MEASURING PHOTOSYNTHETIC ACTIVITV 243 



is oxidized to manganic hydroxide. • By the addition of hydrochloric 

 acid, the latter hydroxide is converted into the unstable manganic chlo- 

 ride. The latter compound breaks down to manganous chloride and free 

 chlorine; this is then determined by means of titration with potassium 

 iodide and potassium thiosulfate in the usual manner. 



Sometimes in water containing organic substances it may be neces- 

 sary to apply a correction because a portion of the chlorine may be 

 absorbed. Also in the presence of organic impurities some of the iodine 

 will be used up after acidulation. lender such circumstances it is advis- 

 able to run a blank on the amount of chlorine which is absorbed. 



If nitrites are present in the water in quantities above 0.2 p.p.m. it 

 is necessary to remove these before proceeding with the analysis. This 

 can be accomplished by treating the water samples with 1 cc. of a 10 per 

 cent solution of urea and 1 cc. 25 per cent sulfuric acid and allowing to 

 stand for 3-4 hours. This is based on the common reaction : 2HNO2 -\- 

 CO(NH2)2 ^ CO2 + 3H2O. 



The effect of nitrites has been investigated by Hale and Melia ^- who 

 recommended the addition of 2 cc. potassium acetate solution (1000 grams 

 per liter) to the bottom of the bottles containing the water after the hydro- 

 chloric acid has been added in the regular course of the analysis. 



For sea water the Winkler method causes some difficulty, because the 

 heavy precipitate which is formed does not permit complete oxidation of 

 the Mn(0H)2- It is necessary therefore to dilute the sea water before 

 making the analysis. ^^ A variety of modifications of the original Winkler 

 method have been proposed for various purposes.^* 



h. Apparatus for Gas Analysis. 



A detailed description of various types of apparatus for gas analysis 

 or a discussion of the principles used in dififerent methods would go be- 

 yond the domain of this book. During recent years a number of very 

 satisfactory instruments for gas analysis have been devised with special 

 consideration for use in investigations of the gaseous exchange of living 

 organisms. Most of these have been applied to studies of the oxygen- 

 carbon dioxide ratio in the respiration of animals. Some of these in- 

 struments are well suited for investigations of photosynthesis and respira- 

 tion in plants, though they have found little application for this purpose. 



In endeavoring to obtain a true measure of the rate of photosynthesis 

 it is absolutely essential that a number of external conditions such as 

 temperature and intensity of illumination be experimentally controlled. 



''Hale, F. E., and Melia, T. W., Jour. Ind. Eng. Chem., 5, 976 (1913). 



"Morissen, W. P., Zeit. anal Chcm., 49, 424 (1910). 



'"Van Eck, J. J., Chcm. Weekhlad., 10, 455 (1913); C. A., 7, 2982 (1913); 

 C A 2,, 540 (1914). Winkler, L. W., Zcit. anal. Chem., 53, 665 (1914). Bruhns, 

 G., C'hcyyi. Ztg., 39, 845 (1915). Clarke, F. W., "The Data of Geochemistry," Bull. 

 770, U. S. Geological Survey, Washington, 1924, p. 143. Krummel, O., "Handbuch 

 der Ozeanographie," Stuttgart, 1907, p. 292. Osterhout, W. J. V., and Haas, 

 A. R. C., Jour. Biol. Chem.. 32, 141 (1917). 



