68 MAINTAINING FISHES 



NaNs to 40 ml. of water; slowly mix the two solu- 

 tions). (3) Starch indicator (2 g. soluble starch dis- 

 solved in 350 ml. hot water). (4) Sulfuric acid (con- 

 centrated H2SO4, sp. gr. 1.84). (5) Sodium thiosulfate 

 solution (dissolve 6.205 g. of NaaSsOg -51120 in 500 

 ml. of freshly-boiled distilled water; dilute to 1 L. ) . 



Sodium thiosulfate solution is unstable. Its stability 

 is greatly improved by storing in a dark bottle, re- 

 frigerating when not in use, and by adding 5.0 ml. of 

 chloroform per liter. Not only must sodium thiosulfate 

 solution be standardized following preparation, but it 

 must also be standardized prior to use depending 

 upon length of time in storage. If it is refrigerated, 

 standardizing at two-week intervals is adequate. If it 

 is not refrigerated, it should be standardized weekly. 

 Standardization is accompHshed by the procedure 

 given below. 



Oven dry a small quantity of reagent grade potas- 

 sium dichromate (K2Cr207 -41120) crystals in oven at 

 130 degrees C. for 30 minutes, cool in a dessicator 

 and weigh out 1.226 g. Dissolve and dilute to 1 L. in a 

 volumetric flask. 



To 10.0 ml. of the above potassium dichromate 

 solution slowly add 1.0 ml. of alkahne-iodide sodium 

 azide reagent then add a drop at a time 1.0 ml. of con- 

 centrated sulfuric acid. The acid should be added 

 slowly enough to avoid any odor of free iodine and 

 heating. Using the thiosulfate to be standardized, 

 titrate this solution as an oxygen sample (see proced- 

 ure below) to a pale straw color, add starch and 

 titrate until the blue color fades completely. The num- 

 ber of milliliters of sodium thiosulfate solution divided 

 into 10 will give a correction factor by which thiosul- 



