MAY 15, 1940
ANALYTICAL EDITION
shows a n inflection. If one knovs the accuracy with which the light absorption can be measured, the maximum attainable accuracy in the determination is immediately available. The curves in Figures 3, 4, and 5 are plotted according to the method suggested b y Ringbom. It will be noted that in most cases the curves have a considerable linear portion around the inflection point, so that the maximum accuracy is attainable over a considerable concentration range. I n some cases, measurements were not followed to sufficiently high concentrations to indicate the inflection point. The curve for the turbidimetric determination of zinc indicates t h a t the same considerations which apply to the colorimetric methods also apply to this turbidimetric method. Table I1 indicates the maximum accuracy obtainable in the determinations previously described; the concentration region over which the maximum accuracy applies is approximate only. The maximum accuracy is figured on the knowledge that the instrument can be read to a n accuracy of 0.1 per cent light absorption. The data in the table are sufficient t o illustrate the importance of proper selection of light filter and concentration range if maximum accuracy is to be obtained. The results obtained with ferrous sulfate indicate the p o b sibility of the application of the thermoelectric absorptiometer in the determination of substances which have little or no visible color, if they show absorption in the infrared. Organic liquids seem to offer good possibilities along this line. Preliminary tests on 45 organic liquids have shown charactel,istic differences between various types of organic compounds, and slight differences between compounds in a given homologous series.
Summary An improved thermoelectric absorptiometer using a balanced circuit has been constructed. It is rugged and compact and therefore portable, and is capable of giving a direct reatling to a n accuracy of 0.1 per cent light absorption. The thermopiles are sensitive and rapid. The ease and speed with which the test t’ubes used for absorption cells can be cleaned and dried, aid in reducing the time required in making measurements. Ten milliliters of liquid are sufficient for a determination. The instrument has been tested with nine systems showing a wide range of visible color and spectral absorption, and a turbidimetric determination has also been made. There seems to be no reason why i t could not be applied to any colorimetric or turbidimetric measurement, as well as to measurements of colorless systems showing infrared absorption. Although thus far the instrument has been used only in series measurements, i t would seem possible to apply it in comparison methods involving duplication or dilution. Literature Cited (1) hyres, G. H., and Smith, F., ISD. Esa. CHEhf., Anal. Ed., 11, 365 (1939). (2) Blanchetibre, A, and Pirlot, J. hI.,Compt. rend. soc. biol., 101, 868 (1929). (3) Bodansky, bf., J. IND. EXQ.CHEM.,13, 696 (1921). (4) Bolton, E. R., and Williams, K. A., Amlust, 60, 447 (1935). ( 5 ) Corning Glass Works, Corning, N. Y., technical bulletin. (6) Huttner, C., 2. anorg. Chem., 86, 351 (1914). (7) Hughes. -i.L., and DuBridge, L. A., “Photoelectric Phenomena”, p. 462, New York, McGraw-Hill Book Co., 1932. (8) Kipp and Zonen, Delft, Holland, Tech. Bull. E x 24 and Exmon 29.
(9) Moll, W. J. H . , Proc. Phys. SOC.London, 35, 257 (1923). (10) hliiller, R. H., IKD. ESG. CHEM., dnal. Ed., 11, 1 (1939). (11) Ringbom, A., 2. anal. Chem., 115, 332 (1939); Ringbom, A., and Sundman, F., Ibid., 115,402 (1939); 116, 104 (1939). (12) Snell, F. D., and Snell, C. T., “Colorimetric Methods of Analysis”, Vol. I, rx 146, New York, D. Van Nostrand Co., 1936. (13) Ibid.,pp. 296-8. (14) Van Tussenbroek, SI.J., Chem. T e e k b l a d , 24, 240 (1927).
291
(15) Ibid., 26, 374 (1929). (16) \Veston Electrical Instrument Corp., Newark, N. J., technical bulletin. (17) Wilcox, L. V., IND.ENG.CHEM.,Anal. Ed., 6 , 167 (1934). (18) Willard, H . H., and Gordon, L., unpublished work, University of Michigan. (19) Willard, H . H., and Greathouse, L. H . , J . Am. Chem. Soc., 39, 2366 (1917). (20) Willard, H . H., and Sheldon, J. L., unpublished work, University of Michigan, (21) Withrow, R . B., Shrewsbqry, C. L., and Kraybill, H. R., IND. EXG.CHEhf., hnal. Ed., 8, 214 (1936).
Modified Rohrig Extraction Tube
T
HE Roese-Got’tlieb method, using a Rohrig extraction tube, is standard for estimation of fat in milk and milk products ( 1 ) . However, as described by Bigelow and Fitzgerald ( 2 ) , it has been criticized because of frequent failiirc to give duplicable results and a tendency to give lon- fat values n-lien compared with other methods. The authors, n-hose experience with the Roese-Gottlieb technique has been principally in the determination of fat in evaporated milk, have found that this method as execut’ed by an experienced technician will give rapid and reliable results. Rohrig tubes are closed during extraction of the samples by a stopper or by the thumb of the operator; in the latter case, the vent hole in the wall of the tube is closed by the first finger. Occasional l o x results may be explained either by persistent adherence of fat particles to the stopper, or by the loss of sample t’hrough the end of the tube or the vent hole, if the thumb or forefinger slips duriue; extraction or during the blowing down of the turbe aftkr extraction. Loss of sample may be due to failure of the operator’s hand to conform to the size of the Rohrig tube, or to fatigue of the thumb or forefinger resulting from a large number of consecutive determinations. 4 These difficulties may be largely obviated by use of a modified form of tube shown in Figure 1. A section of a test tube, A , is sealed t o a glass stopcock, B , and this assembly is in turn sealed to the top of the Rohrig tube, C. The top of the Rohrig tube must be dran-n out somewhat t o eliminate the hole in the wall of the tube and to accommodate the diameter of thestopcock. The test tube hasacapacity of about 20 ml. after sealing; the bore of t,he stopcock is 3 mm. This modified type of tube has been used successfully a t this laboratory for the past 6 months. It permits more thorough mixing and shaking of the sample and also allows control of gas pressure developed within the tube. By careful manipulation of the stopcock, the pressure may be slowly released and the tube blown clown without loss of the sample. Literature Cited (1) Assoc. Official Agr. Cheni., Official and
Tentative Methods of
.\ti:~lysi$, 4th
ell. (lW5)
( 2 ) Bigelow and l:itzgeralcl,
Xational Can-
ners’ Assoc., Bull. 5 , 11 (1916).
FIGURE 1
