20 The Philippine Journal of Science im 



usual manner by titration with standard sodium thiosulphate 

 solution in the presence of phosphoric acid, potassium iodide, 

 and starch solutions, care being taken to keep such factors as 

 temperature and concentration as uniform as possible to en- 

 sure comparable results. Phosphoric acid was used instead of 

 hydrochloric or sulphuric acid, because errors in titration due 

 to the presence of ferric iron and chlorates are thus avoided, (li) 

 The following graduation (4) of this determination, on the basis 

 of the color of the iodine-starch reaction in the presence of 

 hydrochloric acid, serves very well for the estimation of very 

 small quantities of chlorine in 200 cubic centimeter samples: 



Doubtful, when CI content, is 0.11 mg. per liter. 

 Barely noticeable, when CI content is 0.12 mg. per liter. 

 Noticeable, when CI content is 0.13 mg. per liter. 

 Weak, when CI content is 0.17 mg. per liter. 

 Distinct, when CI content is 0.24 mg. per liter. 

 Sharp, when CI content is 0.4 mg. per liter. 



The decomposition rate of calcium hypoclorite solution in the 

 dark. — In order to study the decomposition rate of hypochlorites, 

 known quantities of standard filtered chloride of lime solution 

 were added to water in large glass jars provided with ground 

 glass covers. Aliquot portions were pipetted off from time to 

 time and analyzed. 



Whether the reaction was followed in distilled water, natural- 

 Cartesian or river) waters, in solutions of organic substances, or 

 in sewage, the decomposition curve had the same general trend. 

 At first there was a rapid disappearance of chlorine, followed 

 by an abrupt slowing down of the reaction. This sudden change 

 of velocity generally occurred within from thirty minutes to one 

 hour, after which the reaction proceeded very slowly, but very 

 regularly, showing no sign of reaching an end point, though 

 followed for weeks. Therefore it appears probable that the 

 main reaction between chloride of lime and dissolved substances 

 quickly approaches completion, after which the curve of gradual 

 decomposition in water is followed. The experimental data se- 

 cured are shown in Table I. The results for sewage were the 

 most striking, and as they are apparently typical, on a large 

 scale, of the reactions occurring in the other liquids, they are 

 shown graphically in fig. 1. 



For the sake of completeness, the data for distilled water are 

 plotted in fig. 2 for the entire eight hundred ninety hours during 

 which the reaction was under observation. The curve appears 

 to be typical for all of the reactions in the table. Though the 



