mum values of 0.01 and 1 pM, respectively, Equation 23 yields for the largest measurable rate constant: (24) Under our experimental conditions, it was easily possible to automatically titrate a sample in 100 seconds or less. Consequently, our upper limit of measurable rate constant is about 6 x lo5 M - I sec-l (@ = 6,000). This compares with a limit on the order of 2 X l o 7 M-1 sec-1 for stopped flow methods, over which kinetic titrimetry has advantages of experimental simplicity and directness. Most important is the possibility to utilize a variety of concentration transducers. Thus, e.g., the measurements can be carried out spectrophotometrically as is commonly done in stopped flow methodologies or thermometrically as in our present study. Sensitive electrochemical devices, including rotating electrodes, can also be employed. In the case of spectrophotometric measurements the titration technique has the intrinsic advantage of quite large path lengths (50-100 mm) over stopped flow cells (1-10 mm), thereby gaining an appreciable “Beer’s law amplification” of the signal. Because the concentration is changing rather slowly during a titration, one might employ some transducers which, although too sluggish for measurements in one second, may not be too slow for use on a hundred second time scale. Of course kinetic titrimetry is not competitive with Eigen’s relaxation methods. However, considering both theoretical and experimental advantages, kinetic titrimetry may in many instances be preferable to stopped flow methods. In essence, kinetic titrimetry re-
quires little more than a recorder, a constant rate buret, and a moderately fast concentration sensor. It is instructive to wind up this discussion by exploring whether anything can be gained by using kinetic titrimetry in lieu of classical rate methods, everything else being equal. Let us consider a conventional rate study where reactants A and T’ are instantaneously mixed and CA or CT, is subsequently measured as a function of time. If CAO = CT~O,the famililir relationship holds:
which rearranges to
k , = - - ( l1
c.:t
-
1)
(26)
where all symbols have their previous significance. For CAo = 100 pM, = 0.01 and t = 100 sec, Equation 26 yields k~ = lo4 A4-l sec-I. These assignments for CAO, cA, and t are exactly equivalent to the conditions for which Equation 24 yielded k A = 6 x 105 M - l sec-1 by kinetic titrimetry. Thus, the titrimetric approach to kinetics should offer per se a significant gain-viz., a nearly hundredfold enhancement in accessible rate constants. The authors hope that further work will verify this expectation. Received for review May 30, 1972. Accepted November 22, 1972. Based on a Ph.D. thesis by P. W. Carr. Supported by Research Grants GP 11386 and GP 28581 from the National Science Foundation.
Coulometric Titration of Weak Acids in Tetrahydrofuran C. E. Champion and
D. G. Bush
Research Laboratories, Eastman Kodak Company, Rochester, N. Y. 14650
The use of coulometric titrimetry has been extended to the titration of microequivalents of weak acids in tetrahydrofuran. The solvent has a dielectric constant of 7.4 and is excellent for dissolving a variety of polymers. This advance is important because small amounts of polymer samples from gel permeation chromatographic or precipitimetric fractionations can be examined routinely for their weak acid contents. The titrations can be performed with automatic recording. Such a successful development results from the construction of a small titration cell which enhances the sensitivity of the potentiometric sensing system, and from an effective generation of pure tetrabutylammonium hydroxide in tetrahydrofuran containing 0.2% water. The method is bias-free within the random errors of measurement, and both the precision and accuracy of the method (for amounts of acid between 0.5 and 10 microequivalents) are fl .O%.
The coulometric titration of organic acids in nonaqueous solvents containing a quaternary ammonium halide electrolyte has attracted the attention of several in640
ANALYTICAL CHEMISTRY, VOL. 45, NO. 4, APRIL 1973
vestigators (1-4). They have shown that the method has two principal advantages: pure quaternary ammonium hydroxide, free of carbonates and tertiary amines, is generated with 100% current efficiency; and good precision and accuracy are obtained with acid samples at the microequivalent level. The presence of the electrolyte has the additional advantage that it enhances the acid strength of weak acids (5, 6). Basically, a coulometric titration involves electrogeneration of base at an inert cathode with periodic measurement of the hydrogen ion concentration, which is recorded as a function of the number of coulombs passed. Because the indicating circuit often interacts with the electrical field of the generating circuit in nonaqueous systems, the generation of base is stopped for the measurement of the hydrogen ion concentration. This periodic interruption of the titration has hindered automation. The rate of genera‘(1) C. A. Strueli, J. J . Cincotta, D. L. Maricle, and K. K. Mead, Anal. Chem., 36, 1371 (1964). (2) C. Cotman, W. Shreiner, J. Hickey, and T. Williams, Talanta, 1 2 , 17 (1965). (3) G. S. Fritz and F. E. Gainer, Ta/anta, 15, 939 (1968). (4) G. Johanssen, Talanta, 11, 789 (1964). ( 5 ) H. W. Wharton, Anal. Chem., 37, 730 (1965). (6) G. D. Christian, Anal. Chim. Acta., 46, 77 (1969)
tion of the titrant (base) is determined by the magnitude of constant current which can flow between the anode and cathode, the maximum current being limited by the voltage at which secondary reactions in the electrolyte or solvent impurities begin. In order to generate constant currents of a reasonable magnitude, the resistance of the cell must be as low as possible. This has been accomplished by keeping the distance between the electrodes short and by using solvents with dielectric constants greater than 11. The solvents which have been used for the titration of acids are methanol (2, 7), 2-propanol (4), methyl ethyl ketone (4), acetone ( I , 3) and tert-butyl alcohol (2, 3). In general, these solvents do not cause major complications because reasonably low cell resistances are obtained; however, they are usually poor solvents for polymers. The present investigation of a micromethod involves the nonaqueous coulometric titration of acids in tetrahydrofuran (THF). This solvent has a relatively low dielectric constant of 7.4 and is excelleht for dissolving many polymers. A micromethod is useful for measuring the acid content of research materials which are limited in quantity. One example is the small quantity of sample obtained from the separation of polymer fractions using precipitation and/or gel permeation chromatographic techniques. With this method, it is possible to record the titration curves concurrently with the generation of titrant. The simultaneous generation of titrant and recording of the titration curve have not been reported in earlier investigations of nonaqueous coulometric procedures. In fact, Fritz and Gainer (3) reported that in their work it was impossible to record titration curves during the generation of titrant. The successful development of this method results from the design of the microtitration cell, the use of the potentiometric sensing system, and the effective generation of pure tetrabutylammonium hydroxide in tetrahydrofuran. This method gives precise and accurate titration of 0.5 to 10 Feq of acid. It has been used for the titration of a variety of organic acids varying in strength from a pK,(HzO) of 2.00 to a pK,(H20) of 9.99.
EXPERIMENTAL Design of Coulometric Cell. The microtitration cell is shown in Figure 1. It IS designed with three compartments, cathode ( A ) , anode ( B ) ,and primary reference (C), each with a maximum volume of 15 cm3. The cathode and reference electrode compartments are separated horizontally by a straight tube, in which a fine-porosity, sintered-glass frit ( 7 ) is sealed. A glass joint with an "O-ring" seal (8) and a membrane (9) are used to isolate the anode and cathode compartments. The anode and cathode compartments have Du Pont Teflon covers (1) specifically designed with openings for the following items: electrodes, sample delivery tube, and nitrogen inlet and outlet tubes. The reference electrode (6) is placed in a secondary reference compartment (13) containing a 1: 1 water:THF mixture saturated with KC1. A fine-porosity frit (7) sealed near the bottom.isolates the reference electrode and the aqueous T H F mixture (13) from the primary reference compartment (14) which contains T H F saturated with tetrabutylammonium bromide. The solution in the cathode compartment is stirred with a cylindrical Teflon-encapsulated magnetic stirrer (4). Five-square-centimeter platinum electrodes are used for the generating cathode (3) and auxiliary anode (11). Contacts to the current source are platinum wires spot-welded to the platinum squares. The small size of the cathode compartment provides an increase in the sensitivity of the detection system. The compartmentation isolates the cotholyte from interfering species, such as anode reaction products, and minimizes the electrical interactions between the indicating and generating circuits. Current a n d Time. An E211A Metrohm constant-current coulometer. equipped with a synchronous timer and an electronic ( 7 ) R. 0. Crisler (1962).
and R. D. Conlon, J . Amer. Oil Chem. SOC., 39, 470
6
14
C
9 8
B Figure 1. Coulometric titration cell A, Cathode compartment; B, Anode compartment; C, Reference compartment: 1, Teflon covers: 2, Sample and nitrogen inlet; 3, Pt generator cathode: 4, Stirring magnet: 5, Glass indicating electrode: 6, SCE reference electrode: 7, Glass frit; 8, "O-ring" seal: 9 , Membrane: 10, Spring clamp; 1 1 , Pt auxiliary anode; 12, 0 . 1 M TBABr in T H F (0.2% H20); 13, 1 : l water:THF satd with KCI; 14, T H F satd with TBABr: 15, Methanol satd with T M A B r 4'
A
controller device, is used as the power source. The current outputs of 0.1 to 20 mA are calibrated by measuring the potential drop across a standard resistor and have a performance stability of 0.1%. End-Point Detection. The end point is determined potentiometrically using a glass indicating electrode (Beckman No. 39167) and a saturated calomel reference electrode (Beckman No. 40463). All potential measurements for the manual method are made with the expanded scale of a Corning Model 12 pH meter. A Metrohm E436 potentiograph is used for automatic recording. Requirements for the Use of Tetrahydrofuran as a Solvent. The T H F should be free of acidic inhibitors, which are usually added to prevent peroxide formation. A typical inhibitor is 4methyl-2,6-di-tert-butylphenol, which is titrated as a weak acid in THF. The T H F is purified by distillation. In our laboratory a continuous refluxing still of T H F is set up with a distilling receiver (such as Ace Glass No. 6666) between the distillation flask and the reflux condenser, and small amounts of freshly distilled solvent may be withdrawn a t any time. A supply of inhibitor-free T H F may be stored in glass under nitrogen. The level of peroxides that form after several weeks of such storage is low (200 ppm) (8). This amount of peroxide does not affect its use for this coulometric method. However, hazardous concentrations of hydroperoxides may form in T H F under long-term storage in the absence of an inhibitor (9, 10) P a r a m e t e r s for t h e Titration Cell. The catholyte must contain between 0.2 and 1.0% of water (by volume) to increase the solubility of the electrolyte and to lower the cell resistance. At this level of water the cathode reaction is H20 + e - + OH%HZ(g) (3). However, at lower water levels a secondary cathode reaction is observed. The species generated has an intense red color, which disappears rapidly in the presence of air. Based on the investigation of Horner and Mentrup (Zli, this secondary reaction may be the reduction of the quaternary ammonium salt to a free radical. At water levels above 1%, there may be a loss of the effectiveness of the system for the dissolution of polymer samples and for the analysis of very weak acids, such as phenol. The anode compartment is filled with methanol saturated with tetramethylammonium bromide. This electrolyte-solvent system has a lower cell resistance than the tetrabutylammonium bromide-tetrahydrofuran system. The anode and cathode compartments are separated by a membrane supported by an O-ring seal that facilitates changing the membrane as needed. Of those examined, the Amicon UM-05 ultrafiltration membrane was chosen because it offered the lowest cell resistance. The solvents do not attack this membrane when the filter side is turned toward the methanol. The membrane recommended earlier for other solvents ( 3 ) is rapidly attacked by T H F and produces a very high cell resistance.
+
(8) R. D. Mair and A. J. Graupner. Anal. Chem., 36, 194 (1964). (9) "Du Pont Tetrahydrofuran Handling and Storage," E. I . d u Pont de Nemours and Co., Inc., Wilmington, Del., 1969. (10) "Q. 0. Tetrahydrofuran Unloading, Handling and Storage," Tech. Bull. No. 146-A, The Quaker Oats Co.. Chicago, Ill., 1964. (11) L. Horner and A. Mentrup, Justus Liebigs A n n . Chem.. 646, 49 (1961 ) .
ANALYTICAL CHEMISTRY, VOL. 45, NO. 4, A P R I L 1973
641
t.031
I
I
I
I
I
1
-4001
-.O2I
--0
.
1.0
2.0
3.0
4.0
5.0
Sample size, mg
Materials. All chemicals used were of analytical reagent grade. The catholyte was prepared from distilled tetrahydrofuran, Eastman 5308, to which distilled water (0.2%)and tetrabutylammonium bromide (O.IM), Eastman X7377, were added. It was stored under nitrogen. The anolyte was methanol saturated with tetramethylammonium bromide, Eastman 670. National Bureau of Standards benzoic acid was used to determine the precision and accuracy of the method. The maleic acid (690), o-toluene (2676), and acrylic acid (3588) samples were obtained from Eastman Chemicals. The phenol and bisphenols were obtained from Baker Chemicals and Aldrich Chemicals, respectively. Nitrogen (prepared from liquid nitrogen) was used for purging the solutions. Procedure. The titration cell is assembled and each compartment is filled with its supporting electrolyte. After the catholyte is purged with nitrogen, the indicating system is allowed to equilibrate until the potential drift is less than 1 mV per 5 min; this normally requires about 15 to 20 min. This time requirement is much smaller when the glass electrode is stored in T H F for 24 hr before use. For precise titration results, it is necessary to neutralize acidic species introduced with the catholyte and to predetermine the equivalence point potential for the sample. This is accomplished with a pretitration of a small amount of the sample to be analyzed (approximately 0.5 peq) with electrogenerated base. The sample (weighed as a solid in an aluminum boat or as a liquid in a 1-ml syringe with the needle inserted in a silicone septum) is delivered into the catholyte through the sample inlet of the Teflon cover or by temporarily removing the cover. The titration curves (continuous plots of potential us. time) are made as follows. The recorder drive scans with zero current flowing for several millimeter divisions before the generation current is commenced. The initial surge of current through the cell causes the potential a t the indicating electrode to respond rapidly toward the positive potential (acidic direction). This change in potential returns to the starting potential in 4-8 sec. It causes no problems and is used as a marker for commencing the generation of base. After completing the titration, the generating current is turned off and the electrode potential moves rapidly toward the negative potential (basic direction). This change in potential is a marker for terminating the generation of base. The end point for the potentiometric curve is determined graphically and the equivalents of acid in the sample are determined from the following relationship: peq of acid = ( t x r x S Z ) / S ~where t is the total generation time in seconds, r (current/Faraday’s constant) is the rate of generation of base in microequivalents per second, SI is the linear distance (mm) required for the chart recording of the total generation, and sz is the distance ( m m ) required to record the generation to the end point. Several samples may often be titrated, without changing the catholyte, before the shape of the titration curve deteriorates and end-point detection becomes difficult. 0
’
\\
-6001
Figure 2. Analysis of titration method for bias
642
i
ANALYTICAL CHEMISTRY, VOL. 45, NO. 4, APRIL 1973
G0
OS7
I74 261
348 435
0
087
2 1._
1 7; 261 174
348 435
Generated TCAOH, ueq Figure 3. Typical potentiometric titration curves using the automatic recording system for A , acrylic acid monomer; B, acrylic acid in a copolymer with ethyl acrylate; C, phenol (CI, curve recorded on the expanded scale)
For manual recording. the technique for the titration is similar to that described above. Potentials are usually measured after each 60-sec increment of generation. The pretitration and the titration curves are drawn from a plot of potential us. microequivalents of generated base.
RESULTS AND DISCUSSION Samples of standard benzoic acid ranging in size from 5 mg to 0.1 mg were titrated under the above conditions. The results (based on twelve determinations) give an assay of 99.7 f 1.1%(the uncertainty given is the relative standard deviation). The precision of the method is independent of sample size in this range. The plot of difference in number of milligrams calculated and found as a function of benzoic acid sample size is shown in Figure 2 . Such a plot is quantitatively sensitive to any constant analytical bias that may be present in the method. The magnitude of the bias appears in the zero sample-size intercept, and the deviation from stoichiometry (or absolute purity) is reflected in the slope of the linear relationship. As defined by least-squares analysis, the intercept and slope are -0.0006 mg and 0.005 with standard errors of 0.002 mg and 0.002, respectively. Thus no analytical bias is found in the method but a small deviation from stoichiometry or absolute purity is observed in the standard benzoic acid. This deviation is well within the random error for the method. The indicated assay, 99.7 f 1.1% (the uncertainties given are standard deviations), obtained with these small samples agrees with the certified acidity 99.995 h 0.002% obtained by other investigators (12, 13) using aqueous systems. An over-all examination of these data shows that the accuracy and precision over this sample-size range are represented by the random errors of measurement. In addition, the titration efficiency for 0.5 to 10 Meq of acid is 100.0 h 1.0%. Therefore, it can be assumed that the coulometric generation of base at a platinum cathode is made with 100% current efficiency in the T H F system. Typical potentiometric titration curves (using the automatic recording system) are illustrated in Figure 3 for (12) R. G . Bates (1957).
and E. Wichers, J . Res. Naf. Bur. Sfand. (U.S.), 59, 9
(13) G. Marinenko and J. K . Taylor, Anal. Chem., 40, 1645 (1968).
~
Table I. Various Types of Acids Titrated Coulometrically Half neutralization potentials, Compound titrated mV pKOa Assay, % Maleic acid (K1) +80 2.00 100.0f 1.2 Bis(2-hydroxypheny1)methane -235 .. . 99.2 f 1.0 Benzoic acid -240 4.20 99.7 f 1.1 Acrylic acid -265 4.25 98.5f 0.9 o-Toluenethiol -395 ... 98.5 f 0.5 5-Norbornene-2,3-dicarbox-420 ... 99.0f 1.0
ylic imide Bis(4-hydroxyphenyl)methane Phenol a
...
-480 -490
Table II. Determination of Acid Incorporated in Acrylic Acid-Ethyl Acrylate Copolymer Fractions
NumberaverageC
Fracticn numbera
Acid content, weight %*
weight ( M n )
Unfractioned
18.00 17.87 17.86 17.81 17.84 18.07 18.08 17.90
72,000 120,000 135,000 1 1 0,000 1 1 0,000 43,000 22,000 20,000
1
2 3 4 6 7 8
191.9 f 1.2 98.7 f 1.1
9.99
molecular
pK. values taken from J. Pure Appl. Chem., 1, Nos. 2-3 (1961) ,
Average 17.93f 0.10wt YO
*
a T h e fractions were separated by a precipitation technique. Each value is based on two or more determinations. CDetermined by os-
mometry.
I
- 550
-
40
60
80
I
I
1
I
’....
I
mV I
100
120
I
I
140
160
1
Percent tit rated
Effect of changes in sample size on the shape of the titration curves for acrylic acid-ethyl acrylate copolymer (mol Figure 4.
wt 135,000). A , 1.289 m g ; 0 .4.030 mg; 0.7.157
-5501 40
mg
acrylic acid monomer ( A ) , acrylic acid in a copolymer with ethyl acrylate ( B ) , and phenol (C). It is apparent from the two acrylic acid curves (A, monomer, and B, acid in copolymer) that the polymeric form is less acidic (more negative half neutralization potential, HNP) and that the titration behavior is less sharp owing to buffering effects (14) by the polymer. However, we have shown with the results for the polymer samples (Table 11) that this loss of sharpness does not affect the precision and the accuracy of the titration results. Curve C represents phenol and is the weakest acid we were able to titrate. In order to obtain a meaningful titration curve for such weak acids, it is necessary to use the expanded scale on the recorder. A representation of this (curve C,) is illustrated in the expanded section of Figure 3. This curve has an inflection point sufficiently sharp for the end-point determination by graphical means. A representative list of acids successfully titrated by this method is given in Table I in the order of decreasing acid strength (determined from half neutralization potentials) from pK,’s of 2.0 to 10.0. The precision of 1% is independent of these acid strengths. The titration behavior for bis(4-hydroxypheny1)methane and bis(2-hydroxypheny1)methane is interesting. Only one of the two protons is neutralized in bis(2-hydroxypheny1)methane and its acid strength is similar to that of benzoic acid. The two protons neutralized a t one inflection in bis(4-hydroxypheny1)methane have an acid strength similar to that of phenol. The cause of the stronger acid char(14) A. Katchalsky and P. Spitnik, J . Polym. Sci., 2 , 432 (1947)
I
60
I
80
1
100
I20
140
I
160
Percent titrated in molecular weight on the shape of the titration curves for acrylic acid-ethyl acrylate copolymers Figure 5. Effect of changes
D = Number-average molecular weight, 135,000; E = Number-average molecular weight, 72,000; F = Number-average molecular weight, 20,000
acter observed with bis(2-hydroxypheny1)methane is plained by intramolecular hydrogen-bond formation tween neighboring OH groups in the molecule (15, 16). The direct titration of sulfonic acids in this system not becn successful. Results to date have always been by about 10%.Additional work in this area is planned.
exbehas low
DETERMINATION OF ACRYLIC ACID INCORPORATED IN ACRYLIC ACID-ETHYL ~ . ~ ACRYLATE (1:4 RATIO) COPOLYMER FRACTIONS The usefulness of the method in polymer systems is illustrated by determinations of the acid content of some acrylic acid-ethyl acrylate copolymer fractions, obtained by bulk fractionation. Approximately 30 mg of each sample containing about 20% acrylic acid was available for analysis. The results for 0.5- to 2-mg-size samples are shown in Table I1 in the order of fraction separation. The acid composition of the sample is independent of molecular weight. The average acid content for the copolymer is 17.93 f 0.10%, where the uncertainty represents the standard deviation. A statistical t-test shows that this vari~~
~~
(15) G . R. Sprengling, J. Amer. Chem. Soc.. 7 6 , 1190 (1954) (16) S. K. Chatterjee, Can. J. Chem., 47, 2323 (1969).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 4, APRIL 1973
643
ability agrees with the variability of the method for the standard benzoic acid samples which contained the same level of acid. Therefore, this correlation shows that polymeric acids can be titrated successfully with the same degree of precision and accuracy as benzoic acid. Since the ionic strength is relatively constant during the titration and these polymer fractions provide a broad range of molecular weights, the effects of increases in molecular weight on the shape of the titration curves were investigated. Figure 4 shows typical potentiometric titration curves (recorded manually) for a constant molecular weight (fraction 1) with varying sample size. The HNP (*lo mV) is independent of sample size (0.5to 8.0 mg of copolymer). Figure 5 shows the potentiometric titration curves that were recorded manually using a constant sample size and varying molecular weight. Curves D, E, and F are typical for samples with number-average molecular weights of 135,000,72,000 (unfractionated sample), and 20,000,respectively. These curves show that the sharpest inflection and most positive half-neutralization potentials are obtained for the highest-molecular-weight samples. In fact, the difference between the HNP of the highest-mo-
lecular-weight (Curve D ) and the lowest-molecular-weight (curve F ) samples is about 50 mV, far outside the *IOmV error expected for such titrations. Since the HNP is a measure of the apparent ionization constant, these data suggest that K , increases with molecular weight. This could possibly be explained by changes in nearest-neighbor interactions (1 7) with configurational changes among the polymer fractions.
ACKNOWLEDGMENT The authors are grateful to A. L. Spatorico and J. VanDenBerghe, of the Kodak Research Laboratories, for supplying the ethyl acrylate-acrylic acid copolymers and imide. We are also inthe 5-norbornene-2,3-dicarboxylic debted to A. L. Spatorico for the number-average molecular weight measurements. Received for review October 25, 1972.Accepted December 7, 1972. Presented at the 162nd Meeting of the American Chemical Society, Washington, D.C., September 12-17, 1971. (17) K. Bak, Acta Chem. Scand., 16, 229 (1962)
Determination of Trace Elements in Silicate Matrices by Differential Cathode Ray Polarography E. June Maienthal Electrochemical Analysis Section, Analytical Chemistry Division, National Bureau of Standards, Washington. D. C. 20234
Methods are described for the analysis of some new Trace Elements in Glass Standard Reference Materials by differential cathode ray polarography. Iron and titanium were determined in the 500-, 50-, l - , 0.02-ppm and base glasses after cupferron and ammonium hydroxide separations. Nickel was determined in the 500- and I-ppm samples after extraction with dimethylglyoxime.
During the past few years there has been an increasing need for new Standard Reference Materials especially in the field of trace analysis. There has been a particular demand for standards of glassy or rock-like materials certified for a large number of elements in several ranges of trace concentrations. To meet these needs, the National Bureau of Standards has issued four new glass standards, each containing 61 added elements a t the 500-,50- 1-, and 0.02-ppm level, respectively. These glasses were prepared at Corning Glass Works, Corning, N.Y ., under carefully controlled conditions. The undoped material was prepared from extremely pure starting materials; its composition and the added elements are shown in Figure 1. The analysis of any material to which 61 different elehave been added poses Some very interesting as well as challenging problems to any analytical technique. Polarography, because of its sensitivity, resolution, and relatively inexpensive instrumentation, should be a highly advantageous technique for many of these determinations. Unfortunately, polarography seems to have 644
ANALYTICAL CHEMISTRY, VOL. 45, NO. 4, APRIL 1973
fallen into disfavor in this country; in fact, it has never achieved wide-spread acceptance in the field of glass, mineral, and rock analysis. A cursory survey of the literature showed few applications of polarographic methods of analysis of these materials. It is difficult to understand why this useful analytical tool has not been more routinely used. Work at the National Bureau of Standards over the past few years with differential cathode ray polarography has shown its utility through analyses of matrices such as air particulates, waste water, high purity materials, ores, botanicals, and alloys (1-4). The differential technique potentially allows coverage of concentrations from ppb-amounts to major constituent levels. This range far exceeds the capabilities of most other analytical techniques. Its accuracy and reliability have also been proved by comparisons of results on the same material with other techniques, such as isotopkdilution mass spectrometry and atomic absorption. Most of the added elements could be determined polarographically, in fact over 20 of those added have been determined in this laboratory in the matrices just listed. Many of the determinations were done directly with no separations. For the analysis of the glasses, however, with the known presence of 61 possible interferences, separations were necessary. (1) E. J. Maienthal and J. K . Taylor, (2) E. J. Maienthal, "Polarographic Tech. Note. 505, 1 7 (1969) (3) E. J. Maienthal, "Polarographic Tech. Note, 545, 41 (1970). (4) E. J. Maienthal and J. K. Taylor, Chem. Ser., 73, 172-82 (1968)
Anal. Chem.. 35, 1516 (1965). Analyses" in Nat. Bur. Stand. ( U . S . ) Analyses" in
Nat. Bur. Stand. ( U S . )
"Trace lnorganics in Water," Advan.
