Foods, their composition and analysis : a manual for the use of analytical chemists and others ; with an introductory essay on the history of adulteration / Alexander Wynter Blyth ... and Meredith Wynter Blyth.

  • Alexander Wynter Blyth
Date:
1909
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Licence: In copyright

Credit: Foods, their composition and analysis : a manual for the use of analytical chemists and others ; with an introductory essay on the history of adulteration / Alexander Wynter Blyth ... and Meredith Wynter Blyth. Source: Wellcome Collection.

Provider: This material has been provided by The University of Leeds Library. The original may be consulted at The University of Leeds Library.

    98/680 (page 64)
    absorption proportion for that particular region of the spectrum, and the result equals the gnus, per c.c.—that is, c = e A. In other words, once the absorption proportion for various regions of the spectrum is known, it is easy to ascei'tain the percentage composition The more regions of the spectrum investigated, of course, the more accurate is likely to be the determination. As an example, let us take the absorption spectrum of permanganate. For wave lengths, 680-7 to 650, Kriiss found that a solution containing 0 001 grm. per c.c. gave a coefficient extinction of 0-47238; and for a concentration of 0-00025 per c.c. a coefficient extinction of 0T1351. Therefore dividing c by e in each case, for the one we get as the absorption proportion 0-002116, for the other 0’002202, the mean of which is 0‘002159. In the region embraced between the wave lengths 596‘4 and 582‘8, for solutions containing respectively 0 00025 grm. and 0 000125 grm. per c.c., Kriiss found the coefficient extinction to be 0-40561 and 0T6242; the mean of the results of c divided by e is 0’006845; and so on for various portions of the spectrum. For example, if it were desired to ascertain the strength of a solution of potassic permanganate, the unknown solution would be diluted until, from its colour, it was judged to be somewhere near the strength of the solution whose absorption proportion was known and several extinction coefficients obtained. Thus, in the present instance, supposing for the wave lengths 6807 to 650T an extinction coefficient of 0‘47238, and for the wave lengths 613‘2 to 596‘4 a coefficient extinction of 1‘08093 was obtained, the absorption proportion for the respective wave lengths being known to be 0‘00215 and 0‘0009186, we should have— [e x A = c.] (1) 0-47238 x 0-002159 = ‘00102 (2) 1-08093 x 0-0009186= -00124 giving a mean value of 0-0011, the real value in this case being in a c.c. 001; a nearer approximation could be made to the true value by more de- terminations. For every coloured substance there are special regions of the spectrum most suitable for quantitative estimation, and it is necessary in ascertain- ing the ‘absorption proportion’ to measure carefully the proportions of the spectrum observed ; for example, Kriiss finds that the most suitable regions for the quantitative estimation of potassic permanganate solution are as shown in the following table, which also gives the absorption for those regions :— Wave Lengths. Absorption Proportion. A 494-7 to A 48G-5 A 480 5 ,, A 480-9 A 480-9 ,, A 474-8 0 0001909 1 0-0002251 0*1)003277 1 If oxygen is to be determined by permanganate, then the absorption, in terms of oxygen, is for the wave lengths given—0 00004833, 0 00005699, and 0-0000829(5.