§ 1. EExlGEXTS. 9 



g. Acid, oxalic— H,C,0,. (2H0,C,0,.) This should 

 not present the least appearance of efflorescence ; it 

 should give a perfectly clear solution with water, and 

 sliould leave no residue when ignited in a platinum dish. 

 If the acid does not meet these requirements, it should be 

 purified by repeated recrystallization. 



Stolba {Fresenius^ Zeltschrift 8, 63) recommends subli- 

 mation as a convenient method of purifying oxalic acid. 



Dry the acid thoroughly by keeping it in a warm place 

 for a considerable time, with occasional stirring ; when a 

 small portion of it, gently heated in a test tube, gives off 

 but little water before subliming, it is sufficiently dry. 

 Put it, then, in a large beaker to the depth of 15-20 mm., 

 cover the beaker with a hollow cone of paper, and im- 

 bed it in iron turnings in an iron dish, to the same depth 

 as that of the acid inside, and heat it cautiously, raising 

 the temperature very gradually. Scrape off the outside 

 of the cone of sublimed acid, separate the more solid yel- 

 lowish outer part from the white inner portion, and purify 

 each by itself by crystallization from solution as usual. 



A. Acid, sulphuric— H^SO,. (H0,S03). — Common 

 sulphuric acid usually contains lead, wdiich is precipitated 

 as a fine w^hite powder, when the acid is diluted with con- 

 siderable water, or when mixed with 4 or 5 parts of 

 alcohol; it sometimes gives a red color with a solution of 

 ferrous sulphate, where the two liquids come in contact (§ 

 62), and, when diluted, gives the reaction for chlorine with 

 argentic nitrate (§ 63), and for arsenic by Marsh's test 

 (§ 57). The pure acid should give none of these reac- 

 tions, nor any blue color after dilution with 20 parts of 

 water, when a little starch paste and potassic iodide are 

 added to the cooled liquid ; it should be volatilized com- 

 pletely when heated. 



The dilute acid is prejxired by adding the concentrated 

 acid to 5 parts of water, slowly, and with constant stir- 

 ring, letting the mixture stand a long time if any plumbic 

 1* ^ 



