1604
ANALYTICAL CHEMISTRY
must be used when a sample of an alloy containing silver chloride is first dissolved for analysis. I n experiments in which pure silver chloride and synthetic mixtures containing silver chloride were treated with nitric acid under conditions simulating those that would be used for dissolving such an alloy, the loss of silver chloride was found to range from a minimum of 0.3 mg. t o a maximum of 0 8 mg., with an average of 0 5 mg., the recommended correction. TEST 4YALYSES
I n order to test Procedure I on an alloy of known composition, one &as prepared from 24-karat gold, fine silver of 99.99% purity, electrolytic copper, and reagent grade tin. These ingredients were weighed out on an analytical balance and fused in a graphite crucible under conditions that caused no loss of metal. Metallic lithium was used to deoxidize the melt which w:i* thoroughly stirred before being poured into a mold. Aftei
removal of the rough surface metal, the ingot was reduced to chips in a milling machine, and these were thoroughly mixed to obtain the analytical samples. The calculated composition of the alloy is shown in Table I along with the results of duplicate analysis in which the gold and tin were separated and determined by Procedure I and the other components by conventional procedures. It will be seen t.hat the results for gold and tin are very satisfactory and those for t'he ot'her two metals acceptable. Procedure I1 was test,ed on intimate mixtures containing silver chloride t'hat were prepared from accurately weighed quantities of the several components. I t will be seen from Table I1 t,hat satisfactory results were obtained from the silver chloride as well as for the other components. RECEIVEDfor review J a n u a r y 19, 1955. Accepted June 10, 1955. h b stracted from a dissertation submitted b y Charles D. Oviatt t o T h e Graduate School of T h e Ohio State University in partial fulfillment of thP reriuirementa for t h e Ph.D. degree, 1954.
Determination of Mixed Phthalic Acid Isomers in Alkyd Resins M. H. SWANN, M. L. ADAMS,
and
D. J. WElL
Paint and Chemical Laboratory, Aberdeen Proving Ground,
The meta and para isomers of phthalic acid ha\e recently attained commercial significance in alkyd resin manufacture. An anal?tical method for measuring each of the three phthalic acid isomers in mixture consists of a special saponification technique to recover the acids from resin solution, followed by hydrolysis in methanol solution and measurement of the absorptitity at three ultraviolet wave lengths. Anal) tical control can be exercised on the new compositions.
F
OR a number of years o-phthalic anhydride has been used
in the manufacture of alk?-d resins and these resins in paint. vehicles have been measured by determination of phthalic anhydride content. Recently, the nieta and para isomers, also known as isophthalic and terephthalic acid, have attained commercial significance and may be utilized in alkyd resin manufacture. Investigations were undertaken to determine the effect of these tbr-0 isomers on existing methods of analysis for o-phthalic acid and t,o devise methods of analysis for each isomer. Tivo quantitative methods for determining o-phthalic acid specifically in the presence of other dicarboxylic acids are found in the literature. I n these methods, the phthalic acid is measured by its absorption in the ultraviolet region a t 2 i 6 mp (2) or by the weight of the lead salt formed in glacial acetic acid ( 3 ) . These two methods with slight modification also appear as ASTII methods (1). The lat,ter is not affected by the presence of isoaud terephthalic acids and can he used to measure o-phthalic acid or anhydride in alkyd resins wit,hout int,erference. A l l phthalic acid isomers absorb strongly in the ultraviolet region a t 2 i 6 mp, so that the spectrophotonietric method in its present form is unsuit,able. Horn-ever, it was found that each isomer shorvs a distinctive absorbance curve throughout the ultraviolet range; the iso- and terephthalic acids show strong absorption at the shorter wave lengths and secondary absorption a t the longer wive lengths. Figure1 s h o w the absorptivities of 1to 1methanolwater solutions, made 0.1-Y in hydrochloric acid, of t,he three phthalic acids plotted against the wave length in millimicrons. Sirwe the isomers show different point,s of maximum absorption a t the longer wave lengths, it is possible to treat mixtures of the acid< as three-component R ems. The wave length? chosen for the analysis of the isoiiie phthalic acids, 275, 281, and 287
Md.
mp, are points at which the spread between the curves is large but not so large as to give absorbance readings which are either very high or low. The use of long wave lengths is considered advisable in order to minimize the interference that would he caused by the presence of any water-soluble organic contaminants that might be present in the saponification product of alkyd resins. PROCEDURES
Calibration. The absorbances of the three phthalic acids muit be determined at 275, 281, and 2 8 i mp. The isophthalic and terephthalic acids used were obtained from Eastman Kodnk Co., catalog numbers 3233 and 610, respectively. The Becknian spectrophotometer, Model DU, was used with 1-cm. rell*. Bepause of the low solubility in water of isophthalic and terephthalic acids, a 1 to 1 methanol-water mixture wap used as the solvent throughout. T o calibrate, 25 mg. of each acid are didsolved in 250 ml. of absolute methanol by reflusing. To the methanol solution are added 5 ml. of concentrated hydrochloric acid, and the solution is diluted to 500 ml. with distilled mater. thus giving a final concentration of 50 mg. of acid per liter of solution. The absorbance of each solution is determined a t the three wave lengths using a slit width of 0.6 mm. and a 1 to 1 methanolwater mixture, made 0 . l N in hydrochloric acid, as a blank, following the method of reversing the cells as proposed by Shreve and Heether (2) in the original spectrophotometric method for determining o-phthalic acid. The absorptivity of each acid a t each wave length is calculated, using the equation -4 bc
a = -
where u is the absorptivity a t the particular wave length measured; A is the average absorbance of the acid solution being measured a t the same wave length; b is the cell length in pentimeters: and c is the concentration expressed in grams of acid per liter. Analysis of Acid Mixtures. The analytical data in Table I were obtained by applying the folloa ing procedure to knoxn mixtures of the isomeric phthalic acids. Slixtures totaling a maximum of 50 mg. of acid are refluxed with 50 ml. of absolute ethyl alcohol. When dissolved, 10 ml. of dry benzene are added, followed by 3 ml. of 21-\ alcoholic potassium hydroxide. After refluxing for 1 hour, 150 ml. of dry benzene are added, the flask is stoppered, and the contents are cooled with water and then filtered fhrough a Gooch crucible, benzene being used for tramferring and washing the precipitate. The residue is given a find ether wash and dried a t 105' C.
V O L U M E 2 7 , NO. 10, O C T O B E R 1 9 5 5
1605
The dried salts are dissolved in water and diluted to 100 ml. A 10-ml. aliquot of this aqueous solution is then diluted to 100 ml. with 1 to 1 methanol-water, made 0.LV in hydrochloric acid, and the absorbance of this solution is determined a t 275, 281, and 287 mp as in the calibration procedure. For this three-component system, the following equations apply:
A m = b(anij.ici
+
A?81
= b(a281,
