DEPARTMENT OF AGRICULTURE 



NEW SOUTH WALES. 



SCIENCE BULLETIN, No. 4. 



A Modification of Marsh's Apparatus for the 

 Detection of Arsenic. 



LIONEL COHEN, Chemist's Branch. 



THOSE who have worked with Marsh's apparatus for the detection of arsenic 

 are aware of the difficulty of obtaining a prolonged and uniform current of 

 hydrogen, and have sometimes noticed how, when the stream becomes very 

 sluggish, a series of minute explosions occurs in the heated combustion tube, 

 resulting in the formation of dew or water drops therein. Furthermore, that 

 sine qua non of the original apparatus, arsenic-free zinc, is not always easily 

 obtainable in quantity. 



Many useful suggestions with a view to improvement have been made by 

 chemists from time to time, the chief of these being : (1) The attachment 

 of a reservoir of hydrogen to obtain a stronger flow of gas ; (2) the addition 

 of a copper or platinum salt to very pure samples of zinc in order to promote 

 galvanic action j and (3) the substitution of magnesium for the latter metal. 



The standard electrolytic apparatus, of course, possesses great advantages 

 over the zinc method, but is impracticable in many laboratories owing to 

 lack of suitable source of current, &c. The initial expense is also rather 

 high. The interposition of a hydrogen gasometer or reservoir postulates a 

 supply of arsenic-free gas. 



Where quantities of pure zinc are not available, the use of platinum or 

 copper salts merely expedites the consumption of the small amount on hand. 

 Magnesium is expensive ; its reaction with dilute acid is also very violent, 

 and if sufficiently strong acid is used to promote a stream of gas of requisite 

 volume, so much heat is evolved as to cause the solution to boil and generate 

 large quantities of steam. 



The use of aluminium is open to the same objection ; moreover, it generally 

 contains silicon as an impurity, resulting in a deposit of silica in the heated 

 tube, through decomposition of the silicon hydride. 



