Food : its adulterations, and the methods for their detecton / by Arthur Hill Hassall.

Date:
1876
Catalogue details

Licence: Public Domain Mark

Credit: Food : its adulterations, and the methods for their detecton / by Arthur Hill Hassall. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    to be determined, Wanklyn's method gives satisfactory results; in those cases, however, where the quantity of organic matter, and especially the quantity of carbon, is required, Frankland's method is to be pre- ferred. This latter should, a;.d no doubt will, supersede the perman- ganate process. Both methods for the estimation of nitric acid give very accurate results, if the quantity of nitrates present be but moderate. All the apparatus requii-ed for water analysis by Frankland's method may be obtained of Messrs. Oetti, Holborn. ESTIMATION or THE MIMRAL C0N8T1TTJENT8 OP WATER. Detei-^mination of the total solid residue.—^00 to 500 cc. of the water are to be evaporated in a weighed platinum basin on the water- bath, and the residue is to be dried in the air-bath at about 110° 0 The result is to be calculated for 100,000 of water. The weighed residue is then heated over the open flame, and it ia carefully noted whether it tm-ns brown or black on igniting, indicating the presence of carbonaceous, matter. The loss by ignition may be determined, but it gives no reliable data as to the quantity of organic matter present, smce chloride of calciimi retains, when dried at 110° 0 a not inconsiderable proportion of absorbed water, and, further since' chlonde of magnesium is decomposed by heating, suffering a decrease ot weight which would of course be calculated as organic matter Estimation of chlonne..~0}i\or:m& mav be estimated either gravi- ^A^; ^Inn' ^°l^eti-ically. For the first method of estiniation oOOto 1,000 cc. are evaporated to a small bulk, and the chlorine is then precipitated by means of nitrate of silver. The chloride of silver is hltered, washed, dried, and weighed. 100 parts of AgOl contain 24,724 parts of chlorine. For the volumetric estimation, a solution of nitrate of silver of known strength is required. It is prepared by dissolving in 1 litre of distilled water 0-4/9 gramme of pure crystallised nitrate of silver 1 cc. ol this solution precipitates 0-0001 gramme of chlorine. 100 cc. of the water are measm-ed in a beaker, 2 or 3 drops of a saturated solution of neutral chromate of potassium are added and then from a burette the standard silver solution is dropped into the yellow-coloured water. Each drop produces a brown coloration which at first rapidly disappears. Soon, .however, this coloraS P~ent and the nmnber of cc. of silver solution used ^5 noted and from it the quantity of chlorine is calculated. Ihe pnnciple upon which this method is based is the following •— Silver has a greater affinity for chlorine than for chromic acid ^'if therefore, a solution of silver be added to a liquid containing both chlorine and chromic acid, the former wiU be first perfectly preci- S W r ^'o'Tlff I -ddish-brown preVitatef l^Sns to form. As soon therefore as the brown coloration is permanent. aU