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Analysis. — The method selected as best adapted to our needs is the Mohr method 

 of determination of chlorine by titration with silver nitrate, with a numerical modifica- 

 tion designed to permit the reading of results in parts of chlorine per 1000 parts of 

 water analyzed, from the burette reading of c.c. of silver nitrate solutinn required 

 for reaction, with the least possible amount of computation. 



Preparation of Solutions. — Prepare a standard solution of silver nitrate by dis- 

 solving 23.9545* grams of chemically pure crystals of silver nitrate, which has first 

 been carefully dried, in distilled water to make exactly 1000 c. c. Also prepare a 10 per 

 cent solution of chemically pure potassiiun chromate, which will be used as an indicator 

 to show the end of reaction. 



Testing of Standard Silver Nitrate Solution for Standard Strength. — This should 

 be done at frequent intervals as follows: Dissolve 0.0824 grams of chemically pure 

 dried sodium chloride in any convenient amount of distilled water. This amount of 

 sodium chloride will require exactly 10 c.c. of standard solution of silver nitrate for 

 complete reaction. Add to the sodium chloride solution a few drops of potassium 

 chromate indicator, as above prepared. Then drop by drop, from a burette add the 

 silver nitrate solution to be tested, stirring constantly until the appearance of the 

 permanent red color of silver chromate indicates that the reaction is complete. If 

 more or less than 10 c.c. of the silver nitrate solution is required to complete reaction, 

 repeat the test and record the average ratio of standard to actual amount (e.g., if 

 9.5 c.c. are used, ratio = 10/9.5 = 1.0526), to be applied as a correction factor to all 

 burette readings with that lot of solution. 



It should be borne in mind that there is a large source of possible error in the 

 judgment of the point at which the red indicator color becomes permanent. The 

 titration should proceed slowly and with great caution as soon as red begins to appear. 



To analyze Water Sample for Salinity. 



1. Filter sample of water to be tested, if turbid. 



2. Weigh out 5 grams of the filtered sample, on a balance sensitive to 1 / 10 milli- 

 gram, and add to it 25 or 30 c.c. of distilled water. 



3. Add 10 drops of the 10 per cent potassium chromate indicator. 



4. From a burette graduated to 110 c.c. add standard solution of silver nitrate, 

 a drop at a time, and stir constantly until the end of reaction is indicated by appearance 

 of permanent red color. Then carefully take burette reading. 



This analysis should be repeated at least three times on each sample and the 

 average of results taken, unless the first two analyses agree. 



*0n the basis of 1920 international atomic weights. This solution strength is based on the amount 

 required for a decinormal solution (16.989 grams) multiplied by the factor 5/3.546, which is derived as 

 follows: In the method for anahsis of water, as set forth below, assume the use of a decinormal solution 

 of silver nitrate; then each c.c. of the decinormal solution would represent 0.003546 grams of chlorine 

 in the reaction. Now let a = the number of c.c. of decinormal solution required for complete reaction 

 on b grams of water taken for test. Then 



a X 0.003546 X 1000 a X 3.546 



parts of chlorine in 1000 parts of water as analyzed. If, now, the strength of the silver nitrate solution 

 be increased to 5 '3.546 times that of a decinormal solution, the parts of chlorine per 1000 parts of water 

 will be given by the ratio 



o X 5 



b 



or the number of c.c. of standard silver nitrate solution required for complete reaction multiplied by 

 the ratio of 5 over the number of grams of water analyzed. If, then, 5 grams of water be taken for an- 

 alysis, the parts of chlorine per 1000 of water will be given directly by the number of c.c. of silver nitrate 

 solution required for complete reaction, without further computation, as used in the analysis method. 



