Stability of Wijs Solution for Iodine Number Determinations FRANK A. NORRIS
AND
ROBERT J, BUSWELL, General Mills, Inc., Minneapolis, Minn.
Wijs solution is probably the most satisfactory for THE general use in determining iodine numbers. Its more widespread utilization is hindered mainly by its supposed difficulty of preparation and short life. The first objection is hardly valid if chlorine is available and if the analyst is reasonably careful. The short life would appear to be a much more serious objection, since three standard references on fat analytical methods specifically caution against using this solution when it is more than 30 days old (I, 2, 4)- However, Hilditch (8) does not accept this view, and Wijs himself (o) claims indefinite stability for the reagent. Previous experience by one of the present authors is in agreement with the two last-named investigators. Since a decision on the stability of Wijs solution was considered necessary in connection with some analytical work, the authors measured the stability of the reagent when stored at room temperature in 250-mi. dark bottles. The solution was anil contained 1.5% excess prepared in the standard manner
Stability of Wijs Solution
Table I. Bottle No.
Age of Solution.
Days
Days Since Bottle First Opened
1
6
0
2
45 97 97 132 174 231 277 314 374
0 52 0 35 77
2 3
3
3 3 4 4 4 5 5 5
404.
455 505
134 0
37 97 0 51 101
Iodine No. 177.3 178.0 177.1 176! 5
176.8 176.3 177.0
L77.4 177.0 177.8 L76.8
177.2
-
LITERATURE CITED
(1) Am. Oil Chem. Soc., Methods, 1941. (2) Assoc. Official Agr. Chem., Official and Tentative Methods, 5th ed., p. 430, 1940. (3) Hilditch, T. P., “Industrial Chemistry of Fats and Waxes'p. 4S, London, Bailliere, Tindall Cox, 1941. (4) Jamieson, G. S., “Vegetable Fats and Oils”, p. 343, New York,
equivalents of iodine over chlorine. Linseed oil, stored in the ice box, was used as the test substrate. Over a total period of 505 days, the Wijs solution did not change sufficiently to cause a measurable difference in the iodine number of the substrate. No measurable differences were found when the reagent was taken from bottles that had been previously opened. These facts indicate the validity of storing the solution a year or more, if storage is in small bottles which are
opened
as
Reinhold Publishing Corp., 1932. (5) Wijs, J. J. A., Analyst, 54, 11-14 (1929). Paper
needed.
52, Journal Series, Chemical Research
Department, General Mills,
Inc.
An Aid SELMA L. BANDEMER and
P. J.
in Ashing Certain Materials since the charring proceeds slowly from the outside toward the center. The charred mass tends to retain its cone shape and is
SCHAIBLE
Chemistry Section Michigan Agricultural Experiment Station, East Lansing, Mich.
free of the crucible. Thus, the crucible walls are protected from the action of the materials ashed. Fatty materials bum more evenly and slowly, without spattering. The char is moistened with a solution of magnesium chloride added dropwise over the entire mass and ashed in a muffle overnight at 600° C. The ash obtained is light, fluffy, and voluminous and dissolves readily in dilute hydrochloric acid on heating.
ashing of many products is often difficult and timeTHE consuming, as is evidenced by the number of procedures published for special materials (I). This is especially true for substances that are finely ground, contain oil or fat, or have a high phosphorus-to-base ratio. For such materials, a simple procedure which was recently devised to ash large volumes of liquid egg white (2) is suggested.
If unaided by this procedure, materials which have a high phosphorus-to-base ratio fuse on ignition to a glassy mass which frequently entraps carbon that can then be burned off only with great difficulty. This fusion is caused by the conversion of the dihydrogen and monohydrogen phosphates upon ignition to the metaphosphate (3), which does not dissolve readily in hot dilute hydrochloric acid. In the suggested procedure, magnesium chloride supplies base to produce the tertiary phosphate which is unaffected on heating to 600° C. In the authors’ experience, samples ash poorly if they do not contain ample base for the phosphorus or in the case of plant material if the stalks, stems, hulls, coatings, or bran have been removed. In these materials, the use of the lined crucible and magnesium chloride is beneficial. In comparative trials, the proposed method aided the ashing of fresh tissues such as muscle, liver, intestines, and fat of chicken, pork, and beef as well as fresh egg white and yolk, casein, lecithin, corn gluten meal,
This procedure consists of lining the crucible with filter paper, charring the material over a Meker burner in this lined crucible, moistening the char with a solution of magnesium chloride, and completing the ashing in a muffle furnace. Filter paper of the correct size (Whatman Xo. 40 or equivalent) is folded as for filtering, the tip of the cone folded back, and this truncated cone fitted into the crucible. This liner prevents local overheating because of the air space between it and the crucible and permits the easy escape of the volatile materials,
flour, and starch. LITERATURE CITED
(1) Assoc. Official Agr. Chem., Official and Tentative Methods of Analysis, 5th ed., 1940. (2) Bandemer, S. L.t and Schaible, P. J., Poultry Sci. (in press). (3) Mellor, J. W., “Comprehensive Treatise on Inorganic and Theoretical Chemistry”, Vol. VIII, p. 967, New York, Longmans, Green & Co., 1935. Journal
417
Article No. 697 (n.s.) Michigan Agricultural Experiment Station.
