Determination of Benzoic Acid in Alkyd Resins M. H. SWANN, M. L. ADAMS, and D. J. WElL Paint and Chemical Laboratory, Aberdeen Proving Ground,
Md.
absorption. .I Beckman spectrophotometer, LIodrl DC, n a? used with 1-cm. cells.
The use of benzoic acid in alkyd resin manufacture is increasing. It is particularly useful to control the viscosity of alkyd resins made with isophthalic acid. Its effect on the amount of polyhydric alcohols and dicarboxylic acids needed has increased its value analytically. Benzoic acid can be measured in alkyd resins by ultraviolet spectroscopy after removal of the dicarboxylic and fatty acids. This analytical technique provides a method for its control for qualification purposes.
PROCEDURES
Calibration. To determine the absorptivity of benzoic acid, 50 mg. of the acid of highest purity are carefully weighed and dissolved in absolute methanol and diluted to 100-ml. volume. To a IO-ml. aliquot of this methanol solution are added 1 ml. of concentrated hydrochloric acid and 50 ml. of distilled xat,er. The resulting solution is then diluted to 100 nil. with absolute methanol, giving a final concentration of 50 mg. of benzoic acid per liter. Using a slit width of 0.6 mm. and a blank of 50% methanol, which is 0.1.V in hydrochloric acid, the absorbance of the benzoic acid is read a t 2 i 3 mp. The cells are reversed in position, the absorbances averaged, and the absorptivity is ~ 1 culated from the equation
B
ENZOIC acid has been used in limited amounts for a num-
ber of years in the manufacture of alkyd resins for special purposes. Recently, it has been used in alkyd resins made with isophthalic acid to control the viscosity. dnalytically, its value has increased because of its effect on the amount of polyhydric alcohols and dicarboxylic acids needed. .4study has been made to determine the effect of benzoic acid on existing methods of analysis for alkyd resins in general, and to obtain a quantitative measure of the acid. Because of the solubility of its potassium salt in the anhydrous alcohol from which the salts of the other acids precipitate, benzoic acid offers no interference in the usual analysis for dicarboxylic acids (1). The potassium benzoate formed passes into the filtrate with the polyhydric alcohols and fatty acid soaps. When the solvents are evaporated from this filtrate and the aqueous solution acidified to release fatty acids, benzoic acid is also formed and is extracted along with the fatty acids by the usual ethyl ether extraction. The benzoic acid cannot be washed entirely from the ether layer with water; thus, it interferes with the usual determination of fatty acids. I n order to obtain satisfactory analysis of either the fatty acids or the benzoic acid, a quantitative separation is necessary. Efforts to effect this separation consisted of testing various water-insoluble solvents suital,le for extracting fatty acids in which benzoic acid has limited solubility. Such solvents as benzene. chloroform, naphtha, and carbon tetrachloride have been tested, the best results being obtained with the carbon tetrachloride. Although benzoic acid ha' shown some solubility in all solvent'. tested, its solubility in carlion tetrachloride is sufficiently low to permit its removal with water mashing. The combined water layer and ~ a s h i n g sfrom the fatty acid separation contain benzoic acid, polphvdric alcohols, and large quantities of potassium chloride. Of these, onlv benzoic acid should show appreciable absorption in the ultraviolet region when spectrophotometric measurement is applied. -4s shoivn in Figure 1, this acid shows some absorption throughout most of the ultraviolet region with a well defined peak of maximum absorption a t the 2 i 3 mp wave length. The absorptivity of benzoic acid that has been carried through the entire procedure, as described here, is slightly higher than its absorptivity determined directly. This difference can be corrected either by standardizing with benzoic acid treated through all steps of resin analysis, or by using pure benzoic acid as standard and applying a correction factor t o all analyses. The latter plan was found preferable, because the former requires the running of a blank Lvhich is time-consuming and slightly less accurate. In addition, all alkyd resins tested, which theoretically contain no benzoic acid, show a little absorption a t 273 mp which varies proportionally with the original iesin sample \\eight, but is independent of the type of polyhydric alcohol present. The empirical correction factor (0.0097), applied in all calculations, corrects also for this
in which a is the absorptivity of the acid a t 273 nip, A is thP average absorbance of the acid solution read at the same wave length, b is the cell length in centimeters, and c is the conceritration expressed in grams of acid per liter. Saponification. A resin sample estimated to contain 0.1 gram of benzoic acid is weighed into a 500-ml. Erlenmeyer flask. It is dissolved in 10 ml. of benzene, and 100 ml. of 0 . 5 ~ 1alcoholic ~ potassium hydroxide (made with absolute ethyl alcohol) are added. If it is known that isophthalic acid is absent, the sample may be given the usual 1-hour reflux; otherwise, the flask is warmed in a bath or oven a t 45' C. for a t least 4 hours. An air condenser is att,ached, and the sample refluxed 1 hour. Then 150 ml. of dry benzene are added, the flask is st'oppered and
EIO-
L
k
av)
m
a
6-
4-
2-
I
240
250
I
I
1
I
260 270 280 W A V E L E N G T H mp
290
3 10
Figure 1. Vltraviolet spectra of benzoic acid in 50% methanol made 0.l.Yin hydrochloric acid --Carried through entire procedure
- - - - Dis-dved directly i n solvent
72
73
V O L U M E 28, NO. 1, J A N U A R Y 1 9 5 6 'rable 1.
.inalysis of Some .ilhyd Resins for Benzoic Acid Type of 41kyd
L o n g oil (linseed)
__ Renaoic Present 12.5 12.5 11.2 12.4
Mediiini oil length (soybean) Short oil length (oxidizing) Special isophthalic acid alkyd (26.7rj, isoplithalic)
2,9 16 8 lL6 15.8 14 4 9.1
.acid. Go Found 12.2 12 2 11 J 12.2 3.1
'
lli.4
16.2 16.2 14.4
9.9
cooled, and it- cwntents arp filteirti through a Gooch ciucible using benzene for transferring and n ashing the precipitate. T h e filtrate is collected in a clean flask; the precipitated salts of the dicarboxylic acids are dried, weighed, and analyzed according to available procedures. Separation of Fatty Acids from Benzoic Acid. The filtrate mentioned is transferred to a large beaker and the solvents evaporated on a water bath, with water being added periodically to replace the solvents. The resulting alkaline solution is transferred to a 500-ml. separatory funnel and diluted to 300 ml. with water. It is neutralized with concentrated hydrochloric acid and a 5-ml. excess added. The fatty acids are extracted n ith three 50-ml. portions of carbon tetrachloride, but a fourth extraction is made if the third extract is colored. The solvent layers are combined and the water layer transferred t o a 2-liter volumetric flask. The carbon tetrachloride portion is washed In the following manner: 50-ml. portions are washed through three successive separatory funnels each containing 300 ml. of mater. The 900 ml. of wash water are combined with the first aqueous layer in the 2-liter flask and diluted t o the m a f i with water. The combined carbon tetrachloride portions may be filtered into a weighed beaker and evaporated for determination of the fatty acids. Determination of Benzoic Acid. A 500-nil. separatory funnel is filled with samole from the 2-liter flask and allowed t o stland. If any further separation occurs, the lower laver is drap-n off together with some of the aqueous solution. About 200 ml. of the remaining solution are filtered through a rapid, hardened filter paper, such as Whatman KO.54, and the first 50 ml. are discarded. An e w c t 100-ml. aliquot of the filtered solution is transferred to a 400-nil. beaker, neutralized LTith 2A- potassium hydroxide, and 20 drops in excess ale added. A stirring rod is in-
serted and the sample evaporated to dryness, using a n oil bath a t 110' C. for the final stages of evaporation, after which the beaker is dried in an oven a t 110' C. for 15 minutes. The residue is dissolved in 25 ml. of water, cooled, and neutralized with 6N hydrochloric acid with 1 ml. of concentrated hydrochloric acid added in excess. This acidified solution is transferred t o a 100ml. volumetric flask, the beaker rinsed with another 25 ml. of water, followed by methanol, and the solution diluted t o the mark with methanol. The flask is thoroughly agitated, and the absorbance of the solution read in the spectrophotometer a t 273 mp, slit width 0.6 mm., using a blank prepared by adding 1 ml. of concentrated hydrochloric acid to 50 ml. of water and diluting t o 100 ml. with methanol. The position of the cells is reversed and the readings are repeated. The absorbances are averaged and the benzoic acid is calculated.
% ' benzoic acid Cb =
A (average) a
=
Cb
X 200 (based on 100-nil. aliquot) sample weight X fraction solids
- (sample weight X fraction solids X 0.0007) DISCUSSIOh
In the determination of fatty acids, care must be taken t o avoid contaminating the sample with stopcock grease, as all types of these lubricants are renioved by carbon tetrachloride and will seriously affect the quantitative measurement. The isolated acids may be tested for contamination Kith lubricant bv their solubility in 95% ethyl alcohol. The lubricants are not soluble and can be filtered off. The use of separatory funnels with Teflon or self-lubricating valves is highly recommended for extractions with carbon tetrachloride Slow saponification of the resin sample by warming for several hours prior t o refluxing is necessary when isophthalic acid is used in the alkyd resin. If isophthalic acid is precipitated rapidlv, fatty acid soaps are entrained and difficulty with the analysis for both the isophthalic and fatty acids is encountered. Table I gives the results of analyzing alkyd resins. With the exception of the one alkyd containing isophthalic acid, benzoic acid was added in known amounts t o all the other samples. The average error in the analysis of all known samples tested is about 2.6%. LITERATURE CITED
(1) Kappelmeier. C. P. d.,Farben-Ztg. 40, 1141 (1936); 41, 161 (1936) ; 42, 561 (1937) : Paint Oil Chem. Rev. 6, 10 ( 1 9 3 i ) .
R E C E I ~ E for D review July 26, 19.55. Accepted October 4 , 1955.
Determination of Alpha, Beta-Unsaturated Compounds by Reaction with Sodium Sulfite F R A N K E. CRITCHFIELD and J A M E S B. J O H N S O N Chemical a n d Physical M e t h o d s Laboratory, Carbide a n d Carbon Chemicals Co., Division o f Union Carbide and Carbon Corp., South Charleston, W. V a .
4s a rebulL of a n inrcbtigation of chemical methods for the deterniination of alpha, heta-unsaturated compounds, an acidimetric procedure w-as developed based on the reaction of these compounds with sodium sulfite. Optimum hFdropen ion concentration for the reaction is attained by the addition of a known amount of sulfuric acid. A stoichiometric quantity of acid is consumed in the reaction and the excess is nieasured by titration with standard base using an ilizarin Yellow R-X> lene C j anol FF indicator. Data are presented on the determination of the purity of eight compounds for which this method has been found to give satisfactory results. The advantages and limitations of the sulfite method are discussed and compared with the morpholine method. Also, a guide is given to the selection of the best method for the determination of a compound of this class on the basis of its structure.
A
N ACIDIMETRIC method for the determination of alpha, beta-unsaturated compounds, which is based upon the reaction of these compounds with sodium sulfite, has been developed. For convenience, the reaction is illustrated as taking place with the bisulfite ion. I n this reaction a substituted sodium sulfonate is formed ( 7 ) n i t h a corresponding decrease in acidity according to the equation NaHS03
+ CH2=CII-X
+Na03S-CH2CH2--S
n here X is any strong electron-attracting group.
(1)
This reaction has been utilized commercially in the manufacture of anionic surface-active agents ( 2 ) . It has also been used as an anall-tical procedure by Rosenthaler ( 6 )for the determination of maleic acid, but under the conditions which he established he was unable t o determine fumaric acid by this method. In the method of analysis presented here a known excess of sulfuric acid is added to the sodium sulfite reaction mixture. T h e
