124 PUBLIC HEALTH CHEMISTRY 



Acetic Acid. Take 10 c.c. of sample in a porcelain dish. 

 If dark coloured, dilute well. Titrate with N/2 NaOH, 

 using phenolphthalein as indicator: i c.c. N/i NaOH = 

 o-o6 grm. glacial acetic acid. Should not be less than 

 3 per cent. 



Malic Acid in genuine cider vinegar. Gives a fairly 

 copious precipitate with lead subacetate solution. 



Free Sulphuric Acid. A few drops of sample are taken 

 on a white slab. Place near a drop of methyl-violet and 

 bring into contact with a glass rod. If only a trace of free 

 acid is present, the violet changes to blue ; but if more than 

 i part per iooo be present, a green colour develops. Or 

 test with logwood, as described under " Lime Juice." Or 

 add five drops of methyl-violet solution to 5 c.c. of sample, 

 and compare with control. Dilute sample if dark coloured. 



Nitrogen. Evaporate 25 c.c. to dryness, and proceed as 

 usual (none in wood or distilled vinegars). Less than o-i 

 per cent as a rule. 



Phosphoric Acid is estimated by Stock's method. This 

 consists in treating the ash with nitric acid, adding 

 ammonia, then more nitric, till the precipitate formed 

 dissolves; then more ammonia, until it returns slightly. 

 Some fuming nitric is then added, the solution warmed to 

 70 C, and ammonium molybdate solution added, when a 

 yellow precipitate forms and is collected and weighed. 

 Weight X 0-0373 pa P 2 5 . (None in distilled vinegars.) 



Poisonous Metals. Dissolve ash in boiling distilled 

 water, or this failing, in boiling dilute HC1, or if necessary, 

 in HC1 (2 parts) + HN0 3 (1 part), and then test by usual 

 analytical table. HC1 precipitates Ag, Hg', Pb ; H 2 S 

 precipitates Pb, Hg", Cu, Bi, Cd, As, Sb, Sn, Au, Pt ; AmCl 

 and AmOH precipitate Al, Cr, Fe ; Am 2 S precipitates Zn 

 Mn, Ni, Co ; Am 2 CO 3 precipitates Ba, Sr, Ca ; AmHPO 4 

 precipitates Mg. 



Copper, if present in the vinegar of pickles, may be 

 detected by inserting the bright blade of a steel knife. 



Arsenic may be derived from arsenical malt, caramel, 

 or glucose. Use Reinsch's test (see under " Beer," page 129). 



Copper, lead, and tin may all be derived in the process of 

 manufacture (cider vinegar) or from storage in metallic 

 or metallic-covered vessels. 



