VII. ESTIMATION 451 



two-hole rul)l)er stoppers fitted with gas inlet and outlet tubes. The final 

 trap which contained the alkali for the collection of the hydrogen cyanide 

 was an 18 X 150-mm. test tube closed by a two-hole rubi)er stopp.er The 

 inlet tube reached to the \'ery Iwttom of the tube to ensure complete im- 

 mersion in the small volume of alkali. The collection tubes were coated 

 with silicone Dri-film (C..E. No. 9987) applied in the manner described by 

 Doyle andOmoto.'- The silicone coating prevents continuous film formation 

 and loss of solution through the outlet tube when a rapid stream of gas is 

 bubbled through the alkali, ^^^lenever traps were necessary to remove 

 interfering volatile material, 25 X 200-mm. test tubes were used, fitted 

 exactly like the reaction vessel for a 25-ml. volume. Connections between 

 the various members of the gas train were made with tight-fitting gum 

 rubber tubing. Up to eight individual setups were connected in series and 

 swept by one stream of nitrogen of measured flow rate. The nitrogen, 

 before entering the first reaction vessel, was passed through three traps 

 containing, respectively 20 ml. of acid eerie sulfate, 20 ml. of acid silver 

 sulfate, and 20 ml. of 0.1 N NaOH, and finally through an empty trap. 



Separation of Cyanide from Thiocyanate. The solution in the reaction 

 vessel, irrespective of the volume used, contained between 0.005 and 0.4 

 7 of cyanide. The pH of the solution was adjusted to 5 or less. Aeration at 

 this pH separates hydrogen cyanide quantitatively from even 10,000 times 

 as much thiocj'anate, provided that no oxidizing agent is present in the 

 solution. In all the biological fluids tested in the subsequent papers, the 

 reduction-oxidation potential was so poised that oxidation of thiocyanate 

 to cyanide did not occur. Whenever there is suspicion that oxidation of 

 thiocyanate can occur, it can be prevented with certainty by the addition 

 of some hypophosphite to the solution. 



Collection of Cyanide in Alkali. The hydrogen cyanide swept out of the 

 reaction vessel was in all cases collected in 1 ml. of 0.1 A^ NaOH contained 

 in the 18 X 150-mm. tube. 



Separation of Hydrogen Cyanide from Volatile Interfering Materials. Many 

 substrates contain materials which are swept over with the hydrogen cy- 

 anide. These interfering materials are prevented from reaching the collec- 

 tion tube by placing the following series of traps between the reaction \'essel 

 and the collection tube: (a) an empty tube, (b) a tube containing 20 ml. of 

 0.5 A^ eerie sulfate in 1.0 N sulfuric acid, (c) a tube containing 20% .sulfuric 

 acid saturated with silver sulfate and a few crystals of excess silver sulfate, 

 and (d) a tube containing 20 ml. of 0.1 A'' sulfuric acid. 



A\Tienever these traps are used, 15 minutes must be added to the transfer 

 time. The solutions in the traps should be renewed whenever the eerie ion 

 color fades visibly or the silver trap shows a brownish -black discoloration. 



12 W. L. Doyle and J. H. Omoto, Anal. Chem. 22, 603 (1950). 



