10 The Philippine Journal of Science isis 



oxide and is, therefore, determined with sodium carbonate pellets, 

 using 5 to 10 drops of phenolphthalein solution as indicator. 

 The manipulation is identical with that described for "alkalinity" 

 and "normal carbonates," except that, ordinarily, only one or 

 two tablets, or even less, will be required for a titration. Fur- 

 thermore, since the kaolin in the pellets slightly obscures the 

 end point, the discrepancy between duplicate determinations is 

 usually 0.5 cubic centimeter and often 1 cubic centimeter. 



In the manufacture of the sodium carbonate pellets 4.0 grams 

 of anhydrous sodium carb'onate are used to 130 grams of kaolin. 

 This gives a pellet of approximately the desired reacting value, 

 namely, 1 milligram of carbon dioxide. To standardize, 5 of 

 these pellets are triturated in a mortar with recently boiled 

 distilled water, 5 drops of phenolphthalein solution are added, 

 and the solution is titrated with 0.1 N sulphuric acid. 



If, in a field determination, it is found that the average of 

 two readings taken for the reaction with 1 pellet equivalent to 

 0.95 milligram of carbon dioxide (phenolphthalein being used 

 as indicator) is 24 cubic centimeters of the water, the acidity, 

 expressed in parts per million of carbon dioxide, would equal 



1,000 X Q-95_ .^ 

 24 ~ 



Chlorides. — For the determination of chlorides, "weak" and 

 "strong" pellets of silver nitrate are employed. The former are 

 each equivalent to about 1 milligram of chlorine, the latter to 

 10 milligrams. In the manufacture of the weak pellets, 12.5 

 grams of silver nitrate and 200 grams of kaolin are used, while 

 156 grams of silver nitrate and 250 grams of kaolin are the 

 proportions used for the strong pellets. 



The pellets are standardized with a sodium chloride solution, 

 which is conveniently made to be equivalent to 1 milligram of 

 chlorine per cubic centimeter. Potassium chromate is used as 

 an indicator. 



The determination of chlorides in the field is rapid and 

 simple. A small quantity of water, usually only 10 or 15 cubic 

 centimeters, is introduced from the filled 100 cubic centimeter 

 graduate into the mortar. Five drops of potassium chromate 

 solution (5 per cent) are added as indicator. If the chlorine 

 content of the water is high, "strong" silver nitrate pellets are 

 added one at a time, with thorough mixing, until an excess is 

 indicated by the rose color of silver chromate. If the chlorine 

 content is low, "weak" pellets are added till the end point is 

 passed. If the chlorine content is low, that is, under 10 parts per 

 million, a half or even quarter tablet will be sufficient. In any 



