A Portable Apparatus for the Determina- Tion of Oxygen Dissolved in a Small Volume of Water

It is clear from the equations that each atom of oxygen in solution finally liberates one molecule of iodine. The iodine is titrated against sodium thiosulphate solution using starch as indicator; 1 c.c. of normal sodium thiosulphate solution corresponds to 8 mg. of oxygen, or 5-6 c.c. of oxygen at N.T.P. A number of modifications of the original technique of Winkler has been described. The accuracy both of the original method and also of any of its modifications is necessarily limited by the accuracy of the sampling technique. This factor is especially important when dealing with water of low oxygen concentration, for in order to obtain accurate analyses it is necessary to keep the water sample out of contact with the air. Krogh (1935) described a syringe pipette which could be used in the estimation of the oxygen content of water; by the use of such a pipette contamination of the water sample with atmospheric air was completely avoided. Van Dam (1933, 1935) modified Krogh's syringe pipette, and in the course of our studies of the respiration of aquatic animals we have further modified the syringe pipette of van Dam by replacing the metal collar with two locknuts. By our method