hydrogen, which exhibits the expected anomalous thermal conductivity response. For the analyses of samples taken at 100 mm., the 10-cc. volume was used to obtain increased sensitivity. With more advanced detectors, comparable or improved sensitivity could be obtained with a sample volume consistent with an extended linear range. This sampling method is relatively economical of sample consumption. I n the single-point sampling mode, only about 15 cc. of gas a t the chamber pressure are required for each 10-cc. injection. The analytical results must lag t h e composition of the gas in the system sampled by the time required to flush the manifold, the dryer, and the lines to the test chamber. Because the voids and dead volume of this system were minimal and because small-volume connecting lines were used, this time lag was limited to about one cycle (15 minutes). I n the sequential sampling mode, about 25 cc. of gas are consumed per sampling, the additional volume being
expended in filling the drying tube and the manifold, which was fabricated from l/S-inch pipe fittings. Because sampling from each system is less frequent in this mode, this additional sample consumption was acceptable in this application, and the time lag was reduced to less than one cycle-Le., a change in composition in the test chamber was partially recorded in the next analysis. Gas losses could be reduced to less than 10 cc. by using a drilled-block manifold and moving the driers upstream of the stream valves. This sampling technique is not limited to systems at subatmospheric pressures; it can be applied in its present form to pressurized systems in which sample consumption is a prime consideration. To operate within the linear region at increased pressures, it would be necessary to further reduce the volume of the sample loop or to increase column parameters. This sampling method has been used intermittently for more than 2 years in
the analysis of reduced-pressure systems. With normal maintenance, no atmospheric contamination, cross contamination between samples, or dilution of samples by the carrier was detected. LITERATURE CITED
(1) Casey, K., Edgecombe, F. H. C., Jardine, D. A., Analyst 87,835 (1962). (2) Dal Nonare, S., Juvet, R. S., Jr.. “Gas-LiquTd Chromatography. Theory and Practice,” Interscience, New YorkLondon. 1962. (3) Glew,‘ D. N., Young, D. M., ANAL. CHEM.30,1890 (1958). (4) Nawar, W. W., Sawyer, F. M., Beltran, E. G., Fagerson, I. S., Zbid., 32,1534 (1960). (5) Wilkinson. J.. Hall., D., J . Chromatog. 10,239 (1963).’ RECEIVED for review February 8, 1965. Accepted August 9, 1965. Division of Analytical Chemistry, 144th Meeting, ACS, Los Angeles, Calif., March 31 to April 5, 1962. Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corp.
Polarographic and Spectrophotometric Determination of Acrylamide in Acrylamide Polymers and Copolymers D. C. MACWILLIAMS‘ Physical Research laboratory, The Dow Chemical Co., Midland, Mich.
D. C. KAUFMAN
and 8.
F. WALING
East Analytical laboratory, The Dow Chemical Co., Midland, Mich. The determination of residual acrylamide in polymers and copolymers is described, Acrylamide is extracted with a mixed solvent designed to swell the polymer without dissolving it. The extract is treated with mixed ion exchange resin to remove interferences. The acrylamide is determined by polarography which is highly specific or by ultraviolet spectrophotometry which offers greater simplicity and slightly more sensitivity. The nominal working ranges are 0.01 to 0.5 and 0.005 to 0.10% acrylamide, respectively. The half-wave potential for acrylamide is 1.97 volts VS. S.C.E. in 0.091M tetra-nbutylammonium hydroxide in a 70/30 (v./v.) methanol-water solvent. The acrylamide i s determined from the wave height directly or by difference after reaction with dodecyl mercaptan. A discrepancy in the electrode process chemistry has been detected but not resolved. The ultraviolet absorption
-
1546
0
48640
ANALYTICAL CHEMISTRY
48640
is measured at 240 mp and conforms to Beer’s law in the concentration range of 5 to 100 mg. per liter.
T
of acrylamide monomer (AA) in polyacrylamides to 0.005 to 0.01% is required to ensure that very low levels of this toxic monomer are maintained (8). Of the existing methods for acrylamide in solution when little or no polyacrylamide is present, bromination (1) is perhaps most useful, though not specific. Other methods are based on the addition of morpholine (3) or dodecyl mercaptan (8) to the acrylamide double bond. The polarographic reduction of AA has been described (IO),and utilized (9) to follow its decrease during polymerization. Ultraviolet spectrophotometry has also been applied to the determination of acrylamide (6, 11). These methods have limited sensitivity in the presence of high molecular weight polyacrylamide HE DETERMINATION
because even 1% solutions are viscous gels (4). For lower monomer levelsLe., 0.1% or less-they are insensitive or subject to severe interference. The basis of the present procedure is the extraction of the monomer from the polymer using a mixed methanol-water solvent followed by treatment of the extract with ion exchange resin to remove interferences. The extract is then analyzed by polarography or ultraviolet spectrophotometry. The polarographic wave height was measured at 0.95 i d (-2.1 volts us. S.C.E.) directly or by difference after removal of the AA by reaction with dodecyl mercaptan in situ. The ultraviolet absorbance was measured a t 240 mp on the side of the AA absorption band. The range of composition of the systems to be considered is shown in Table I. The anticipated concentration 1 Present address, The Dow Chemical Co., Pittsburg, Calif. 94565.
L B N
L e N
ELECTROCHEMOGRAPH
pH METER
astic Workinq
( a ) =Fine Frits
Figure 1 . Polarographic cell (approximately to scale)
EXPERIMENTAL
The majority of the polarographic study was conducted with a Leeds & Northrup Electrochemograph Type E operated on the No. 2 damp position. The Sargent Model XV has been successfully applied with no added damping. The polarographic cell must have suitable bridges to separate the calomel electrode from the cell. Potassium ion interferes with the AA wave and hydroxide ion is injurious to calomel electrodes. Mercury ion cannot be present in the Difference Method. The cell design used in this study is shown in Figure 1. Accurate control of the d.m.e. potential made the use of a nonworking reference electrode a virtual necessity for routine work with the Difference Method. The circuit employed is shown in Figure 2. All potentials were referred to the slidewire of the polarograph. The negative potential of the mercury cell ( E B )used to bias the pH meter was measured against the polarographic slidewire. The reading on the pH meter (E.M)was measured on the 1400-mv. scale against the slidewire set a t -1.35 volts. The dissymmetry of the reference electrodes (Eo) was measured by connecting the d.m.e. lead to the nonworking reference electrode and determining the slidewire reading, sign included a t zero current. The diffusion of potassium ion from conventional calomel electrodes into the solution section of the cell was totally eliminated by using 0.1M tetran-butyl ammonium chloride (TBAC1) in place of saturated potassium chloride. Three such electrodes constructed had potentials 0.08, 0.08, and 0.10 volt more negative than the S.C.E. To determine the potential to be maintained on the pH meter by adjustment of the polarographic slidewire such that the d.m.e. would be -2.18 volts us. the 0.lM TBAC1-calomel nonworking reference electrode, Equation 1 was used. Apparatus.
Reference.
Figure 2. Circuit for absolute measurement of potential of dropping mercury electrode
25'C.
of each species and the polarographic and ultraviolet behavior are shown. Where not otherwise specified, the polarographic behavior reported was determined under the conditions of this study.
Non Working
Note that E,w and EB are negative values as determined. All absorbance measurements were made with a Beckman Model DU spectrophotometer equipped with matched 1-cm. quartz cells and a hydrogen lamp. Extraction times and recovery are based on the use of a Burrell Wrist-Action Shaker or an Eberbach Reciprocating Bottle Shaker operated a t a vigorous rate. Materials. All materials were reagent grade unless otherwise noted and were used without further purification. Acrylamide was purified by double subliming the American Cyanamid Co. product a t 85' C., 1 mm. H g pressure. T h e product was stored a t -20' C. Commercial polyacrylamides were obtained from various suppliers as dry solids. Research samples in the dry and gel forms were obtained from The Dow Chemical Co. The quaternary ammonium supporting electrolytes were polarographic grade materials from Southwestern Analytical Chemicals except tetramethylammonium iodide which came from Matheson, Coleman & Bell. Dodecyl mercaptan and mercaptoethanol for the Difference Method were Eastman Kodak Blue Label products redistilled t o give heart cuts of 10-90%. 3,3',3"-Nitrilotrispropionamide was prepared by reacting
Table I.
AA with an aqueous solution of ammonia. The ion exchange resin was a mixture of Dowex 1-X8, OH- form, 20-50 mesh and l / 3 Dowex 50W-X8, H + form, 20-50 mesh, by weight. These resins were technical grade material and were rinsed with methanol and air dried until damp prior to use in the spectrophotometric procedure. Mixed resin is available in a form especially prepared for analytical use from Bio-Rad Laboratories, designated as XG501-X8, Catalog S o . 47000. The 4CS grade methanol was regularly checked for impurities electroreducible below -2.3 volts us. S.C.E. and/or for ultraviolet absorbers contributing in excess of 0.05 absorbance a t 240 mp, The extraction solvent was prepared by mixing 80 volumes of methanol with 20 volumes of water. A standard stock solution of AA was prepared by dissolving 50 mg. of sublimed AA in 100 ml. of extraction solvent. All of the standard solutions used were dilutions of this stock solution. Sample Preparation. Weigh 10 grams of 100-120 mesh dry polyacrylamide into a 4-oz, wide mouth bottle or a 250-ml. Erlenmeyer flask and add 100.0 ml. of extraction solvent. Stopper securely and shake vigorously for 3 hours on a mechanical shaker. Remove the flask from the shaker and allow the polymer to settle. Carefully decant 25 ml. of
Composition, Polarographic, and Ultraviolet Behavior of Various Components of Typical Polyacrylamides
Component Acrylamide Polyacrylamide Sodium ion Transition metals Cu-I1 Fe-I1 Ammonium ion Ammonia Acrylate ion Sulfate ion 3-Nitrilopropionamides Acetone Formaldehyde
Level, % ' 0-2.5 70-100 0-15
0-0.1 0-0.1 0-10 &lo 0-5 0-5 0-2 0- 1 0-0.1
Behavior at d.m.e.' Red., 1.97 volts Distorts waves Red., 2 . 1 volts Red., 0.6 voltc Red., 0.0 voltc Red., 2.1 voltsd Red., 2.3 voltsd Not red. Not red.' Interference Red., 2.4 voltse Red., 1.7 voltse
Ultraviolet behavior at 240 mp Absorbs strongly Absorbs weakly No absorption May absorb" May absorbc No absorption No absorption Absorbs stronglyb No absorption Absorbs weakly Absorbs strongly No absorption
Negative half-wave potentials us. S.C.E. See text. Complexed. Reference (IS). e Reference (6). b
VOL. 37, NO. 12, NOVEMBER 1965
0
1547
the clear extract into a glass stoppered flask. Measure the apparent pH of the extract and add either 1:4 sulfuric acid or 10% sodium hydroxide to obtain a n apparent pH of 5.5 to 6.5. Add 1 gram of mixed resin per 10 ml. of extract for the polarographic procedures or 2 grams per 10 ml. for the spectrophotometric method and stir for 10 & 0.5 minutes on a magnetic stirrer. Insulate the top of the stirrer with l / 4 to '/2 inch of foam plastic. Filter the treated solution through a glass wool plug well packed in the vertex of a 7.5-cm. funnel. Cover the funnel to retard evaporation. This solution is stable for a t least one day, usually for a week or more. Prepare a reagent blank by resin treating a similar volume of extraction solvent. If the above extraction procedure is applied to highly hydrolyzed polymer, difficulties may arise which are attributed mainly to soluble polymer fractions. The problem is avoided by making the system slightly acidic. For example, carefully add 1 ml. of concentrated sulfuric acid to 100 ml. of solvent plus sample before extraction. Afterward, adjust the final apparent pH as above. With the use of the concentrated acid, no volume correction is made, as the ionic material is subsequently removed by the resin. Use twice as much mixed resin as specified above for ordinary polyacrylamide and prepare new standards because more AA is removed by the extra resin. Gel samples containing 1-10 grams of polymer and ranging from 5 to 35% solids are macerated in 80 ml. of methanol with a Virtis Homogenizer. Transfer the macerated gel to an Erlenmeyer flask along with sufficient water to bring the total water present to 20 ml. Shake the sample for 3 hours and continue the analysis as with a dry sample. Include a 4% correction for volume contraction in the calculation. Direct Polarographic Method of Measuring Acrylamide Concentration. Prepare a standard curve using solutions of AA in 80/20 solvent containing 0, 10, 30, and 50 mg of AA per 100 ml. Resin treat the standards in the same manner as the sample. These solutions are stable for one week. Set up a decinormal tetra - n - butylammonium chloride (TBXC1)-calomel reference electrode and fill the intermediate bridges with 0.1X TBACl (Figure 1). Place 8 ml. of extract and 0.8 ml. of 1M tetra-nbutylammonium hydroxide (TBAOH) in the cell. Bubble 5-10 minutes with nitrogen saturated with S0/20 solvent. Determine the C.V. curves from -1.6 to -2.5 volts at appropriate sensitivity. Measure t h e apparent half-wave potentials and calculate the cell resistance, the true half-wave potential being independent of concentration ('7). Prepare a standard curve based on the currents a t the resistance corrected potential corresponding to 0.95 i d . The C.V. curves for sample extracts are measured in the same fashion and the current read off the waves corrected 1548
ANALYTICAL CHEMISTRY
\UNTR
E ATE D
W A V E L E N G T H , mp
Figure 3. Ultraviolet spectra of polymer extract in
80/20 methanol-water vs. resin treated and untreated reference solution showing base line extrapolation correction
for the cell resistance a t the same potential used for the standards. Verify the identity of the polarographic wave by comparing the half-wave potential against t h a t of the standards. Difference Polarographic Method of Measuring Acrylamide Concentration. Assemble the nonworking reference electrode circuit (Figure 2). Prepare extracts and standards and determine a C.V. curve as above. Calculate the value of E to be set on the p H meter from Equation 1, and set the polarograph slidewire to give this reading a t maximum drop life. Add 0.025 ml. of dodecyl mercaptan (DDM) by microsyringe. Protect the alkaline solution in the cell from air during this addition by purge and continue the purge for a n additional 6 minutes. Follow the decay of the wave correcting the slidewire at frequent intervals to maintain the potential on the pH meter constant =!=0.002 volt. Continue the reaction until a steady current is observed (20-50 minutes). Measure the decrease in wave height and calculate the AA content by comparison with standards. Verify the identity of the reaction by measuring the first order rate constant of the decay process and comparing it to that of a standard. Alternatively, plot the C.V. curve represented by the difference between the initial and final C.V. curves and compare the resulting wave with an AA wave. Wash the cell parts in 100% methanol followed by water. If the d.m.e. is erratic or has been exposed to solutions with residual currents greater than 5 pa., immerse the flowing electrode in 10M nitric acid for 5 minutes and rinse again with water. Spectrophotometric Method of Measuring Acrylamide Concentrations. The sample preparation for ultraviolet measurement is similar in principle to t h a t for the polarographic method but differs in detail because the interferences are not identical. The extraction time is limited to 3 hours. Follow the general method of sample preparation above, treating the sample extract and a reference
solution as directed, including filtration, and transfer a portion of each to matched 1-cm. cells. Record the absorbance of the sample vs. the treated blank every 10 mp from 230 to 300 mp. Plot the absorbance vs. the wavelength and extrapolate the base line from 300 to 240 mp (Figure 3). Determine the absorptivity (a) for acrylamide from a standard stock solution diluted to a concentration of 5 mg. per 100 ml. Treat the solution with mixed resin, and plot the absorbance as above. The absorptivity should have a value of about 10.3 liter/gramcm. a t 240 mp when treated with 2 gram resin/lO ml. of extract. The absorption conforms to Beer's law a t 240 mp in concentrations of 0.5 to 10 mg./100 ml. The AA content is calculated using Equation 2.
yoAcrylamide = (A240 - A240 extrapolated)100 (a)
x sample concn. (grams/liter)
(2)
RESULTS A N D DISCUSSION
Sample Extraction. A mixture of 80 volumes of methanol and 20 volumes of water gave the optimum balance between insolubility of the polymer and inorganic salts and comfrom the polymer. plete removal of -4-4 It was necessary to grind nonporous samples to about 100 to 120 mesh avoiding fines which cloud the extract if assay was by spectrophotometry. Porous samples of size up to 6 mesh were successfully extracted. The 80/20 solvent gave quantitative yields of monomer in 3 hours from samples containing up to 0.3% monomer, and samples containing up to 2% monomer were extracted in 16 hours. The extraction time for the spectrophotometric procedures should be as short as possible consistent with complete removal of AA as longer extraction increases the amount of interferences. A solvent ratio of 70/30 made the polymer too
Table 11. Acrylic Acid in 80-20 (v./v.) Methanol-Water Solutions
(Absorbance before and after Treatment with Mixed Ion Exchange Resin) Absorbance at Acrylic Resin acid concn. treatment 250 mp 1.14 113.8 mg./100 ml. None 0.000 113.8 mg./100 ml. 15 min. 0.116 11.38 mg./100 ml. None 0.000 11.38 mg./100 ml. 15 min.
sticky and a ratio of 85/15 failed to extract all of the monomer in 3 hours. Resin Treatment. The mixed resin treatment was predicated upon the removal of recognized interferences. Acrylic acid which has a spectrum almost identical with that of acrylamide (11, 1%) was totally removed by mixed resin treatment (Table 11). Sodium is present principally as sodium sulfate and polyacrglate. The sodium ion in 100 ml. of a saturated solution of sodium sulfate in the 80/20 solvent is equivalent to 0.4 meq. of mixed resin. Sodium polyacrylate and other sodium containing impurities equivalent to as much as 1.7 meq. of sodium ion have been detected in unacidified extracts. Ammonium salts are soluble to about the same extent as sodium salts. The ammonia addition products of Ah‘have the formula H~-nK((CH.&H&ONH2), where n = 1, 2, or 3 and are known occur in samples where acrylamide monomer is exposed to ammonia1 The extraction of :3,3’,3”-nitrilotrispropionamide with the mixed resin was incomplete. Varying the crosslinkage of the Dowex 50 resin portion from X8 to X4 to X2 and decreasing the bead size from 20-50 mesh to 50-100 and 100-200 mesh did not improve the degree of removal. Assuming the extract contains 0.3% of 3-nitrilopropionamide in the worst case, 3.41 meq. of resin capacity would be required and less if the bis or tris adducts were present. Although 3,3’,3”-nitrilotrispropionamide mould be troublesome if present in excess of about I%, experience indicates little or no difficulty of this kind. The total demands on the resin are therefore about 5.5 meq. equivalent to 8 grams of resin per 100 ml. of extract. Ten grams of resin per 100 ml. of extract was employed to provide a n excess and to account for the ammonium ion neglected in this summation. No unacidified extract to date has exceeded these allowances. The completeness of removal of ionic material from an extract not adjusted for p H was confirmed by following the specific resistance of the extract during the resin treatment. The specific resistance rose from 400 to 400,000 ohm cm. in less than 10 minutes and steadied.
Figure 4
Figure 4.
Polarograms of acrylamide.
1. 7 0 / 3 0 (v./v.) methanol/water, TBABr, 0 . 1 7 4 mg./ml. AA, corr. for i, 2. 70/30 (v./v.) methanol/water, TBAOH, 0.1 74 mg./ml. AA, corr. for i, 3. 70/30 (v./v.) methanol/water, TBABr 4. 70/30 (v./v.) methanol/water, TBABr, 0 . 0 0 4 mg./ml. AA 5. 7 0 / 3 0 (v./v.) methanol/water, TBAOH 6. 70/30 (v./v.) methanol/water, TBAOH, 0 . 0 0 4 mg./ml. AA
0.091 M 0.068M 0.091 M
0.091 M 0.091 M 0.091 M
Loss of A4 during the resin treatment was compensated by treating standards with resin under identical conditions. The loss after 90 minutes measured polarographically was 30yo which by linear interpolation to 10 minutes represents a 3% loss. The resin for the polarographic procedure contained about 50yo water but’ this is variable. The resin for the ultraviolet procedure contained a similar amount of methanol. d s these solvents represent dilution factors, new standards were run with each batch of resin. d carboxy-carboxyamide containing fraction soluble in the mixed solvent was identified from infrared spectra of the residues from the evaporated, resin treated extracts of some samples. Subsequent to the majority of the polarographic study it was found that the solubility of this fraction, which produced a premat’ure hydrogen discharge, could be suppressed by cautiously adding l ml. of concentrated sulfuric acid to the 100 nil. of the estraction solvent. extraction the apparent pH was adjusted to 5.5 to 6.5 with 10% sodium hydroxide of 1:4 sulfuric acid and the resin level increased to 1 gram per 5 ml. of extract to provide removal of colloidally dispersed sodium sulfate by resolution. Treatment of the extract with technical grade resin may add a slight, ultraviolet interference to the solution. This interference can be reduced by washing the resin with methanol or a methanol-water solution. Any remaining interference is eliminated by external compensation. The absorb-
ance of the sample is measured us. a resin treated reference solution. Figure 3 shows the ultraviolet spectrum of a treated sample extract us. treated and untreated reference solutions. Direct Polarographic Method of Measuring Acrylamide Concentration. A typical C.V. curve for .Ad corrected for resistance and residual current is shown in Figure 4, curve 1. This wave was obtained with TBABr support,ing elect’rolyte. The C.V. curves for ALA in T B h O H supporting electrolyte were similar up t o 0.1 mg. A4A/ml. At higher concentrations of in TBAOH supporting electrolyte a complex wave form similar to curve 2 was usually observed. The inflection was sensitive t o the deaeration history of the sample. Repurging a sample would suppress the inflection toward a single wave but usually did not eliminate it completely. The advantage of the alkaline supporting electrolyte is shown by curves 3 through 6 of Figure 4. The residual current a t -2.1 volts was reduced from 0.5 pa. (curve 3) to 0.2 pa, (curve 5). An extract which showed a premature hydrogen discharge commencing a t -2.1 volts in neutral medium (curve 4) showed the discharge a t -2.25 volts in alkaline medium (curve 6) and the A h wave became well defined. I n practice severe interference may mask the A h wave even in alkaline supporting electrolyte. Determinations made under these conditions valid provided the half-wave potential based on the current a t -2.1 volts agrees with that of the concurrent standards. maximum such as is shown on curve 1, Figure 4 has been observed with impure T B d O H solutions. *inalpis of the polarographic waves by plotting log ( i / i d - i) us. E for several supporting electrolytes gave nonintegral values of n,, the apparent number of electrons involved in the rate controlling step. Curve 3 of Figure 5 is the plot for the wave shown as curve 1 of Figure 4. E,;, was -1.977 volts and na was 0.60. Curves 4 and 5 are plots for the single wave and complex wave (curve 2 of Figure 4) in TBAOH supporting electrolyte. The values of and n, agree closely with those found in neutral medium. The upper part of the complex wave is identical in form and position to the simple wave. The half-wave potentials were independent of p H in the neutral and alkaline solutions and have ranged, over a period of eight years, from -1.96 to -1.98 volts us. S.C.E. The currentconcentration relationship was linear. For an electrode having the constants m = 1.29 mg./second, t = 3.16 seconds, i d / C was 5.3 pa./mmole/liter. Skobets, Kestyuk, and Shapoval (10) reported a reversible, one-electron, rate controlling process ( n = 1.02) in VOL. 3 7 , NO. 1 2 , NOVEMBER 1 9 6 5
1549
their study of AA in aqueous 0.1 M tetramethylammonium iodide using a capillary with m = 28 mg./second, t = 2.88 seconds. A repeat of their experiment using a capillary with m = 1.29 mg./second, t = 3.16 seconds (determined a t -2.1 volts in 70/30 methanol/ water solvent, 0.091M in TABOH) gave a slope constant of 0.71 (curve 1, Figure 5 ) . The wave for AA in 0.091M aqueous TBAOH was likewise irreversible, n, being 0.62 (curve 2, Figure 5). The physical chemistry of the electrode process is not completely understood. The serious discrepancy in the reversibility of the rate controlling electrode process as reported by Skobets et al., and as found in this study, appears to be real and is not a n artifact of the recorder circuitry. The value of i,/C of 5.3 pa./mmole/liter is large and is hard to match with the 0.3 electron process reported by Skobets, et al. from coulometric data. The methanol to water ratio does not exert a dramatic effect on the AA wave from 0 to 95% methanol. The principal difference in the two studies is the much higher mass flow of mercury in the electrode used by Skobets et al. I n spite of irreversibility, the present observations are reproducible and the results from several capillaries of similar characteristics to that described in detail have shown no unusual differences. The determination of the current a t 0.95 id was unconventional and was done to avoid the premature hydrogen discharge which occurred in some samples. The technique was accurate provided the potentials were carefully corrected for cell resistance and the AA concentration was kept below 0.5 mg./ml. in the extract. Higher concentrations of AA which enhanced the tendency to form maxima were rarely found and, when found, were diluted with 80/20 solvent prior to resin treatment. By measuring the cell resistance directly the shift in half-wave potential with wave height was shown to be a resistance effect. The cell shown in Figure 1 had a resistance of 6200 ohms. Bromination has been used as a direct method for AA determination in
Table ill.
Sample 1
2 3 4 Blend 4
1550
b
Analysis of acrylamide waves
0.1 OM tetramethylammonium iodide, 0 . 2 5 mg./ml. AA, H g poo 0 . 0 9 1 M TBAOH, 0 . 1 8 2 mg./ml. AA, H g pool (v,/v.) methanol/water, 0.091 M TBABr, 0.1 7 4 mg./ml. AA, (v./v.l methanol/water, O . 0 9 1 M TBAOH, 0.174 mg./ml. AA, (v./v.)
methanol/water, 0.068M TBAOH, 0 . 1 7 4 mg./ml. AA,
acrylamide where monomer levels exceed about 0.1%. Samples containing up to 0.25 mmole of AA in 100 ml. of aqueous solution were reacted for 30 minutes with 1.25 mmole of bromine, generated from bromate plus bromide in 0.125M sulfuric acid. Excess potassium iodide was added and the liberated iodine determined with 0.112.1 sodium thiosulfate. The difference from a control containing no sample was a measure of the acrylamide. A comparison of analyses by the two methods using neutral (TBABr) and alkaline (TBAOH) polarographic media is shown in Table 111. The polarographic determinations usually give lower AA contents than bromination determinations, a result to be expected because of the specificity of the polarographic procedure. The neutral and alkaline supporting electrolytes give almost identical results where there is no interference. Sample 5 showed interference in neutral
Determination of Acrylamide in Polyacrylamide by Bromination and Polarography
Bromination, % AA 0.72
0.13-0.18 0.17 0.16
4B 4c 5
Figure 5. 1 . Water, 2. Water, 3. 7 0 / 3 0 S.C.E. 4. 7 0 / 3 0 S.C.E. 5. 7 0 / 3 0 S.C.E.
0.88
ANALYTICAL CHEMISTRY
Polarography Neutral Alkaline medium, medium, % AA % AA 0.88 0.11 0.11 0.16 0.12 0.19
(Interference)
0.12 0.18 0.80
Standard addition Amt. Net % AA assay 0.40
0.87
0.05
0.79
solution but was successfully assayed in alkaline solution. Standard additions were made to the solvent prior to extraction of samples 1 and 5 . After correcting the observed concentrations for the contribution to be expected from the standard addition, the results shown in the last column were obtained. These results indicate that the AX is not held preferentially in the solid phase after extraction. A brief statistical analysis of the method was made. The residual current a t -2.1 volts based on 5 determinations over a period of days reproduced 10.07 Fa. a t 95% confidence limit. This corresponds to 0.0003% AX in solution (0,0037G in a 10-gram polymer sample extracted with 100 ml. of solvent). The limit of detection based upon duplication of the results of 5 samples providing 0.0005 to 0.001% AA to the extract was 0.007% XA at 9.570 confidence limit based on a 10gram sample. The precision of the methods based upon 5 pairs of duplicates assaying 0.1 to 0.9% Art (0.005 to 0.O45yGAX in the extract) was 6.8% based upon 5-gram samples extracted with 100 ml. of solvent. The systematic errors which give low results are incomplete extraction and failure to correct for cell resistance. High results are observed from premature hydrogen discharge and from contamination by sodium or potassium ions. These errors can be identified and, with the exception of premature hydrogen discharge, controlled.
Difference Polarographic Method
of Measuring Acrylamide Concentration. The importance of potential control is shown by the data of Table IV. The large residual currents shown in column 2 are characteristic of difficult samples. The currents due to AA (column 3) are still significantly higher than the residual current. The AA contents of the samples calculated with and without correcticin of the potential for iR drop are shown in columns 4 and 5. The slope of the residual current trace a t -2.18 volts us. 0.1M TBAC1-calomel in volts per microampere is converted to volts per 0.01% AA in a 10-gram sample extracted with 100 ml. of solvent and the results appear in column 6. The data of column 6 show that control of the potential to =k0.002 volt is necessary to achieve O . O l ~ oaccuracy. The absolute potential difference between two separate experiments need be no more accurate than for the direct procedure, nominally zk0.01 volt. Keat dodecyl mercaptan (DDM) was difficult to dissolve in alkaline 70/30 solvent mixture in the polarographic cell. Solutions of DDAI in methanol were not stable. Mercaptoethanol failed as a replacement for DDM because of a premature discharge. To increase the methanol content of the solution, 10-gram samples of polymer were extracted with 25 ml. of 80/20 solvent for three hours, diluted with 75 ml. of methanol, and extracted one additional hour. Values obtained with the two solvent systems by the direct and difference procedures using samples showing various degrees of interference are shown in Table V. Sample 9 showed no interference and the results were consistent for both solvents and both polarographic procedures. Sample 10 showing a mild interference gave a slightly lower assay by the difference method. Samples 11, 12, and 13 showed considerable interference as they were predicted to do on the basis of their history. The high assays by the direct procedure as compared to the difference procedure are characteristic of this interference. Samples 9, 10, and 11 gave the same results by both extraction procedures within the experimental error. The first order rate constant [k(min,-1)] for 0.025 ml. of D D N added to 8.8 ml. of solution with an average composition of 70 parts methanol to 30 parts water, 0.091M in TBAOH was 0.20 =t 0.02 min.-l a t 35' zk 1' C. for 80/20 solvent. The rate constant dropped to 0.16 zk 0.02 min.-* for 95/5 solvent similarly diluted. The reaction is almost certainly first-order in D D M also. These data show no particular advantage of the 95/5 solvent system in eliminating interferences. The D D M reaction is slowed in 95/5 solvent. Not shown by
Table IV.
Effect of Potential Correction for Cell Resistance upon Assays of Low Hydrolysis Polyacrylamides Showing Interference
Solvent: 80/20 (v./v.) methanol/water
Sample
a
AA (%) Not corr. Corr. 0.116 0.152 0.195 0.232 0.084 0.099
Current Residual" Totalo
6 7 8
2.55 1.62 1.82
+ 0.1 volume M/liter TBAOH
10.30 13.34 6.87
Slope of residual current
(v01t/0.01% AA) 0,0018 0.0025 0.0023
pa. at -2.18 volts us. 0.1M TBAC1-calomel, corrected for iR drop.
Table V.
Effect of Varying Solvent System on Acrylamide Determination by Difference Procedure
Supporting Electrolyte: TBAOH Difference procedure Dir. Drocedure 95/5
9 10 11 12 13 a
0.017 0.012
0.017 0.011 0.029 0.020 0.021
1.05 1.25 1.10
0.014 0.005 0.007
0.!2 0.185
1.20 1.27 2.43 1.66 1.64
0.013 0.004
0.005 0.008 0.008
0.18 I,
0.15 0,:s
All samples were less than 10% hydrolyzed. Too small to measure.
the table is the increased fouling of the lumen of the d.m.e. capillary which was observed in 95/5 solvent. Since the D D M would dissolve in the 80/20 solvent, given time, the use of 95/5 solvent was abandoned. The DDMAB reaction was verified by measuring the rate constant where possible. I n cases of doubt the difference in currents before and after the D D M reaction were plotted as a function of potential to reveal the acrylamide wave. The recovery of standard additions to the extractions using the difference method are shown in Table VI. The recoveries from low level samples show a precision greater than hO.Olyo AA. The accuracy of recoveries from five samples containing 0.1% or more A*% (including samples 16 and 17 of Table VI) was 8%. The absolute deviation from the average of six pairs of duplicate results from samples containing 0.02'30 or less AA was 0.003?$0 AA a t 95% confidence limit. The deviation from the average of duplicate results for four pairs of samples assaying greater than 0.1% AA was 4y0. The limiting factor on the assay of higher concentrations of AA is in the accuracy rather than the precision when interference is severe. Systematic errors are similar to those for the Direct Method except that the premature hydrogen discharge will give high or low results only if there is a change in its magnitude during the assay. Incomplete reaction with D D M will give low results.
Table VI. Recovery of Standard Additions of Acrylamide by Difference Procedure
+
Solvent: 80/20 methanol/water 0.1 volume Mlliter TBAOH Analysis (corr. Original Std. for std. analysis addn. addn.) Sample AA To AA yo yo AA 13 14 15 16 17a
0.00s
o.ooS 0.013 0.282 0.147
0.013 0.013 0.125 0.125 0.083
0.01n
o.oii
0 012 0.304 0.150
Vinyl pyrrolidone copolymer.
Ultraviolet Method of Measuring Acrylamide Concentration. Charac-
teristic ultraviolet absorbance curves for acrylamide are shown in Figure 3. The base line of the spectrum may not be perfectly flat because of traces of colloidal matter in the solution. This error is compensated by extrapolating the base line from 300 mp to 240 mp as shown in Figure 3 and subtracting the extrapolated absorbance from the total absorbance. Some color interference can b6 eliminated by treating the extract with a small amount of activated carbon. This treatment also helps eliminate interferences from certain aromatics (4). If activated carbon is used, the standard solutions must be VOL. 37, NO. 12, NOVEMBER 1965
1551
Table VII. Comparison of Results by Ultraviolet and Polarographic Methods
Sample 18 19 20 21 22 23 24
--Residual acrylamide, % Polarographic Ultraviolet 0.51 0.043 0.025 0.020 0.011 0.008 0.006
0.050 0.041 0.026 0.020 0.012 0.008 0.007
treated in the same manner, as the acrylamide recovery will be lowered about 10%. Any ultraviolet absorbers not separated during the extraction step or removed by the resin treatment would be expected to interfere. The presence of interfering substances can be readily detected by examination of the sample spectrum. The spectrum should be a smooth curve free of peaks and the absorbance a t 240 mp should normally be 3.0 times the absorbance at 250 mp. Data obtained by this method indicate a standard deviation of 0.003% in the 0.01 to 0.1% range. Table VI1 compares results obtained by this method with those obtained by polarography.
The ultraviolet spectrophotometric procedure is subject to the systematic errors of incomplete extraction which will yield low results, the presence of collidial matter which will cause high results if not compensated, and contamination by other ultraviolet absorbers most of which will distort the spectrum, usually in the direction of high results. ’ Choice of Method. The final choice of the measurement procedure is a compromise between simplicity and specificity. For samples of similar character and known interferences the spectrophotometric procedure is preferred. The much greater specificity of the polarographic procedures dictates the use of these procedures for samples of unknown character and as reference methods. The Difference Method can be applied directly to a sample which is found to give severe interference using the Direct Method by connecting the auxiliary circuity and adding the DDM. ACKNOWLEDGMENT
The advice and consultation furnished by R. E. Friedrich during the development of these methods is gratefully acknowledged. R. G. Beattie, R. D.
Bicknell, and K. E. Werth rendered assistance in portions of the experimental studies. LITERATURE CITED
(1) American Cyanamid Co. New Product Bulletin, Collective Volume 111, p. 27, American Cyanamid Co., New York 20, N. Y. (2) Beesing, D. W., Tyler, W. P., Kurta, D. X, Harrison, S. A., ANAL. CHEM. 21, 1073 (1949). (3) Critchfield, F. E., Funk, G. L., Johnson, J. B., Ibzd., 28,76 (1956). (4) Friedrich, R. E., Jones, G. D., Heiny, S. M., (To The Dow Chemical Co.) U. S. Patent 3,130,229(April 21, 1964). (5) Hopke, E. H., The Dow Chemical
Co., Midland, Mich., private communication, 1955. (6) Kolthoff, I. >I., Lingane, J. J., “Polarography,” Interscience, New York, 1952.
(7) Ibzd., p. 374. (8),McCollister, D. D., Oyen, F., Rowe, \ . K., Tosicol. A p p l . Pharmacol., 6 (2) 172 (1964). (9) Skobets, E. >I., Nestyuk, G. S., Ukr. Khzm. Zh. 28, No. 8, 934 (1962). (10) Skobets, E. M., Nestyuk, G. S., Shapoval, V. I., Ibzd., No. 1, 72-6. (11) Skoda, W., Schura, J., Makromol. Chem. 29, 187 (1959). (12) Ungnande, H. E., Ortega, I., J . Am. Chem. Soc. 73, 1568 (1951). (13) Zuman, P., Collectzon Czech. Chem. Commun. 15, 1109 (1950).
RECEIVEDfor review March 4, 1965. Accepted August 30, 1965.
Analysis of Mixtures of Aluminum, Gallium, and Indium by Solvent Extraction and Gas Chromatography GERALD P. MORIE and THOMAS R. SWEET McPherson Chemical laboratory, The Ohio State Universify, Columbus, Ohio Solvent extraction is combined with gas chromatography to produce an effective method of analysis for aluminum, gallium, and indium mixtures. The extraction of these three metals with trifluoroacetylacetone into benzene was studied. The three chelates in the organic phase are withdrawn from the extraction flasks, injected into a gas chromatograph, separated, and determined quantitatively. Aluminum, gallium, and indium were determined with a relative mean error of 2.32, 2.34, and 5.20%, respectively.
B
have been used extensively as reagents for solvent extraction of metals (18). Trifluoroacetylacetone (TFA) has been used in relatively few of-these studies. Schultz and Larsen extracted zirconium and hafnium from aqueous solutions with ETA DIKETONES
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0
ANALYTICAL CHEMISTRY
TFA into benzene (13). Omori et al. have studied the extraction of scandium (111) with T F A and other fl diketones (9). Recently, Scribner, Treat, Weis, and Moshier reported the extraction of several metals with TFh into chloroform (16). A TFA isobutylamine system was also used for the extraction of some divalent metals (15). The application of gas chromatography to metal chelate analysis, especially fluoroacetylacetonates has received much attention in recent years. A few quantitative studies of the synthesized chelates have been made using electron capture (10-12). flame ionization (1, 6), and thermal conductivity (14) detectors. Moshier and Sievers have summarized much of the progress in this field in a recent book ( 8 ) . The present paper describes the extraction of aluminum, gallium, and indium with T F A into benzene. The pH,
concentration of metal ion, equilibration time, and effect of diverse ions were studied. Using optimum conditions for extraction, and chromatographic conditions similar to those described by Schwarberg, Moshier, and Walsh ( 1 4 ) , accurate analyses of aluminum, gallium, and indium mixtures were performed. EXPERIMENTAL
Apparatus. An F&M model 720 gas chromatograph equipped with a thermal conductivity detector and a Brown-Honeywell recorder was used for this investigation. The chromatograph was modified by placing a piece of 4 mm. (0.d.) borosilicate glass tubing into the injection port (17 ) . Helium was used as the carrier gas and borosilicate glass columns were used throughout this work. A 50-J. Hamilton syringe with a Chaney adaptor was
