Tables V and VI show summation analyses for various materials. Vanadium results were obtained using differential spectrophotometry and nitrogen values were obtained by means of the micro-Kjeldahl method. Table V shows results obtained for Vz03, VO, and VOz. For vzo3 and VO the mean oxygen value is within 0.7 and 1 . 8 z , respectively, of theoretical. For VOz the oxygen content found is higher than theoretical but is validated by the summation analysis. In addition, the precision for the VOz results is about 1%. Table VI summarizes the results obtained for the total analyses of samples of other oxide matrices. The summations indicate good accuracy for both vanadium and oxygen
methods. Both methods of analysis are at least as accurate as prior methods, and the time for analyses is markedly reduced. In the case of the oxygen method the dovetailing of samples reduces the analysis time per sample to about 10 minutes. ACKNOWLEDGMENT
The authors thank Frank V. Durkin and William D. Shelby for their assistance in performing the analyses. RECEIVED for review October 24, 1967. Accepted December 29,1967.
Composition and Structure of Acrylonitrile-Methacrylic Acid-Methyl Methacrylate System Ferencz Denes, Nicolae N. Asandei, and Cristofor I . Simionescu Institute of Macromoleculare Chemistry “ P . Poni,” Jassy, Rumania
DETERMINATION of the composition of copolymers presents many difficulties, stemming from the nature of the functional groups, the number of monomers taking part in the reactions, and the limited sensitivity of many physicochemical methods of analysis to a relatively small number of functional groups present in the macromolecule. To avoid these difficulties we suggest a titration system, using solvents and organic bases that react within the homogeneous phase both with the low molecular weight compounds, and polymers or copolymers with acid functions (1-8). In this study the composition of the acrylonitrile (AN)methacrylic acid (MAC)-methyl methacrylate (MMA) ternary copolymer was determined using potentiometric titration, with sodium hydroxide and tetraethyl ammonium hydroxide (TEAH) in different solvents. EXPERIMENTAL
Titration with NaOH. In preliminary determinations, dimethylformamide (DMF) and dimethylsulfoxide (DMSO) were used as polymer solvents; the titration was carried out potentiometrically in an aqueous solution of sodium hydroxide. The copolymer starts precipitating on addition of 0 . W aqueous NaOH solution and the greater the number of acid groups in the polymer chain, the faster the precipitation takes place. Use of organic solvents for dissolution of copolymers and for preparation sf base solutions permits titration of the carboxylic groups. Titration of these groups in the ternary copolymer dissolved in DMSO takes place more satisfactorily than the titration of the carboxylic groups of the acid homo~
~~~
(1) G. A. Harlow, C. M. Noble, and Garrard E. A. Wyld, ANAL. CHEM., 28,787 (1956). (2) Robert H. Cundiff and Peter C. Markunas, Zbid., p 792. (3) J. E. Burleigh, 0. F. McKinney, and hl. G. Barker, ANAL. CHEM.31, 1684 (1959). (4) A. H. Beckett and E. H. Timley, “Titration in Nonaqueous Solvents,” 3rd ed., BDH Laboratory Chemicals Division, Poole, Dorset, England. ( 5 ) T. Iasinski and S . Krystyna, Chem. Anal. (Warsaw), 10, 211 (1965). (6) Mitsuru Nagasawa, J. Phys. Chem., 11,4005 (1965). (7) S. J. Fritz and N. 31.Lisicki, ANAL.CHEM., 23, 589 (1951). (8) Ichiro Sakurada and Yukihiko Osurni, Kobunshi Kuguhu, 19, 620 (1962).
polymer where precipitation takes place near the equivalence point (Figure 1). However, the NaOH solutions in D M F and DMSO are unstable; NaOH solutions in D M F decompose faster. Titration with TEAH. The difficulties in using NaOH for the titration of carboxylic groups in the ternary copolymers of acrylonitrile can be overcome by using organic bases. Results in satisfactory agreement with the total concentration of carboxylic acid groups present, have been obtained for the titration of carboxylic groups of the AN-MAC-MMA copolymer with tetraethylammonium hydroxide in DMSO. Results of potentiometric titration of carboxylic groups by TEAH both in polyacid, and in the physical mixture of polymethacrylic acid and binary copolymer AN-MMA, indicate that in both cases half of the number of carboxylic groups existing in the polymers were titrated (Figure 2). The other functional groups of the copolymer (-CN and -0-CH3) are not affected in this reaction. Because of the large size of the quaternary ammonium ions as compared to the sodium ions steric hindrance occurs. Gregor and Frederick (9) titrating polymethacrylic acids with quaternary ammonium bases in aqueous solutions, noticed a decrease of acid strength with increasing size of the basic molecule. These authors concerned themselves primarily with the qualitative aspects of these phenomena. Based on these results, we found it possible to determine the structure of the polyacids and the acrylic copolymers with functional acid groups. Potentiometric titration with sodium hydroxide of the polymethacrylic acid in water gave a base consumption equivalent to carboxylic groups existing in the polymer. When the same polyacid was titrated under similar conditions with TEAH in water, only half the carboxylic groups were titrated. It thus appears that DMSO does not interfere, and that steric factors prevent TEAH from reacting with all polyacid carboxylic groups. The carboxylic groups are likely to be arranged in a certain configuration in the chain. By reacting the carboxylic groups of the ternary AN-MAC-MMA copolymer with a 0.05N solution of TEAH in DMSO, the same results were obtained as by titrating the copolymers in a 0.05N solution of sodium hydroxide in DMSO. Thus the same amount of organic base and NaOH is consumed by the carboxylic groups of the copolymer (Figure I). (9) Harry P. Gregor and hIichael Frederick, J . Polymer Sci., 23, 451 (1957). VOL 40, NO. 3, MARCH 1966
629
4 Figure 1. Potentiometric titration of functional acid groups in polymer-DMSO medium Acid groups in polymethacrylic acid with NaOH b. Acid groups in ternary copolymer AN-AMc-MMA with NaOH c. Acid groups in the ternary copolymer with TEAH a.
Table I. Analysis of Ternary Copolymers Initial composition, molar fractions AN: MAc:MMA.
MACdetn., Monomer Length of Initia- conc. in polymer- Conversion, With NaOH With TEAH in DMSO in DMSO tor,a solvent,* Temp. "C ization h 1 43.1 29.1 28.9 3 62.2 29.4 29.3 0.33:O. 33:O. 33 3 14 70 5 82.1 33.8 34.0 7 97.3 37.2 37.0 9 88.5 40.3 39.4 1 40.2 23.1 24.0 3 62.8 22.2 22.3 0.8:O.l:O.l 3 14 70 5 68.6 20.8 21.1 7 75.5 18.2 20.2 9 70.5 21.6 21 .o a Initiator :lauryl peroxide. Solvent.:ethyleneglycol carbonate.
z
b Figure 2. Potentiometric titration of carboxylic groups in polymer with TEAH in DMSO medium a.
Acid groups in polymethacrylicacid
b. Acid groups in mixture of poly-
methacrylic acid and binary copolymer AN-MMA
630
ANALYTICAL CHEMISTRY
z
z By elementary analysis 33.17 30.40 36.51 37.48 42.70 26.2 25.3
...
20.5 24.4
AN detn... .Z_ BY BY Dumas' Kjeldhal's method method 10.9 11.2 13.4 14.3 18.7 19.7 21.7 23.1 17.9 18.4 39.9 41.8 56.1 58.4 62.0 63.4 61.1 62.3 56.5 58.4
Table I shows the values of the carboxylic groups determined by potentiometric titration with TEAH, and by other methods. DISCUSSION The reactions of the carboxylic groups of copolymers with NaOH in water or organic solvents produce gradual precipitation of products as a result of the formation of sodium salts. Assuming the head-head structure of the polyacid by analogy with the alpha-halogenated acrylic derivatives, we have the three possible structures for polyrnethacrylic acid shown in Figure 3(10-14). The examination of these models leads to the conclusion that because TEAH reacts with half the number of carboxylic groups, only structure 3a and 3c are possible. In the case of structure 3a, however, TEAH would react with every other carboxylic group only by a deformation of the molecule of the quaternary ion. For the structure 3b, where the position of carboxylic groups is alternating, these groups should be fully titratable. Because this is pot possible experimentally, the 3b structure must be excluded. The single structure which does not require TEAH to be deformed for satisfying the experimental conditions is 3c. Because the carboxylic groups in the ternary copolymer could be reacted to equivalence with TEAH, it follows that the acid monomers in the AN-MAC-MMA copolymer are in a sequence form. Only under these conditions is the titration with TEAH complete and no acid homopolymer is present with the copolymer. By knowing the total amount of acid groups in the copolymer and titrating these groups with TEAH in DMSO, the percentage of the acid groups in the chain in either its “sequence” or block form can be established. It is not possible to establish the “mer” order in the ternary copolymer, but this can be accomplished for the binary copolymer. As a typical example the reaction of TEAH with such a copolymer containing 64.98 % polyacid and 35.42 % poly (methylmethacrylate) shows that for the total neutralization of the acid groups, 2.06 ml of 0.05N TEAH are required, but only 1.58 ml are consumed. The difference (0.48 ml), represents the amount of 0.05N TEAH corresponding to half of the polyacid in a block form and 0.96 ml corresponds to the total quantity of polyacid in block form. Thus 30.28% of the polyacid is in block form and 34.70% is in sequence form.
a.
OH
@O=
b0H
H
H
COOH
b.
(10) A. A. Strepiheev and V. A. Derevitkaia, Chim. compusilor macromoleculari, Ed. Tehn. Bucuresti, 1962, p 31 3. (11) C. S. Marvel and J. C. Cowan, J. Am. Chem. SOC..61, 3156 (1939). (12) Llasaru Ibonai and Shaji Iwatsuki, Kogyo Kagaku Zasshi,
67, 824 (1964). (13) I. P. Losev and G. S. Petrov, Chimia risinelor sintetice, Ed. Tehrt. Bucuresti, 1954, p 311. (14) C . Gentilhomme, thesis for the degree of Doctor of Engineering, University of Lyon, 1961.
H
H
CH3
COOH
H
C.
b Figure 3. Possible structure of polymethacrylic acids polymerized according to head-head type a. By deformation of quaternary ammonium ion 6. With alternating position of carboxylic acid groups c. Deformation of TEAH ion in reaction with carboxylic acid groups is unnecessary
u
OH
@o-
-
0-
@c
VOL. 40, NO. 3, MARCH 1968
631
The order of “mers” in the binary copolymer is shown in the figure above. Thus, correlation of analytical data with the copolymerization kinetics processes permits certain conclusions regarding
the structural arrangement of the acrylic copolymers containing carboxylic groups. RECEIVED for review January 23, 1967. Accepted August 25,1967.
Reproducibility of Mass Spectrographic Analyses of Steel and Aluminum Using Ion Beam Chopper P. G . T. Vossen Scientific Apparatus Division, Consultant Laboratory, GEC-AEI(Electronics) Ltd., Manchester, England
THE ANALYSIS of metals has been carried out for many years using the spark source mass spectrograph. Although this method has given excellent results for many materials under examination, the reproducibility of analyses of nonhomogeneous materials could have been better. Halliday, Swift, and Wolstenholme (1) investigated a procedure whereby the source conditions were maintained constant and relative sensitivity factors were established with a sample of the same matrix examined under the same source conditions. It was reported that for homogeneous metallic and nonmetallic materials the reproducibility on repeat analyses was normally of the order of 2O-30%, and in very isolated cases as good as 10%. Aulinger, Reerink, and Riepe ( 2 ) have shown that when applying a rotating electrode technique instead of the normal fixed electrodes, a relative standard deviation of 10 to 15% could be obtained. Despite attempts to improve the reproducibility of analysis of metals by maintaining instrumental and source parameters constant and by rotating the electrodes, no drastic improvement was made. Apart from the variable parameters mentioned above, the question arose whether a sufficient amount of the sample under examination was consumed during the analysis to obtain the best possible reproducibility for all impurities concerned. Jackson, Vossen, and Whitehead (3) have carried out reproducibility work on titanium dioxide using the ion beam chopper and quote a relative standard deviation on these analyses of which is an improvement of a factor of 2 to 3. Hence, it was decided to apply this technique to the analysis of steel and aluminum.
5z,
EXPERIMENTAL
Apparatus. All spectra were taken with a solid spark source mass spectrograph Type MS702 of Associated Electrical Industries, Ltd.
(1) J. S. Halliday, P. Swift, and W. A. Wolstenholme, paper presented at the Conference on Mass Spectrometry,Paris, September 1964. (2) F. Aulinger, W. Reerink, and W. Riepe, “Methodische Untersuchungen zur Analyse mit dem Massenspektrometer,” Forschungsberichte des Landes Nordrhein-Westfalen, Westduetscherverlag, Koln und Opladen, Nr. 1793 (1967). (3) P. F. S. Jackson, P. G . T. Vossen, and J. Whitehead, ANAL. CHEM., 39,1737 (1967).
632
ANALYTICAL CHEMISTRY
t
,1 A
I
IO0
I
792-
Ii
I
#
I
1
I
o
1
4--OTAL A” SOX 5FL I G T V i IF\ 1 h 5i ~ C G J L O I35 ElPlC
Figure 1. Variations of per cent standard deviation with chopping frequency A = Cr, 0 = Sn, X = Pb, 0 = Bi The photoplate density curves were recorded with a JoyceLoebl double-beam microdensitometer MK I11 B. The computer used for plotting the photoplate characteristic curves and calculating the exposure ratios of standard and impurities for a prefixed density, as well as the standard deviations, was the Associated Electrical Industries 1010 computer using Mercury CHLF 3 language. Only points on the linear part of the photoplate density curve were included as data to construct a straight line using the least squares method. The photographic plates utilized for this work were Ilford Type 4 2 , thin glass. Sample Preparation. The l / d n c h diameter by ’/*-inch length sample electrodes were degreased in laboratory grade ether and rinsed thoroughly in deionized water after machining to size. The steel sample electrodes were etched for 2 minutes in dilute acid (40% HCl, 6 0 x HzO), the aluminum sample electrodes for 1 minute in dilute acid (25% HCl, 7 5 x HzO). All the electrodes were thoroughly rinsed in deionized water. Procedure. The sample electrodes were mounted in the source sample clamps, and positioned as close to the first slit as possible for sparking and 24 repeat plates were recorded on each of the two samples under examination, plates one to eight in one day, and plates nine to 24 during 8 days on a two plates per day basis, For every two plates, a new pair of sample electrodes of the same bulk material was analyzed. The selected exposure ranges were for the steel sample from 0.0003 to 10 ncoulombs and for the aluminum sample from 0.0003 to 50 ncoulombs. Spark parameters were: voltage,
