ALUMINUM-MERCURY COUPLE FOR COPPER-ZINC 449 



above, and the distillation continued until the 150 cubic centi- 

 meter cylinder is filled. The titration is made in the usual way. 



389. Preparation of the Copper-Zinc Couple. For 800 cubic 

 centimeters of boiled rain water, prepared as described, six strips 

 of zinc foil, four inches long by one and a quarter inches wide, 

 are bent at right angles along their center to obtain stiffness. 

 The zinc strip is cleansed and coated with copper by washing in 

 a series of five beakers containing, respectively, dilute solution 

 of sodium hydroxid, very dilute sulfuric acid, a three per cent, 

 solution of copper sulfate, ordinary distilled water, and distilled 

 water free from ammonia. Through these five beakers the zinc 

 foil is successively passed. It is rinsed both after the alkali and 

 the acid, but after the copper has been deposited, the strips are 

 simply drained and carefully placed in the distilled water, it 

 being difficult to rinse without removing the copper. The couples 

 should be entirely submerged when placed in the rain water. The 

 strips should remain in the copper sulfate solution long enough 

 to be well covered with copper. 



390. Substitution of an Aluminum-Mercury Couple for Copper- 

 Zinc. Ormandy and Cohen have proposed to use an alumi- 

 num-mercury couple for the copper-zinc in the process described 

 above. 72 



This couple acts more quickly than the copper-zinc, and the 

 results are equally accurate. Nitrites are reduced in about one 

 hour by this apparatus, while the zinc-copper couple of Glad- 

 stone and Tribe requires about six times as long. Aluminum foil, 

 free of grease, should be used. The foil should be heated over 

 a bunsen just before amalgamation. The clean, very thin foil is 

 coated with mercury by shaking with a concentrated solution of 

 mercuric chlorid. It should be prepared immediately before use. 



The amalgamated foil is introduced into the sample of solution 

 to be analyzed, and left until all the aluminum is converted into 

 oxid. The presence of the oxid favors the prevention of bump- 

 ing during the subsequent distillation. The distilled ammonia, 

 collected in dilute acid, is determined by nesslerizing, the free 



72 Journal of the Chemical Society, 1890, 57 : 811. 

 15 



