A Text-book of Tanning / A treatise on the conversion of skins into leather, both practical and theoretical. - H. R. Procter

Lead (and copper) may be detected by passing sulphuretted hydrogen through the water acidified with HCl, or by adding a drop of fresh ammonium or sodium sulphide to the slightly acidified water, when a brownish coloration clearly visible in a deep beaker set on a sheet of white paper will be produced. Iron also gives a black with sulphides in alkaline solution. Copper may be distinguished from lead by the blue given with ammonia, and by a reddish-brown precipitate with potassium ferrocyanide.

For accurate quantitative estimation of these impurities, the regular works on the subject, such as Thorpe's 'Quantitative Analysis,' Sutton's 'Volumetric Analysis,' or Fresenius' 'Quantitative Analysis,' must be consulted.

EXAMINATION OF COMMERCIAL ACIDS.

Sulphuric acid 10 grm. may be made up to 100 c.c. and well mixed, and of this 10 c.c. may be tested with normal sodic carbonate in presence of methyl orange. Each 1 c.c. of soda solution used corresponds to 0·049 grm. or 4·9 per cent. of H₂SO₄. For most purposes, the strength may be ascertained from the specific gravity, as measured by a hydrometer or weighed in a specific gravity bottle. The following table gives the strength at 59° F. (15° C.):—

Specific Gravity.	Degrees Twaddell.[H]	Per cent. H₂SO₄
1·8426	168·5	100
1·8376	167·5	95
1·822	164	90
1·786	157	85
1·734	147	80
1·675	135	75
1·615	123	70
1·557	111	65
1·501	100	60
1·448	90	55

Specific Gravity.	Degrees Twaddell.[H]	Per cent. H₂SO₄
1·398	80	50
1·351	70	45
1·306	61	40
1·264	53	35
1·223	45	30
1·182	36	25
1·144	29	20
1·106	21	15
1·068	14	10
1·032	6	5

[H] Degrees of Twaddell's hydrometer may be reduced to specific gravity by multiplying by ·005 and adding 1·, thus 10° Tw. = 1·050 sp. gr.

The impurities of sulphuric acid most common and injurious for tanning purposes are iron and nitrous acid. Iron is detected on neutralising with soda or ammonia, when it falls as a yellowish precipitate, which may be recognised by the ordinary tests ([p. 100]). Nitric and nitrous acids are detected by pouring a strong solution of ferrous sulphate cautiously on to the top of the strong cold acid, when a dark ring is formed at the junction of the two liquids.

Hydrochloric acid may be tested with soda solution like sulphuric. 1 c.c. of normal soda = 0·0365 grm. or 3·65 per cent. HCl. It may also be calculated from specific gravity.

Specific Gravity, 15° C.	Per cent. HCl.
1·200	40
1·177	35
1·151	30
1·126	25
1·100	20
1·075	15
1·050	10
1·025	5