4 8o PRACTICAL ORGANIC AND BIO-CHEMISTRY 



(c) Carboxyhaemoglobin. COHb. 



Haemoglobin combines with carbon monoxide forming carboxy- 

 haemoglobin. Carboxyhaemoglobin differs from oxyhaemoglobin in 

 being more stable ; it can only be dissociated by prolonged treatment 

 with oxygen. It is formed in cases of coal-gas poisoning by the 

 action of the carbon monoxide in the gas. Owing to its stability cases 

 of coal-gas poisoning are generally fatal. 



A stream of coal gas is passed through a solution of defibrinated 

 blood. It assumes a cherry red colour as compared with the yellowish- 

 red of oxyhaemoglobin, due to the smaller absorption of light in the 

 blue and violet regions. 



Spectroscopic examination will show two bands like those of 

 oxyhaemoglobin, but they are situated slightly nearer the violet end 

 of the spectrum. The difference in position can only be determined 

 by the use of a spectroscope with a scale of wave lengths. 



Differentiation between Carboxy hemoglobin and Oxyhcemoglobin. 



Oxyhaemoglobin is reduced to haemoglobin by reducing agents, 

 but Carboxyhaemoglobin is not reduced. If therefore some ammonium 

 sulphide be added to a solution of Carboxyhaemoglobin and the solu- 

 tion be gently warmed, no change is noticed in the absorption bands. 



The blood in cases of suspected coal-gas poisoning must always be 

 tested by the reduction. 



Solutions of Carboxyhaemoglobin (saturated with carbon monoxide) become 

 light red in colour on treatment with an equal volume of 40 per cent, caustic 

 soda; a red precipitate is deposited. Solutions of oxyhaemoglobin give a 

 brown coloration and brownish-black precipitate. 



Tannic acid gives a red precipitate with Carboxyhaemoglobin solutions, 

 a greenish-brown precipitate with oxyhaemoglobin. 



On boiling a pure solution of Carboxyhaemoglobin, a red precipitate is 

 obtained ; oxyhaemoglobin solutions give a brown precipitate. 



Nitric Oxide Haemoglobin. NOHb. 



Haemoglobin combines with nitric oxide in the same way as with carbon 

 monoxide ; this compound is more stable than Carboxyhaemoglobin. It is 

 prepared from Carboxyhaemoglobin by passing nitric oxide into the solution. 

 The solution is placed in a vessel so that the air is first expelled by passing a 

 current of neutral gas (nitrogen or hydrogen) ; the nitric oxide is passed in 

 and the excess expelled in the same way. If prepared from oxyhaemoglobin 

 the solution must contain alkali to absorb nitric acid produced by combination 

 of oxygen and nitric oxide. 



Solutions of nitric oxide haemoglobin possess a bright red colour and show 

 two absorption bands like those of Carboxyhaemoglobin. On boiling they give 

 a red precipitate. 



Nitric oxide haemoglobin is formed together with methaemoglobin when 

 nitrates are taken by the mouth ; the red colour of meat pickled with potassium 

 nitrate is due to nitric oxide haemoglobin (Haldane). 



