Colorimetric Determination of Rosin and Rosin Esters iMELVIN H. SWANN Paint & Chemical Laboratory, Aberdeen Proving Ground, M d .
This work was undertaken to develop a satisfactory qualitative test for rosin in paints and varnishes, for quality control and acceptance, and to develop quantitative methods for either free rosin or rosin esters in paint products. Certain types of rosin modification are desirable in some paints used for Army Ordnance materiel, but in the absence of means of determining the type of rosin modification, i t has been necessary to prohibit use
R
of rosin in most paints. Procedures for diatinguishing between free rosin and rosin esters provide a means of determining either type of product quantitatively under certain conditions. Analysts will be able not only to determine the nature of the rosin products present, but in most cases to determine it quantitatively, thus providing a way to control the formulation of products containing desirable rosin modification and prohibit undesirable forms.
OSIN is the most widely used natural resin in both the var-
by nicans of an electrophotometer. In the procedure for rosin esters, the color is developed in the benzene medium and is sufficiently stable to permit comparison with standard potassium permanganate colors. I t is not possible to distinguish by thcse procedures between different types of rosin esters. Cumar and dammar, natural rt~41iswhich interfere in the usual qualitative Liebermann spot trst, do not interfere with either of the newly developed procedurw;. Some rosin products, such as hydrogenated rosin, previously reported as giving a color different than rosin by the usual qualitative spot test, have been found actually to produce the same color as any other form; only the intensity is different. S o actud color variations between different rosin products have k w n discovered in these hivest,igations. IIowever, certain oils arid (Jthw paint constituents char readily in the presence of SUIfuric :tc.iti as used in the original spot test, and form color!: which make thc t,est uncertain. Thr diluted conditions used i i i the new ~r(Jwdlll'~'.5 (~1imiri:rtt~ this t j p of error.
nish and plastic fields; rosin is cheap and easily available and by reason of its acid character it reacts to form salts and catern.
ThcJrr has been no completely suitable method of determining rosui and rosin derivatives in paint products, either qualitatively or quantitatively alt'hough many methods have been recom-
mended for detecting rosin in other resins, in varnishes, and in a wide variety of materials. None of these qualitative tests seems reliable, and many of the tests have been subjected to numerous modifications ( 3 ) . The Lieberniann test, ( 2 ) has been the most widrxly used, although many interfering substances have been rianied. One investigator ( 7 )has concluded that the Liebermam t e d ~is significant only if a negative result is obtained. In devcloping a systematic procedure for identifying synthetic resins and plastics, Shaw ( 4 ) employed the Licbermaxin test along with *ome tvienty or more other confirmatory tests to identify most rosins of commercial importance. &[any of the synthetic resins containing no rosin or rosin rsters gave color reactions similar to that o f rosin products. The reaction with butyl phenol formaldehyde resin is identical with the reaction of some rosin products. There is a definite need for ii satisfactory qualitative test for rosin and its products. In a recent study of the varied methods of detecting rosin, it was rioted that under certain conditions it was possible to distinguish between the colors formed by free rosin and rosin esters, employiiig the color reactions characteristic of the Liebermam test. As 2 result of further investigation, two quantitative colorimetric prowdures, one for rosin and the other for rosin esters, were developed. Free rosin c u i be determined quantitatively in any typv of vehicle, with the exception of lacquers containing cellulose derivatives. S o n e of tlie esterified or modified rosin products interferes with the determination of free rosin; the method can bc applied to incompletely esterified rosin products such as nionoand diglycerides, and the results agree reasonably well with the acid number of such esters. Esterified and modified rosin products can be determined quantitatively by a separate colorimetric procedure, but the method is limited to nonalkyds and samples in which the rosin has been completely esterified. Simple qualitative tests will indicate whether or not the procedure is applicable to the sample under analysis. 130th methods are based on the violet color formed by the react ion of abietic acid (the principal ingredient of rosin), sulfuric acid, and acetic anhydride. When rosin products in benzene are shaken with 507, sulfuric acid and acetic anhydride in certain proportions, the violet color formed by free rosin or rosin soaps goes into the sulfuric acid layer, while the color formed by rosin esters remains in the benzene layer. In t,he procedure for free iosiu arid rosin soaps, t,he color of the reaction is extracted n-ith 50qc sulfuric acid from benzene and compared with a standard
ANALYTICAL PROCEDURE
Qualitative Method for Detecting Rosin and Rosin Esters. The following instructions apply to resin solutions such as paint vehicles. If dry resins are to be tested qualitatively, 5 to 10 mg. are dissolved ill the specified volume of benzene.
R o s ~ xESTERS. Two drops of the resin solution to be tested are added to 50 ml. of benzene in a 10O-ml. glass-stoppered graduate, and 0.5 ml. of acetic anhydride is added and mixed with the sample. One large drop of concentrated sulfuric acid is added and the graduate is shakeri until color develops. If no color develops in 15 seconds of vigorous shaking, one more drop of acid is added and the agit,ation is continued I O to 15 seconds longer. If no red to violet color forms throughout the benzene. rosin esters are absent. FREEROSINOR RESINATE DKIERS. Two drops of the resin solution to be tested are added to 75 ml. of benzene in a 100-ml. glass-stoppered graduate. Five milliliters of 18 iV sulfurir acid are added and mixed with the sample by shaking. Acetic anhydride is added in 1-ml. portions, follon-ed by vigorous shaking and cooling in cold water for approximately 30 seconds between addi4 red to violet color in the lower layer tions of the anhydride. . of sulfuric acid, after the addition of 5 to 10 ml. of the anhydride, indicates the presence of free rosin or resinate driers. However, a faint coloration in this test, when applied to a sample which shows rosin esters from the previous test, does not indicate m b stantial amounts of free rosin, and the test should be confirmed by the quantitative method for free rosin, as outlined. Preparation of Sample for Quantitative Analysis. The following instructions apply to resin solutions such as paint vehiclcs. Dry resins may be weighed directly or dissolved in benzene so that aliquots may be taken. In either case, the recommended sample size is indicated. The degree of accuracy obtained by either method of analysis is t l e t e i ~ n i n ~I dJ two ~ fact.ors: the accuracy nith which the small
885
ANALYTICAL CHEMISTRY
886 sample is weighed, and the number of aliquots of varying size withdrawn for confirmatory runs. The sample used is necessarily small and should be weighed t o the nearest 0.0001 gram by difference from a special dropping vial. The recommended sample bottles have molded screw cap and pipet of clear glass, capacity 15 or 30 ml. (Fisher Scientific Co. catalog Xo. 3-337). Before pigmented materials are analyzed, the vehicle must be isolated by supercentrifuging until clear. A sample of the resin solution, weighing not more than 0.5 gram (0.1 to 0.2 gram of nonvolatile solid), is weighed by difference from the special sample bottle into a 100-ml. volumetric flask containing benzene, ACS reagent grade. It is then diluted to volume with benzene and aliquot,~ are withdrawn for analysis as indicated in the detailed procedure. COLORIMETRIC PROCEDURE FOR FREE ROSINAND ROSISSoaps. A sample aliquot, estimated to contain 4 mg. or less of rosin, is transferred to a dry 250-nil. separatory funnel (Squibb, pearshaped) containing benzene, -4CS reagent grade. The volume is t'hen made up to 100 ml. with benzene. After mixing, 5 ml. of 18 N sulfuric acid are added from a buret. This mixture is thoroughly shaken and 1 nil. of acetic anhydride (c.P., 98% minimum) is added from a 1O-ni1. buret. The separatory funnel is immediately and vigorously shaken for approximately 5 seconds and immediately immersed in a reservoir of water at, room temperature. The level of water in the bath should be adjusted so that it equals or slightly exceeds the level of benzene in the separatory funnel. When the funnel has been in the bath 45 seconds, it is withdrawn, 1 ml. of acetic anhydride is added quickly, and the funnel is immediately shaken for 5 seconds and returned to the water bath for 45 seconds. This process is continued until 10 ml. of acetic anhydride have been added. Upon removal from the bath after the tenth addition of acetic anhydride, excess water is quickly blotted from the outside of the funnel. Eighty milliliters of IS A' sulfuric acid are added from a graduate or dispensing buret and the funnel is tumbled (not shaken) five times to mix the contents. As soon as the liquid phases have separated, which takoi only a few seconds, the lower or sulfuric acid layer is withdrawn directly into a dry frittedglass filter crucible, medium porosity, of 30-ml. capdity, prepmed with an additional thin mat of medium-fiber-length filtering asbestos. The use of ib Fisher Piltrator is recommended and the sample is collected in a dry 250-nil. beaker. Porosity of the crucible should be such that, the 55 nil. of sulfuric acid will be filtered in 3 or 4 minutes. The sample is transferred to a 10O-ml. volumetric flask, diluted to volume with 18 rl: sulfuric acid, and compared at once on t,he Fisher electrophotometer with a green light filter (nave length 525 nip). The rosin content of the sample aliquot is determined by consulting a graph previously prepared in like manner, using 1-, 2-, 3-, and 4-mg. samples of purified rosin. If the color intensity of a sample aliquot equals or exceeds the color equivalent to 1 mg. of rosin, the test should be repeated using a smaller aliquot sample. If rosin esters are present in the sample, as indicated either by previous qualitative test or by coloring of the benzene layer in the above procedure, the test must be repeated, using a proportionately smaller aliquot until analysis of the aliquot shows approximately 1 += 0.5 mg. of rosin. Details of preparing the rosin standard are discussed in a later paragraph. Total time for conducting the extraction procedure, from drawing of aliquot to comparison of color on electrophotometer, should not exceed 20 minutes. COLORIMET~R~C PROCEDURE FOR ROSIK ESTERS. A sample aliquot, estimated to contain the equivalent of 2 to 15 mg. of abietic acid, is withdrawn from a predried buret into a dry 100-ml. glass-stoppered graduated cylinder and diluted to the 50-ml. mark with benzene, ACS reagent grade, and 0.500 ml. of acetic anhydride (c.P.,98% minimum) is added from a dry 10-ml. buret. The contents of the graduate are thoroughly mixed and placed in a reservoir of cold water maintained at a tem erature of 15" to 18" C. by means of ice. After 3 minutes the grafuate is withdrawn and quickly wiped dry, and one drop of concentrated sulfuric acid (95.5% minimum) is added, The graduate is immediately stoppered and shaken with maximum speed and vigor until color develops and for 3 seconds longer. I t is immediately placed in an upright position before a light source and matched at once with potassium permanganate utandards previously prepared as outlined below. Fading usually starts in 10 to 15 seconds and the test should be repeated with varied aliquot sizes unt'il one or more of the permanganate standards has been perfectly matched. The abietic acid equivalent content of the sample aliquot is determined from Table I, or, if desired and if the type of rosin ester is known, it may be calculated in terms of the ester present. PREPARATION O F POTASSIUM PERV.4NGANATE COLOR STANDC.P. potassium permanganate (0.100 gram) is dissolved in
.IRDS.
I liter of distilled water. If the solution is not free of suspended matter, it .is filtered through a fritted-glass filter funnel. Ten aliquots, ranging in volume from 1 to 10 ml., are withdrawn from a buret into separate 100-nil. glass-stoppered, graduated cylinders of equal size, shape, and height of graduations. These graduates match those used to develop color in t,he sample aliquots. The permanganate aliquots are diluted to 50 ml. with distilled water. Both the diluted and undiluted permanganate standard solutions are unstahle and must be prepared fresh the same day they are used. The usual methods of stabilizing permanganate 3tandard solutions are unsuitable for these dilutions.
Calculation of Rosin Esters. 3Iatching of a large number of coniinercial samples of high qualit,y and low acid number has established the relations shown ill Table I. The equivalent weights of abietic acid are calculaf cd from the theoretical coiitpnt of completely reacted ester gum antl nialeic. rosin ester. ACCURA
