Ag per litre. By 

 inspecting the equation 



Ag + CI = AgCl 



108 + 35-5 = i43'5 



we see that 108 of silver are equal to 35-5 of chlorine, 



