POLYOXYETHYLENE STEARATE Colorimetric Determination in Dilute Solutions ROBERT V. MAcALLISTER AND RAYMOND J. LISK Central Laboratories, Division of General Foods Corp., Hoboken, N . J .
of the high equivalent weight of the polyosyethylene steamtv :ind the low concentrations which were of interest. Heald ( 1 ) found that the viscosity of starcah sols is nffc~ctrtl markedly by low concentrations of various soaps and tvotting agents. The effect of polyoxyethylene stearate on the vi?;cosity of starch sols was investigated, and results similar to thost, rcported by Heald ( 1j were obtained. However, it was found to I)(, impractical to exploit these effects as an analytical procedurr to determine polyoxyethylene stearate concmtration, because tht. measurements were not sufficientlj- reproducible a t thc ClJlI('el1trations which were of int,erest. Schoch and Rilliams ( 5 )had notcd that fatty acids repress th(. iodine binding power of cornstarch as shown by the starch-iodine titration. Mikus, Hixon, and Rundle ( 3 )extended thesp studies by an investigation of the x-ray patterns of the starch-fatty acid complexes and by a more detailed study of the starch-iodine titration. Kerr and Trubell(2j studied the iodine-starch complex spectrophotometrically. They showed that the unbranched portion (amylose) of the starch combines with iodine to form a complex which has a maximum absorption at about 610 mp. The amyloseiodine complex follows Beer's law fairly well. The presencr of the branched molecules of starch (amylopectin) ha$ little cffcct on tho formation of the amylose-iodine complex. The present method was developed by studying the effeet of various amounts of polyoxyethylene stearate on the formation of the potato starch amylose-iodine complex as determined spectrophotometrically. The method is sufficiently sensitive to permit the analysis of solutions as dilute as 0.05% with reproducit)ility of about *O.OlO%. The potato starch-iodine complex has a minimum absorption a t 590 mp. The discrepancy between this value and the one reported by Kerr and Trubell ( 2 ) for cornstarch may be due to differencesin degree of polymerization of the amylose portions of the starches. However, the curve Lvhich relate? per cent transmittance to wave length is so flat in the region of 590 to 610 nip that the exact choice of wave length for the analysis is not critical.
During an investigation of the effects of emulsifiers on food products, the need arose for a rapid and con\ enient method for determining the concentration of dilute solutions of polyoxyethylene stearate. A simple and convenient method was developed which depends upon the formation of a complex between the polyoxyethylene stearate and the aniylose fraction of potato starch. The amylose, which is not inbolved in the complex with polyoxyethylene stearate, is free to form an amylose-iodine complex which is determined colorimetrically. This method is sufficiently sensitive to permit the analysis of solutions of polyoxyethylene stearate as dilute as 0.05% with a reproducibility of about *0.010%. The use of its general principles for analysis of other compounds which form complexes with amylose is suggested.
D
U R I S G the investigation of the effect of polyoxyethylene
stearate on a number of processes arid products, it became iircessary to develop a convenient and rapid method for the determination of the concentration of dilute solutions of the strarat,e ester. (The polyoxyethylene stearate is manufactured t)y the -4tlas Powder Co., Wilmington, Del., under the code designation of G-2152. This product contains approximately 40 oxyethylene mole units per mole of stearic acid.) The method proposed by Morgan (Q), based on a modified Ltiwl procedure, is not adaptable as a rapid routine analytical mothod. Morgan's method has a disadvantage in that it does not distinguish between polyoxyethylene stearate and its hydrolysis products, if one wishes to make such a distinction. llethods based upon saponification were not attractive because 100 90
i p 2
80
POLYOXYETHYLENE STEARATE METHOD '0 60 50
!=
2
a E
40
30
ep PO 10
Starch Solution. The starch solution is prepared by suspending 0.400 gram (dry basis) of potato starch in 500 ml. of an acetate buffer of p H 5.0. (The buffer solution is prepared by neutralizing 800 ml. of 0.025 N acetic acid to pH 5.0 with sodium hydroxide, followed by dilution to 1000 ml.) The starch suspension is heated to a boil and held at the boiling point' for 5 minutes. The solution is then cooled, filtered through asbestos, and diluted t o 1000 ml. Iodine Solution. -4solution is prepared containing 0.500 gram of iodine and 1.00 gram of potassium iodide per liter. This solution is prepared as needed by dilution of a standardized stock solution. Spectrophotometer. The per cent transmittance at 590 nip is determined on a Coleman hlodel I1 universal spectropliotorneter. Matched square cuvettes (13 X 13 X 105 mm.) are used. PROCEDURE
0 .IO .PO -30 .40 -50 .60 POLYOXYETHYLENE STEARATE CONCENTRATION (GRAMS/I 00 ML.)
In a typical analysis, a 15.00-ml. sample of the polyolcyeth~I ( ~ P stearate solution (containing a maximum of 0.0050 gram of polyoxyethylene stearate per ml.) is filtered if necessary and added to 5.00 ml. of the starch solution. Then 2.00 ml. of the iodine arr added, and the per cent transmittance is determined 1 minute
Figure 1. Variation of Per Cent Transmittance at 590 mp with Polyoxyethylene Stearate Concentration
609
ANALYTICAL CHEMISTRY
610 Table I.
Determination of Polyoxyethylene Stearate
Concn. of ~ ~ 7j't& ~Transmittance ~ , a t 590 Gram,'100 111. 1 2 3 0,500 81 0 80.3 0.400 72 7 73.0 0.300 57.5 60.0 34.0 0.200 37.0 38 0 13.2 15.0 15 8 0.100 0.050 8.8 8.5 8 8 3.5 3 5 0.000 3.5
m p for 5 Detns.
4
80.7 71.7 59.3 37.2 16.0 8.5 4 .5
5 81.3 73.3 61.3 37.5 15.3 9 0 4.0
hverage,
70 80.60 72.64 59.52 36.74
15.06 8.72 3.80
after the addition of the iodine solution. The concentration of the polyoxyethylene stearate solution is then determined from a calibration curve which relates polyoxyethylene stearate concentration to per cent transmittance. DISCUSSION
Figure 1 is the calibration curve based upon these data which relates per cent transmittance to polyoxyethylene stearate concentration. I n view of the variations u-hich occur among starches, it is suggested that a calibration curve be established for each batch of starch used. Often the conditions under which the polyoxyethylene stearate solutions are used bring about a hydrolysis of the ester and a precipitation of the stearic acid formed. The polyoxyethylene glycol formed by the hydrolysis remains in solution, and does not interfere with the formation of the amyloseiodine complex. Since the precipitated stearic acid is removed by filtration prior to analysis, only the unhydrolyzed polyoxyethylene stearate remaining in solution is determined by this procedure. Although polyoxyethylene stearate was the only compound studied, this procedure could be adapted to the analysis of many of the compounds which form complexes with amylose. ACKNOWLEDGMENT
Table I illustrates the precision with which the polyoxyethylene steaiate concentration may be determined. These determinations were carried out by one analyst over a period of 2 days. An analysis of these data reveals that the 95% limit of confidence (L.C.93) in determining per cent transmittance under these conditions is +1.41. (The limit of confidence is a measure of the limits about the average of a large number of determinations, within which the means of !%Yoof all duplicate determinations will fall.) This corresponds to about *0.01 gram of polyoxyethylene stearate per 100 ml. over the larger part of the range studied.
The authors wish to thank Steve Harrison for the statistical analysis of the data in this report. LITERATURE CITED (1) Heald, 9. M.,Paper Trade J . , 113, 39 (1941). 12) Kerr. R. X.. and Trubell. 0. R.. Ibid..117. 25 (1943). i 3 j Blikus, F. F., Hixon, R. M., and Rundle, 'R. E., J.'Arn. Chem. SOC.,68, 1115-23 (1946). (4) Morgan, P. IT..IND.ESG.CHEY.,4h-AL.ED.,18, 500 (1946). ( 5 ) Schoch, T., and ITilliams, C., J . Am. Chem. SOC.,66, 1232 (1944).
RECEIVED August 30, 1950.
Determination of Saturation Temperatures of Inorganic Salt Solutions Refractive Index Measurements CHARLES R. PARKERSON Naval Research Laboratory, Washington, D . C. A s a preliminary step in the study of supersaturation of inorganic salt solutions i t was necessary to devise a method for rapidly and accurately determining saturation temperatures. Using a modified Bausch & Lomb dipping refractometer, a complete analysis was made of the variation of refractive index as a function of temperature and concentration of potassium chloride and potassium bromate solutions. Solutions studied had concentrations such t h a t they were saturated between 25" and 65" C. The temperature range investigated was from 25' to 75" C.
AS
A preliminarj step in the study of supersaturation of inorganic salt solutions i t was necessary to devise a method for accurately determining saturation temperatures. The author desired a quick as well as accurate method. The Bausch & Lomb dipping refractometer was selected as an instrument for the analysis of solutions because of its high accuracy and ease of manipulation. Refractive index measurements as a means of analyzing the concentration of solutions have been a great aid to the sugar industry in the analysis of sugar solutions. Urban and Meloche ( 3 ) used refractive index measurements to analyze solutions of telluric acid, selenious acid, and potassium ferro-
Graphs and tables were obtained from which t h e saturation temperature can be determined from t h e refractive index of the solution and the temperature a t which t h e measurement is taken. The saturation temperature determined is estimated to be within +0.2" C. of the true saturation temperature as indicated by the solubility data used. The main value of this paper lies not in t h e refractive index data presented for the two salts b u t rather in presentation of a technique for precise refractive index measurements of solutions a t elevated temperatures.
cyanide. Washburn and Olsen (4)used refractive index measurements to determine the concentration of sodium hydroxide and hydrochloric acid solutions. Potassium chloride and potassium bromate were selected for study because solutions of these salts were to be used in an investigation of supersaturation phenomena. The solutions investigated were saturated a t temperatures ranging from 25' to 65' C. APPARATUS
A diagram of the apparatus used in taking refractive index measurements is shown in Figure 1.
