526 PHYSIOLOGICAL CHEMISTRY 



ter should extend to about 2 mm. from the bottom of the flask and the 35 mark 

 on the thermometer stem should be visible above the stopper. The flask is placed 

 on an asbestos gauze supported by an adjustable ring stand. A Bunsen or Meker 

 burner is used and is placed at from 3-5 cm. beneath the lower surface of the asbestos 

 gauze. Use a large flame and allow the ring and gauze to become thoroughly 

 heated. Then place the flask, into which 60 c.c. of distilled water has been intro- 

 duced, in the center of the gauze and observe the time required for the temperature 

 in the flask to rise from 35 to 95C. By several trials the flame and position of the 

 gauze are so adjusted that it requires (within a few seconds either way) just 120 

 seconds for the temperature to rise from 35-95C. This standard flame is then 

 used in the determinations which follow. 



B. The Reduction Process. Into a 200 c.c. Jena Erlenmeyer flask fitted as 

 above, introduce 20 c.c. of alkaline tartrate solution, 1 20 c.c. of the copper sulphate 

 solution 1 and the sugar solution to be analyzed which should first be made up to 

 20 c.c. so that the total volume of the fluid in the reduction flask is always 60 c.c. 

 Place the flask over the standard flame and note when the temperature of the mix- 

 ture reaches Q5C. Allow to boil for exactly 20 seconds after the temperature 

 reaches 95. Then the flask is promptly removed with the stopper still in place 

 and held under the tap for a moment to stop the reduction but not to cool the mix- 

 ture more than a few degrees below. the boiling-point. Filter hot through a pre- 

 viously prepared Gooch crucible with a heavy asbestos mat through which sufficient 

 of a suspension of talcum has been filtered so that with suction, not a stream but a 

 rapid succession of drops comes through the filter. (A calcium chloride tube 

 packed with glass wool and asbestos may also be used.) The suction flask should 

 have a capacity of about 200 c.c. so that the titration may be carried out in it di- 

 rectly. Wash the filter with a fine stream of water using not more than 15-20 c.c. 

 of water in all. To the filtrate add 4 c.c. of concentrated sulphuric acid and cool 

 to 20C. Add 6-7 c.c. of a saturated solution of potassium iodide. Titrate the 

 liberated iodine with standard thiosulphate solution, 2 adding a few drops of a solu- 

 tion of soluble starch as an indicator near the close of the titration. The "spot 

 test" may be used in determining the end point. As long as a drop of thiosulphate 

 produces a perceptible white area in falling upon the quiet solution the end point 

 has not been reached. The chocolate-brown cplor of the mixture changes to a 

 light cream white as the last necessary drop of thiosulphate is added. 



A blank should be run in exactly the same way but with the omission of the 

 sugar solution. y 



Calculation. Subtract the number of cubic centimeters of thiosulphate re- 

 quired for the titration of unreduced copper from the number of cubic centimeters 

 required for the blank. This gives the amount of thiosulphate equivalent to copper 



1 Copper Solution. Dissolve 34,639 grams of highest purity crystallized copper sulphate 

 (Kahlbaum's "zur Analyse mit Garantieschein) in water to make 500 c.c. 



Alkaline Tartrate Solution. Dissolve 173 grams of sodium potassium tartrate and 125 

 grams of potassium hydroxide in water to make 500 c.c. 



2 N/5 Sodium Thiosulphate. Dissolve about 50 grams of ordinary c.p. sodium thio- 

 sulphate or exactly 49.66 grams of the pure, dry, recrystallized salt, in enough boiled-out 

 distilled water to make a liter. Allow to stand for several days. The solution should be 

 standardized against the copper solution prepared as above. For this purpose introduce 

 20 c.c. of the copper solution into a 200 c.c. Erlenmeyer flask, add 20 c.c.^of 30 per cent 

 acetic acid and 40 c.c. of water. Add about 7 grams of a saturated solution of KI and 

 titrate with the thiosulphate using starch as an indicator. Calculate the equivalent of i 

 c.c. of thiosulphate in Cu. One c.c. of the copper sulphate solution contains 17.647 mg. 

 of Cu. The thiosulphate remains constant for some months. It should be kept in a dark 

 bottle. 



