February 15, 1941
103
ANALYTICAL EDITION
mination of very small quantities of nickel, cobalt, and bismuth in organic materials. The spectrographic method used for nickel and cobalt has almost as good agreement and precision but requires somewhat larger quantities of the materials. The latter is more rapid, however, if many samples are to be analyzed. The close agreement of the analyses by the two methods indicates that the results are very accurate.
solution of potassium iodide were then added before diluting to a volume of 25 ml. The concentration was plotted against per cent transmission on a semilogarithmic scala; the graph was a straight line passing through the ordinate at 100 per cent, showing that Beer’s law held for these dilute solutions, and that the compound was suitable for use in the photometer. TABLE IV. BISMUTH IN BISMUTH PECTINATE
Literature Cited
Bismuth Found by Photometric Method
Weight of Sample Grain 0.2155 0.20GO
(1) Griggs, M. A., Johnstin, R., and’Elledge, B. E., IND.ENO.CEEM., Anal. Ed., 13,99 (1941). (2) Lundeghrdh, H.,“Die quantitative Spektralanalyse der El& mente”, Teil I and 11, Jena, Verlag von Gustav Fischer, 1929 and 1934. (3) Lundegbrdh, H., Lantbruks-HGgakol. Ann., 3,49 (1936). (4) Lundeghrdh, H.,and Philipson, T., Ann. Agr. College Sweden, 5, 249 (1938). ( 5 ) MacPherson, H. T., and Stewart, J., Biochem. J . , 32,763 (1938). (6) Murray, W. M., Jr., and Ashley, 5. E. Q,,IND. ENO.CHEM., Anal. Ed., 10,1 (1938). (7) Rasmussen, H.B.,Jackerott, K. A., and Schou, S. A., Damk Tida. Farm., 1, 391 (1927); Biochem. Z.,193,53 (1928).
% 0.57 0 A3 0.53 0 A:,
0.1801 0.2840 0.2834
0.55 0.5:
Av.
Conclusions The photometric methods presented in this paper show excellent agreement and therefore high precision for the deter-
Use of Thorium Nitrate to Distinguish between Pectin and Certain Gums E. F. BRYANT, California Fruit Growers Exchange, Research Department, Ontario, Calif.
I
precipitate with 50 per cent alcohol; gum tragacanth gives only a slight precipitate; but pectin, quince seed, and locustbean gums give good precipitates. Of the gums listed in Table I, only Irish moss, quince seed, and pectin give precipitates with thorium nitrate. The precipitate from Irish moss is stringy and opaque and easily differentiated, but those from quince seed and pectin are alike, being of a firm, gelatinous nature. The pectin precipitate is easily dispersed or in some cases actually dissolves on adding an excess of the thorium nitrate solution or dilute acetic acid; the quince-seed precipitate with thorium nitrate is unaffected by this treatment. If the dilute acetic acid is first added to the gum sol, the quince seed will yield a firm gel and the pectin shows only a thickening or very slight gel. A confirmation of the thorium nitrate test for pectin may be made with neutral lead acetate. In unacidified sols pectin and quince seed yield gels, while if acetic acid is added to the sol before the lead acetate, pectin forms a firm, clear, brittle gel but quince-seed gum gives only a very weak gel or a visEous sol, This is just the-reverse of the thorium nitrate test. To 10 ml. of the aqueous solution of the gum (1 in 100) add 1 ml. of 10 per cent thorium nitrate TABLE I. REACTIONS OF GUMSAND RELATED SUBSTANCES WITH THORIUM solution, stir, and allow to stand 2 minutes. If a NITRATE AND NEUTRAL LEADACETATE gel results, the gum is either pectin or quinceseed gum. If no gel results it is not pectin. Thorium Nitrate Neutral Lead Acetate To differentiste between the two gums: To 10 10% solution 10% solution Material 10% arid 5 N 10% and B N rnl. of the sol add 1 rnl. of 5 N acetic acid, then Used solution ncctic acid solution acetic acid 1 ml. of 10 per cent thorium nitrate solution, stir, Gum nrnbio a a 4 and allow to stand 2 minutes. If no firm gel reLocust-bean 4 0 Slight thicka sults the gum is pectin; if a gel forms it is quincegum ening seed gum. a 4 a Gum To check the reaction, a 10 per cent solution tragacanth Irish moas Stringy, white White granyCloudy Cloudy of neutral lead acetate is used and the same proceprecipitate lnr Dreciw dure carried out as for thorium nitrate. tate -
DENTIFICATION of the individuals commonly grouped as water-soluble gums is a t times difficult, as many of the substances react alike to the same reagent. Pectin is one of the most difficult of this group to identify, because it is usually precipitated by the reagents which also precipitate the other gums. Differentiating between pectin and other gums (2,s) requires more time and labor than a simple precipitation test. In attempting to carry out work on pectin similar to that done by Bonner (1) on pectate it was found that precipitation with thorium nitrate exhibits a novel property of pectin not shown by the other gums tested. If the substance under examination is a powder containing no added materials, such as sugar or acid, it is made up to a 1 per cent aqueous sol. Any admixture should first be extracted with 50 per cent alcohol, or can be eliminated by dissolving the substance and then precipitating with an equal volume of alcohol. If the sample is a liquid preparation, the gum should be precipitated as above, so that a pure sol of known strength can be made. Irish moss, agar, gum arabic, and karaya gum give no
(1
(1
Karaya gum Quinee-seed gum
Pectin Agar ( 0 . 5 ini3t&B:’ of
0
Firm opaque gel Firm, transpurent gel Slight haze
4 M&l cellulose Starch (I No apparent reaction. (I
0
Firm gel
0
Fairly firm gel
Very weak pel a thickening(
Firm. transparent pel
a
Very wenk gel u thickenink Firm, brittle, clear gel 4
(1
(1
4
a
a
4
a
Table I shows the gums tested and the results obtained .
Literature Cited
(1) Bonner, J., Proc. Acad. Sei. Amsterdam, 38, 349, 350 (1935). (2) Jacobs, M. B., “Chemical Analysis of Foods and Food Products”, p. 283, New York, D. Van Nostrand Co., 1938. (3) Savini, G., and Gianferrara, S., Ann. chirn. 4p plicata, 29, 111-15 (1939).
