Simplified Preparation of Cadmium Iodide-Linear Starch Reagent for Colorimetric Iodimetry
SIR: Previous studies (1, 3- 10) have eitablished the usefulness of the cadmium iodide-linear starch reagent in colorimetric iodimetry. The reagent is stable, reproducible, and has a molar absorptivity of about 17,000 per equivalent of oxidizing agent ( 5 ) . Reagent stability is attributed to the touicity of the cadmium, which inhibits bacterial growth, and to the formation of cadmium iodide auto-complexes, hich allparently prevent retrogradation of the starch. The greatest stability and swsitivity are obtained with the linear A, or amylose, fraction from potato starch. The soluble blue htarch-triiodide incluqion complex produced by rmction nith oxidizing agents has a n absorption maximum a t 615 mp. Despite these advantages the reagent ha5 not found wide usage, and many n orkers continue to use starch solutions nhich produce inferior hues and are much le\, stable. This is probably due to the lengthy butanol-1 or pentanol-1 recrystallization procedures previously required to isolate a pure linear starch fraction. Recently, a n amyloie potato starch fraction of high purity has become available a t low cost under the trade name Superlose, marketed by Stein Hall &C'o., 285 Madison -4ve., Kew York 17, h-.Y. The authors have found that this product ($anbe used to prepare a cadmium iodide-linear starch reagent comparable to the one prel iously reported.
WAVELENGTH, mp
Figure 1 . Absorption curves in visible region of blue starch-triiodide ion complex formed from cadmium iodide-linear starch reagent
EXPERIMENTAL
Dissolve 11.0 grams of cadmium iodide in 400 ml. of water and boil gently for 15 minutes t o expel1 any iodine. Dilute to 800 ml., and while stirring slowly add 15.0 grams of Superlose to the gently boiling solution. Continue boiling and stirring for 10 minutes. Filter with suction through fine filter paper, such as llJhatman's No. 42, on a Buchner funnel, while maintaining the unfiltered solution a t about 65" C. (The rate of filtration can be accelerated if fresh filters are inserted a t regular intervals. This step should be completed in less than 1 hour if five or six changes of filter paper are made.) Dilute the filtrate with water to one liter. If a high-speed centrifuge is available, the time-consuming filtration step could be eliminated. DISCUSSION
For all practical purposes, the reagent described herein is comparable to that obtained prel iously with the linear fraction precipitated from butanol-1 and pentanol-1 solutions. On long standing i t may develop some cloudiness which can not readily be removed by filtration or centrifugation, but n-hich dissolves upon dilution. Absorption curves and concentration determinations made with the reagent more than four months after its preparation were identical to those obtained from fresh solutions. Presented in Figure 1 are a series of ahsorption curves in the visible region, of the blue starch-triiodide complex formed from this reagent. The curves shorn a maximum extending from about 600 to 630 mp, n-hich agrees clo,ely nith prei ious r e d t s (5). FIypochlorite, at a pH of about 5.2, m s uked aq the oxidizing agent ( I O ) . Figure 2 shows the results of a typical concentration-absoi bance plot, recorded a t 615 mp, for rarious amounts of iodate ion. Samples prepared for use in obtaining the data presented nere made up in a 50-nil. volumetric flask from a 10-p.p.m. iodate stock solution and distilled nater to gire a volume of about 40 ml. The solutions were then acidified n-ith 1.0 ml. of 2.5Y hydrochloric acid, and 1.0 ml. of the cadmium iodide-linear starch reagent was added. The samples vere then diluted to 50 ml. with distilled water, and absorbance readings Tvere taken after 10 minutes' standing. (Final concentration was based on the 50-ml. volume.)
1.41
Figure 2. Concentration-absorbance relationship for iodate ion, recorded at
615 mp As noted earlier (6, I O ) , the curve is linear, but can not be said to obey Beer's law as it does not pass through the origin. This behavior is due t o the fact that color development results from a complex formed b y initial iodine penetration, followed b y adsorption on the exterior, of the amylose helices ( 2 ) . Under these conditions more than one triiodide ion per starch helix is required to produce color. The molar absorptivity calculated from Figure 2 is 18,500 per equivalent of oxidizing agent, which is slightly larger than the value of 17,000 reported earlier (5). ACKNOWLEDGMENT
The authors are grateful to Stein Hall &. Co. for providing samples of Superlow. LITERATURE CITED
( i ) Arthur, P., hloore, T. E., Imnbert,
r,.,
J. J . -4?,%. soc.71,3260 (1949). ( 2 ) Hollo, J., Szejtli, J., (Polytech Univ. of Budapest), Periodica Polytech. 1, 223-35 (1957); C.A. 5 2 , 12347d (1958). (3) Lambert, J . L., Ibid., 25, 271 (19%). (4) Ibid., p. 954. ( 5 ) I b i d . , 23, 1247 (1951). ( 6 ) Ibid., p. 1251.
( 7 ) Lambert, J. L., Arthur, P., Moore, T. E., Ibid., 23, 1101 (1951). (8) Lambert, J. L., Rhoads, S. C., hsa~.
CHEM.28,1629 (1956).
(9) Lambert, J. I,., Yasuda, S.I
