495 



oily acid appearing in the neck of the retort when the temperature 

 rises to 203 C., and producing crystals on coming in contact with the 

 first distillate. These crystals are likewise formed when water is added 

 to the volatile liquid ; and this is, in fact, the best mode of preparing 

 them in the pure state. 



1. Perchloric Acid, C1O 4 H. 



Pure perchloric acid may be obtained in quantity by the process 

 above described, or, more easily, according to the following method. 

 Perchlorate of potassium is distilled together with four equivalents 

 of strong sulphuric acid, until the drops of distillate no longer soli- 

 dify on falling into the receiver ; in this way 1 00 grammes of per- 

 chlorate yield 14 grammes of crystallized perchloric acid containing 

 traces of sulphuric acid. The impure crystals are then melted, placed 

 in a small bulb-retort, and gently heated. Ebullition commences at 

 110 C., and the decomposition continues for some time without fur- 

 ther aid of heat ; dense white fumes, accompanied by small quantities 

 of chlorine, issue from the mouth of the apparatus, and a mobile, 

 easily condensible, heavy liquid is found in the receiver. This liquid 

 is pure perchloric acid, C1O 4 H, which, if the distillation has been 

 conducted slowly, is perfectly colourless, but often has a slightly 

 yellow colour, owing to the presence of lower oxides of chlorine. As 

 soon as the first appearance of crystals in the neck of the retort is 

 noticed, the distillation is stopped. The liquid distillate should then 

 be at once sealed up in a bulb with long capillary ends, out of which 

 any wished-for quantity may be obtained at pleasure. The greatest 

 care is necessary in working with this body, owing to its explosive and 

 corrosive properties. 



The composition of the liquid thus prepared was determined in 

 two ways : in the first place, by the estimation of the quantity of 

 the potassium salt which a given weight of the acid yielded, as also 

 of the quantity of chlorine, oxygen, and potassium which this salt 

 contained ; and secondly, by estimation of the quantity of barium 

 needed to form a neutral salt with a given weight of the acid. As 

 the acid on exposure to the air absorbs moisture with the greatest 

 avidity, producing the crystals, it is necessary that the acid for 

 analysis should be quickly sealed up in weighed bulbs. 



Analysis No. 1. 0'7840 grm. of pure perchloric acid, prepared as 



