CHAPTER XV. 



DETERMINATION OF BLOOD GASES BY THE 

 CHEMICAL METHOD 



Instead of pumping the gases out of blood, they may be dis- 

 placed by chemical means. Each method has its own advantages. 

 By the pump the gases are obtained unmixed with air so that their 

 analysis tells us directly how much of each is contained in the blood. 

 By the chemical method the oxygen that is loosely combined with 

 haemoglobin is expelled by shaking the laked blood with potassium 

 ferricyanide (the oxygen liberated during the reduction of this 

 salt to ferrocyanide displaces the oxygen which is loosely com- 

 bined with the haemoglobin, and takes its place to form methaemo- 

 globin). The carbon dioxide is expelled by adding a non-volatile 

 acid (saturated solution of tartaric acid). The volume of the 

 expelled gases may be measured in a suitable burette, if proper 

 care is exercised to avoid change in volume due to variation in 

 temperature. The chemical method is the more practical for most 

 work, but since the gases become mixed with the air originally 

 present in the apparatus, it is not suitable for proving what these 

 gases may be. Logically, therefore, in studying the blood gases 

 the pump method should precede the chemical. 



Experiment 52. The technique is as follows: The water 

 jacket of the burette (Fig. 35) (G) and the water bath of the blood 

 bottle (D) are filled with water that has been standing for some 

 time in the same room, and the temperature is noted. With the 

 stopper removed from the bottle the fluid (solution of calcium 

 chloride) in the burette is adjusted to the zero mark by raising or 

 lowering the levelling tube (H). 20 c.c. of CCVfree weak ammonia 

 water* i then placed in the bottle (C), and 2.5 c.c. (indicated by file 

 mark) of a freshly prepared saturated solution of potassium ferri- 



*0.5 c.c. Aq. Ammonia per 1000 c.c. of distilled water to which some barium 

 hydroxide has been added and then some ammonium sulphate, the resulting pre- 

 cipitates of BaCO 3 and BaSO 4 being allowed to settle. 



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