MEASUEEMENT OF SALINITY OF SEA WATER 5 



CHEMICAL TITRATION METHOD 



This method consists in determining how much silver nitrate solu- 

 tion of known strength must be added to a measured sample of sea 

 water to precipitate the chlorine in the sample as silver chloride. 

 The proportion of chlorine in the total dissolved solids is nearly con- 

 stant in ocean waters, so that the amount of nitrate solution re- 

 quired is a measure of the salinity of the sample. The observer is 

 enabled to detect the completion of the precipitation by the addition 

 of a small quantity of potassium chromate solution to the sea-water 

 sample before any silver nitrate is added. As soon as the last trace 

 of chlorine has been precipitated the next drop of nitrate solution 

 added begins the precipitation of silver chromate, which is of a deep 

 red color and brings about a sharp color change. 



It is customary to let the silver nitrate solution run down into 

 the measured sea-water sample from a burette. The level of solu- 

 tion in the burette is set at zero at the beginning, and the reading of 

 the burette at the change of color will be the number of cubic centi- 

 meters of nitrate solution required to precipitate all the chlorine 

 from the sample. If 10 c. c. of sample is used and the nitrate solu- 

 tion has been made up by dissolving 27.09 grams of chemically 

 pure, dry silver nitrate crystals per liter of solution, then the read- 

 ing of the burette when the color change occurs will be numerically 

 equal to the salinity of the sample. 



The assumption is made here that the silver nitrate crystals are 

 100 per cent silver nitrate and that 55.25 per cent of the total dis- 

 solved solids is chlorine. This proportion is in close agreement with 

 Dittmar's figures on the composition of average sea water and with 

 the data in Clarke's The Data of Geochemistry, and in fair agree- 

 ment with Knudsen's formula: 



Salinity = 0.030+ 1.8050 X chlorine content 

 where salinity and chlorine content are both expressed in parts per 

 thousand by weight. 



Figure 3 shows the outfit needed for the chemical titration method. 

 The locker can be constructed by the ship's carpenter from Figure 3. 

 The dimensions given in the figure correspond to those of the locker 

 made up in the office for tryout on board the Lydoma; the height 

 can be reduced by 4 inches, the width by 2 inches, and the depth by 

 1 inch, if necessary ; all measurements are inside measurements. The 

 following chemical apparatus will be needed and can be purchased 

 from any laboratory supply house: 



One half-gallon aspirator bottle, plain. 



One one-hole rubber stopper for aspirator bottle. 



Note. — Some aspirator bottles require 2 rubber stoppers, 1 at the mouth 

 and 1 at the outlet. 

 One rubber bulb, with hard-rubber valves, for pressure. 



103509—28 2 - I 



