ACKNOWLEDGMENT
The authors thank Sierra Metals Corp. for permission to publish this work and IC. R. Johnson for his assistance in carrying out spectrographic analyses. LITERATURE CITED
(1) Am. Soc. Testing Materials Methods,
“Chemical -4nalysis of Metals,” pp. 116-8 (1956). ( 2 ) Andermann, G., Kemp, J. W., ANAL. CHEM.30, 1306-9 (1958). (3) Bruch, J., Arch. f. Eisen. 33, No. 1, 5 (1962). (4) Campbell, W. J., Leon, M., Thatcher, J. W.,U . S . Bzireau of Mines Rept. of Investzgations 5497, 24 (1959). (5) Hillebrand, W. F., Lundell, G. E. F.,
Bright, H. A., Hoffman, J. I., “Applied Inorganic Analysis,” 2nd ed., pp. 11622, Wley, New York (1953).
(6) Ibid..
I
DD. 609-10. r - - ~-
C
( 7 ) Ibid., pp. 689-93, (8) Niekerk, J. N. van, Wybenza, F. T., A p p l . Spectroscopy 14, 56-(1960). (9) Pigot, E. C., “Ferrous Analysis,” pp. 505-10, Wiley, New York (1953). (10) Rothmann. H.. Schneider. H.. Nie’ buhr, J., Pothrnann, C., Arch. f. Eisen. 33, KO.1, 17 (1962).
RECEIVED for review February 23, 1962. Accepted May 28, 1962.
Determination of Maior and Minor Constituents in Ceramic Materials by X-Ray Spectrometry RITA LONGOBUCCO Electron Tube Division, Radio Corp. o f America, Harrison,
b X-ray spectrometric methods have been developed for the determination of magnesium, aluminum, silicon, potassium, calcium, titanium, iron, and barium in forsterite and in the materials used in making it (talc, magnesium oxide, feldspar, and clay). Major constituents are determined by comparison with standards using a glass disk technique. Minor constituents (less than 5%) are determined as powders by a standard-addition method. The precision in most cases appears to b e excellent. The accuracy, although difficult to evaluate, compares favorably with that of classical wet methods.
A
THE result of the introduction by RCA of Suvistor and other tubes using ceramic rather than glass stem wafers, forsterite-type ceramics have assumed an important role in the receiving tube industry today. Forsterite is prepared from five raw materials-talc, magnesium oxide, barium carbonate, feldspar, and clay. Complete analyses of these materials are desirable because their composition affects both the physical dimensions and the final properties of the finished parts. However, analyses of these raw materials by the classical analytical techniques are tedious and a t times inaccurate. Despite difficulties generally encountered in the direct analysis of powders by x-ray methods, the convenience and rapidity of this method warranted an investigation. This approach was abandoned after preliminary experiments gave highly inaccurate results. Solution techniques were not investigated because of the extreme difficulty of dissolving inert, refractory ceramic materials and the consequent loss of intensity on dilution of these light matrix materials.
s
N. J.
Several workers have successfully applied various versions of the borax fusion technique first introduced by Applied Research Laboratories (8) and developed by Claisse ( 7 ) . Maneval and Love11 (9) used the fused glass beads obtained by this method without further processing. Others (1, 3) found it advantageous to grind and briquet the glass. Of all the reported methods, two seemed to offer promise for determination of the major constituents. The direct analysis of the powder was still possible if the as-received materials were ground to a fine, reproducible particle size (-300 to -400 mesh) and an internal standard was added. Alternatively, the powders could be converted to a solid solution-a glass -by dissolving them in a glass-forming agent such as borax. Of the two methods, the glass fusion technique appeared to offer greater potential accuracy and reliability and was therefore adopted for the major constituents. The borax fusion technique, developed by Claisse for minerals and ores, has been described in detail ( 7 ) . For the more numerous minor constituents (less than 5%) a method which is faster and simpler than the fusion technique was desired. The method of standard addition, discussed in detail by Campbell and Carl (5), was found suitable.
Table
I.
This paper describes s-ray spectrometric methods based on the glassbead and standard-addition methods for the analysis of MgO, talc, clay, feldspar, and forsterite for MgO, A1203, SiOz, KzO, CaO, TiOe, Fez03, and BaO in the weight per cent concentrations shown in Table I. EXPERIMENTAL
Instrumental Conditions. A General Electric flat-crystal x-ray spectrometer with conventional-i.e., not inverted-optics, equipped with a Machlett AEG-50s chromium-target x-ray tube, was used for t h e analyses. A 0.625 X 0.070 inch collimator was used a t t h e source, and a 3.5 X 0.020 inch collimator a t the detector. A General Electric SPG-4 flow proportional counter with a 0.00025-inch Mylar window was used. The instrumental operating conditions for each element are given in Table 11. A fixed count method was used, and the number of counts taken for each element is listed in Table 11. The esposure time for an element, of course, varied with each material, but in general ran about 200 seconds. One timing is usually sufficient. For the determination of .41203, the conventional aluminum face plate on the sample drawer was replaced by a similar one made of Lucite. Since the standard addition method used for minor constituents involves the direct use of powers, a Spex rotating sample
Composition of Ceramic Materials for Which Methods W e r e Established
Material Magnesium oxide Feldspar Talc Clay Forsterite
MgO 98
... 35
...
46
A1103
...
18 1.7 35 4
Si02 0.9 68 62 60 40
(Per cent) K20 CaO ...
11
...
1 2 1
0.9 0.2 0.5 0.4 0.4
Ti02
FesOa
BaO
...
0.2 0.1 0.9 1 0.6
...
,..
...
1.8 ...
VOL 34, NO. 10, SEPTEMBER 1962
... ...
... 7
1263
Table 11.
Element Atomic No., 2 Spectral line Wavelength, A, A. 28, line, deg. X-ray tube operating potential, kv. X-ray tube current Crystal Radiation path No. of counts taken Major constituent Minor constituent
Table 111.
Ka
Ka
Ka
8.339 142.44 50 40
7.126 108.00 50 40
3.744 136.64 50 10
3.360 113.06 50
50 40
ADP
He
16,384
...
Table IV.
40
LiF He
LiF He
102,400
102,400
819,200 204,800
102; 400
I ,6:i8 400
...
X X X X X
Oxide, yo Concn. Std. dev. 34.6 17.1 3 81 60.3 0.58 10.8 4.03 0.35 0 56 7.9
0.29 0.097 0.440 0.194 0.066 0.014 0.138 0.004 0.003 0.038
Reproducibility of Sample Analyses
3-21-61 4-26-61 5-10-61 5-25-61 2-17-61 5-10-61 6-6-61 8-21-61
...
...
... ... 34.0 34.6 33.9 34.7
drawer (Model 3520) was used. Specially designed Lucite rings (ll/ainch I.D.) (4) were used as powder sample holders. Chemicals. All chemicals were Fisher certified reagents. When practicable, standards were prepared from the oxides of the elements of interest. When these oxides were unsuitable or unavailable-e.g., CaO, KzO-an equivalent amount of nitrate or carbonate was used. Fused borax (Na2B,O7) was the glass-forming agent. Sample Treatment. Each of the minerals studied shows a loss on ignition rangin from 0.4 to 17%. Because of the k g h temperatures involved in the fusion process, this loss can lead t o serious errors in the analysis. Consequently, the sample material was ignited or calcined a t 1350' C. before use. FUSION METHOD
The low intensities obtained from the light elements require the largest practicable sample-to-flux ratio. A ratio Ratio.
ANALVTICAL CHEMISTRY
Analysis, yo AWs Si02
K20
18.3 18.2 18.6 17.6
10.5 10.1 10.6 10.6
... ...
... ...
70.7 71.0 70.6 71.4 65.2 65.5 64.5 64.5
... ...
...
...
of 1 to 1.5 (5 grams of sample, 7.5 grams of flux) is the most favorable. Addition of a small amount of LisCOs greatly aids the fusion process. Background Correction. The background is measured at the peak, using a standard glass disk prepared so t h a t the element of interest is replaced with a n equivalent amount of a n element one or perhaps two atomic numbers away. The substitution is made so t h a t these 0% disks will have nearly the same scattering properties as the other standard disks. Preparation of Disks. The preparation of the disks involves several critical steps. If the aluminum block is too hot, the disk will stick t o it; if it is not hot enough, the surface of the disk will be irregular. If the hot disk comes in contact with a cold metallic object, i t will crack; if it is cooled too rapidly, it will be stressed so severely t h a t i t will "explode" when handled. I n the preparation of the standards, the various oxides dissolve preferentially
56
1.937 57.49 50
2.567 79.15 50
Air
204; 800
8 IO, 200 ...
LiF
;
W1 40 IdF He
40
LiF He
X
Algo
86.12 50 40
Ba
35
Ka
Ka 2 .i50
EDDT He
X
Date
22
EDDT He
s,m
Fe
Ti
20
Ka
X
Feldspar Felhpar Forsterite
0
Ca
19
9.889 136.47
X
Clay
1264
K
Si 14
Ka
X
hla nesium oxidc Feltspar
Sample-to-Flux
13
Constituent Major Minor
Talc Clay
Ref. talc
12
Instrumental Conditions
A1
Standard Deviations of Instrumental Errors
Material Talc Feldspar
Material Ref. feldspar
Mg
in the flux. In adtlition, a ratlicr v i w . u melt is obtained as R result of the high sample-to-flu.; ratio. These two factors can result in an inhomogeneous glass. Constant swirling of the molten glass in the crucible is necessary tci ohtain a homogeneous melt. Often small gas bubbles form in the molten glass during the fusion and are not expelled by heating and swilling. A large number of these m a l l gas pockets can cause serious errors in the analysis. They are eliminated by placing the crucible with the melt in a muffle furnace a t 1350' C. for 3 minutes. The glass, free from bubbles is then ready to be poured. The following procedure minimizes the difficulties discussed above and produces disks of good quality. -4ppropriate amounts of dried chemicals for preparation of the standards or 5 grams of calcined sample material are weighed into a suitable vial. Then 5.5 grams of fused borax and 2 grams of LizCOa are added, and the contents of the vial are mixed for 15 minutes in a mechanical mixer. The powder is transferred to a platinum crucible and heated slowly over a Meker burner. As fusion begins, the temperature is gradually increased until the crucible is red hot. After the powder is dissolved, the crucible must be wirled constantly for 3 minutes to ensure homogeneity. The crucible is heated in a muffle furnace a t 1350" C. for 3 minutes; the melt is then poured onto a n aluminum slab previously heated to about 260" C. and the resulting glass disk is allowed t o cool very slowly to room temperature. The flat side of the disk is then polished on a metallographic pregrinder and polishing H heel. The disks should be transparent, free from bubbles, and of uniform color. If a disk should crack because of uneven Table V. Analytical Reproducibility of Five Samples of Same Material
1
Z
3 4 5
17 2 70 1 ii.0 70.9 17.2 70.7 16.7 69.3 16.1 71.4
10 3
10.3 10.2 10.3
10.2
2 3 ~. 2.5 2.2 2.3 2 5
0 03s
0.040 0.040 0.039 0.039
cooling, the pieces can be recast. Excess glass remaining in the platinum crucible can be conveniently removed by fusing with potassium pyrosulfate and leaching with hot water. ’4ny glass remaining on the outside is removed by placing the entire crucible in a hot solution of 10% HF in a polypropylene beaker for about an hour. STANDARD ADDITION METHOD
A composite mixture is prepared containing certain known amounts of the oxides or other suitable compounds of the elements of interest. The particle sizes of all the chemicals used for the composite mixture must be similar. Coarser chemicals must be ground before use. The final composite mixture must be perfectly homogeneous. An appropriate amount, C, of the composite mixture is added to 10 - C grams of calcined sample material in a suitable vial. Another vial is filled with approximately 10 grams of sample. The contents of each vial are then mixed simultaneously in a mechanical mixer for 15 minutes. Each powder is packed from the back side into a Ii/8-inch Lucite ring so that the surface is plane and smooth; the packing densities of the two powders should be about the same. The rings are rotated in the spectrometer, and the intensity of each element in the sample and in its corresponding standard are measured alternately. Background measurements are made near the peaks. The data are then applied to the equation 1: = (Ry)/(l - R) (derived in the Discussion) and per cent oxide is calculated. DIRECT ANALYSIS OF POWDERS
For the sake of time and convenience, the direct analysis of powder samples is highly desirable, although, according t o the literature, the success of the method is unlikely @,la). The direct analysis of feldspar powder was investigated as follows. For the determination of major constituents, standards were prepared by weighing appropriate amounts of the oxides or other compounds of the elements of interest (SiOz, A1203,Kh’O,), mixing with 25% binder (1 to 1 starch, stearic acid), and pressing into pellets at 20,000 p.s.i. Calibration curves were drawn from data obtained by irradiating each of the standards. A feldspar sample was analyzed a t the same time with the following re-
Table VI.
Effect of Sample Position Net Intensity, AlzOa, % Posi- IL - I B , Concn., Std. dev., tion Counta/Sec. C U 1 2 3 4 5
106.0 106.5 105.9 106.8 105.6
16.66 16.72 16.64 16.76 16.60
0.064
Table VII.
Detn.
1
2
Effect of Sample Rotation 3
4
5
Av.
0
Concentration, % A1,03 Stationary Rotated Reloaded
2.38 2.19 2.07
2.14 2.22 2.09
Stationary Rotated Reloaded
0 0303 0 0393 0 0387
0 0385 0 0395 0 0397
2.17 2.18 2.07
2.37 2.19 2.14
2.07 2.22 1.96
2.23 2.20 2.07
0 14 0 04 0 07
0 0385 0 0397 0 0403
0 0389 0 0395 0 0395
0 0004 0 0001 0 0006
Concentration, % ’ CaO 0 0391 0 0395 0 0393
sults: 25.5% AI&, 51.0% Sios, and 18.8% KzO as compared with the vendor’s values of 17.501, Al2O3, 86.1% SiOa and 10.9% KzO. These results indicate that gross errors are introduced by such factors as segregation, surface irregularities, particle size variations, and packing density. RESULTS
Precision. INSTRUMEKTAL STANDDEVIATION. -4standard deviation for a determination based on a single measurement was calculated for each element. Rlaterials representing the most unfavorable conditions were selected. For major constituents, five measurements were taken a t one time on a sample disk of each material, and t h e standard deviation was calculated using a n For established calibration curve. minor constituents, five alternate measurements were taken on each sample and its standard and the concentration of the element was calculated for each measurement. A standard deviation was obtained from the five concentrations. Since this involves removing the sample and standard rings after each measurement, the possibility of slight damage to the surface of the powder is increased. I n addition, since the total exposure time is considerably longer than that used for the major ,constituents, instrumental variations are possible. This partly explains why the standard deviation values for A1203 and K20 (Table 111) determined as minor constituents are much larger than when these same oxides are determined as major constituents. REPRODUCIBILITY OF SAMPLEANALYSES. A useful evaluation of the method used for major constituents is obtained from the data in Table IV compiled as follows. For each mineral used in the fabrication of forsterite, a “reference” material was obtained which mas known to produce good ceramic wafers. Each material was cast into a glass disk in the usual manner and analyzed for major Constituents repeatedly over a period of many months. Results of these analyses for two typical ARD
0 0391 0 0395 0 0380
materials, feldspar and talc, given in Table IV, indicate that the ~ C ~ ~ O ~ L ibility of sample analyses is good. ANALYTICALREPRODUCIBILITY OF FIVESAMPLES O F SAME MATERIAL.The reproducibility of analysis from sample to sample of the same material was established as follows. Five sample disks from the same batch of “reference” feldspar were prepared and analyzed for A1203, SiOz, and KzO in the usual manner. Five powder sampIes from a batch of “reference” talc were analyzed for & 0 3 and CaO by the standard addition method. Results for each of the five analyses are shon-n in Table V. EFFECTOF SAMPLEPOSITION. An important factor affecting the reproducibility of the method used for major constituents is the homogeneity of the final glass disk. Because the sample-toflux ratio is extremely low, i t is possible, especially in the preparation of the standards, to obtain a glass nith an uneven distribution of constituents. To determine whether such segregation occurred, five intensity measurements were made for A1 on the 17y0 &03 standard. Between readings, the disk was removed and repbitioned. The intensity values were then applied to a calibration curve, and the concentration of A1203 was derived for each of the five positions. Table VI shows that sample position has little effect on the intensity measurements and, therefore, that the disks are homogeneous. EFFECTOF SAMPLEROTATION. Segregations in powder samples cause serious analytical errors. Rotation of the sample minimizes this error. I n the methods described, rotating powder samples are used for the determination of minor constituents. T o establish the error introduced by segregation in the powders, a sample of talc was analyzed for AlZOaand CaO five times b y each of three methods: (1) The sample was measured while stationary, but rotated between measurements. (2) The sample was rotated continuously during measurements. (3) The sample was rotated continuously during measurements and reloaded into the holder between measurements. The results shown in Table VI1 indicate VOL 34, NO. 10, SEPTEMBER 1962
1265
I C -
that the most reproducible results are obtained when the sample is rotated continuously during irradiation, as in the second case. Accuracy. T h c Xational Bureau of Standards has available a variety of minwals a n d ores for which t h e comF)osition is accurately known. From its list, a clay standard (NBS S o . 9 s plastic clay) was selected which nearly duplicates the composition of the mineral used in this work. A feldspar standard (NBS KO.99 soda feldspar) was selected in which the K20 present in the feldspar used is replaced by NasO. Because of this difference in matrix, analysis of the NBS feldspar for major constituents by the method described previously would be invalid. However, the method of standard addition for minor constituents can be readily evaluated using both these standard materials. Data for NBS feldspar and clay are given in Table VIII. I n some cases the accuracy is poor and the deviation between x-ray and chemical values is much greater than the over-all x-ray method reproducibility. KO comprehensive explanation can be offered a t this time. I n one case (A1203 in clay), the composition of the NBS sample is outside the range of the calibration curve and extrapolation of the curve was necessary. I n other cases where the standard addition method was used, errors were most likely introduced in the physical treatment of the sample and addition of somewhat nonoptimum amounts of the elements sought. Further refinements would undoubtedly improve the analytical accuracy. However, the precision and accuracy requirements for this particular problem
Table VIII.
are not rigorous and the results obtained by these methods are entirely adequate. Vendors generally supply a chemical analysis along with a shipment of materials. KO information is given about the analysis, and the reliability of these figures is often questionable. Nevertheless, because no more reliable data are available, this information is also included in Table IX. Table IX compares the vendor's chemical analysis of each material with typical x-ray analyses by the methods described. DISCUSSION
Spectral Interferences. Spectralline interference can be a serious problem in t h e analysis of light elements because of high-order Bragg reflections of heavier elements. The interferences encountered in t h e work described were limited to one material-forsterite, which contains BaO. Third-order Ba La(% = 141.99') interferes with first-order A1 Ra(% = 142.44'). Pulse-height discrimination is the most effective means of eliminating this Ba interference. Ba La(28 = 87.13') interferes with T i Ka(28 = 86.12'). Because these lines are both first-order lines and unfavorable concentrations exist (0.08% TiOz, 7.5% BaO), there are no practical means for eliminating this Ba interference. With the chromium target tube, a n additional problem is encountered. A fourthorder Cr K p line (28 = 142.48') interferes with first-order A1 Ka(28 = 142.44'). The only possible means of correcting this condition is to use a pulse-height analyzer. However, according to Liebhafsky, Pfeiffer, Winslow, and Zemany ( 8 ) ,interference from
Comparison of NBS and X-Ray Analysis of Feldspar and Clay
.4l2O5, Material Soda feldspar, NBS 99 Plastic clay, NBS 98
Table IX.
Material Forvterite Feldspar Clay Talc MgO
Analysis Nominal X-ray Vendor X-ray Vendor X-ray Vendor X-ray Vendor X-ray
Analysis
%
KBS
X-ray
... ...
IiBS
25.5 26.5
X-ray
SiO?, %
KQ,
%
?4
... ... 63.8 63.9
... ...
0.36 0.28 0.21 0.19
Ti02, Fe205,
9 6 %
... ...
0.067 0.066 2.05 3.19
1.43 1.32
Comparison of Vendor and X-Ray Analyses" Al2O3, MgO, Fe2O8, CaO, IGO, TiO,,
SOn, %
40.2 40.9 68.3 68.6 59.7 60.4 61.6 63.6 0.93 0.87
%
%
%
3.87
46.2 47.5
17.5 16.5 35.3 34.2 1.8 2.0 0.16
... ...
0.56 0.54 0.09 0.11 1.14 1.21 0.82 0.80 0.2 0.08
...
...
0.3
...
35.4 35.0 96.2 ...
Vendors' values corrected for ignition loss. 1266
3.57 3.17
CaO,
ANALYTICAL CHEMISTRY
%
0.44 0.21 0.20 0.34 0.35 0.37 0.38 0.06 0.86 0.74
%
0.85 0.50 10.9 10.4 1.20 0.98
... ...
...
..
fifth and higher order lines may be neglected. Because this interference is caused by a relatively weak fourthorder KB line, i t was expected to be insignificant, and experiments proved this assumption to be true. Absorption and Enhancement Effects. Absorption and enhancement effects (interelement, matrix effects) which arise within t h e sample are among t h e most serious for quantitative work. Scott (IS), among others, claims that ceramic materials are especially prone to deviations due to matrix effects and that careful sample preparation is necessary. His results for the SiOz-Alz03 system show marked absorption and enhancement effects. I n the present problem, matrix effects are relatively insignificant because analyses have confirmed that major constituents occur in relatively narrow concentration limits, and the accuracy requirements are not rigorous. Where accuracy requirements are high, corrections may be applied based on a variety of techniques such a s addition of an internal standard, use of absorption coefficients, either mathematically ( I 0 ) or by establishing a family of curves ( I l ) , and, in the fusion technique, increasing the absorption coefficient of the flux ( 7 ) . The borax fusion technique used for the major constituents involves dilution of the matrix with borax, which also tends to minimize matrix effects. Interelement effects are compensated for when the standard addition method is used for minor constituents. The most troublesome interelement effects were investigated. Standards containing the same amount of the element of interest, but increasing amounts of the interfering element, were prepared and analyzed. It was observed in these cases ' that matrix effects do not present a serious problem. Standard-Addition Method. The standard-addition method involves correlation of intensity d a t a obtained from a sample before and after addition of a small increment of the element of interest. The simplified equation derived below is used t o perform the necessary calculations (see Figure 1).
BaO' %
%
0.08
X - = -
x
7.5 7.2
...
x
. . . . . .
. . . .
. . . .
. . . .
. . . .
+ Ry
- RX = Ry
~ ( -l R ) R = y
..
..
. . . .
'I = R I=+,
x = Rx
......
1.78 1.80
+Y
x = - R, 1 - R
where x
=
y
=
weight of oxide in 10 grams of sample weight of oxide added to 10 C grams of sample
I,
net intensity of element before addition Is+# = net intensity of element after addition R = ratio of net intensities of element before and after addition Composite C = yl yz yr+. .. =
+ +
Care must be taken to add an appropriate amount of the element of interest. If too much is added, the relationship loses its linearity and erroneous results are obtained. If too little is added, experimental errors may be large. For optimum results, the amount of added element was chosen so that the value of R in the equation was between 0.7 and 0.8. Although the correct amount can be determined only by trial, a general rule is to add about 25% of the expected amount present. For most accurate results, the line intensities from the elements of interest are measured on the same sample before (10 grams) and after addition of C grams), and a the composite (10 dilution factor is applied in the calculations. However, this method involves a time lapse of possibly several hours between measurements on the sample and its standard, during which the instrument may not remain stable. Therefore, because of instrumental variables and for increased simplicity of operation, it is more convenient to assume that the weight of the sample k
+
equal to the combined weight of the sample and the composite mixture, and to measure the intensity of the sample and its standard at the same time. To justify this approach, the powder sample and ita standard must be packed into the Lucite rings in such a manner that the packing densities of each are about the same. This is accomplished by adding C grams of composite to 10- C grams of sample. If this is done carefully, the error introduced by the equation is small, because the two rings contain nearly identical amounta of sample, one of which contains known excess amounts of the elements of interest. LITERATURE CITED
(1) Anderman, G.,ARL Spedrographer’s News Le& 13, No. 3, 1-3 (1960); Proc. 9th Ann. Cmf. A p lications of X-Ray Analysis, Denver, 8olo., 1960 (2) Applied Research Laboratones, AR’L Spectrographer’s News Letter 7, No. 3, 1-3 (1954). (3) Baird, A. K., MacCall, R. S., McIntyre, D. B., 10th Ann. Cod. Applications of X-Ray Analysis, Denver, 1961; A&. X-Ray A d . (to be published). (4) Berth, E.P., Longobucco, R. J., 10th Annual C o d Apphcations of X-Ray Analysis, Denver, 1961; A&. X-Ray Anal. (to be ublished). ( 5 ) Campbell, J., Carl, H. F., ANAL. CHEM.26, 800-5 (1954). (6) Carl, H. F., Campbell, W. J., ASTM Spec. Tech. Publ. 157, 63-8 (1953). (7) Claisse, F., Norelco Replr. 4, 3-7, 17,
%.
x Figure 1.
x+y
Standard addition method
19, 20 (1957); Province of Quebec, Canada, Dept. Mines, Rept. 327 (1956). (8)Liebhafsky, H. A., Pfeiffer, H. G., Winslow, E. H., Zemany, P. D., “X-ray Absorption and Emission in Analytical Chemlstry,” p. 218, Wdey, New York, 1960. (9) Maneval, D. R.,Lovell, H. L., ANAL. CHEM. 32, 1289-92 (1960). (10)Meyer, J. W., Zbid., 33,692-6 (1961). (11) Mitchell, B. J., Ibid., 33, 917-21 (1961). (12) Neff, H., Togei, K., Siemens and Halske AG, Karlsruhe, Germany, Rept. SH8203 (1959). (13) Scott, R. K., Fall Meeting, Refractories Div., Am. Ceramic SOC.,Bedford Springs, Pa., 1956; Pittaburgh Conference on Analytical Chemistry and A plied Spectroscopy, Pittaburgh, Pa., d r c h 1957. RECEIVEDfor review February 5, 1962. Accepted July 5, 1962.
Funda menta Is of Coulostatic Ana lysis PAUL DELAHAY Coates Chemical Laboratory, Louisiana State University, Baton Rouge, La.
b Fundamentals are discussed for an electroanalytical method in which POtential-time variations are determined at open circuit after abrupt change of the charge on the electrode. The most favorable range is approximately 10-6 to TO-’ mole per liter, and the method appears applicable to any substance which is reduced or oxidized under polarographic or voltammetric conditions. Methodology is briefly covered.
A
ELECTROANALYTICAL method is offered, based on a principle reported in a recent communication ( 1 ) . (However, for priority of idea, see this issue, page 1344.) The principle is as follows: The charge on the working electrode o & m electro-chemical cell is changed-in a short time (a few tenths of a microsecond to a few milliseconds) with an instrument which allows flow of the charging curN
rent but prevents %ow of the reverse current. Charging is achieved, for instance, by discharge of a capacitor, initially charged a t a known voltage, across the electrochemical cell (cf. Methodology). The potential, E, of the working electrode, before charging, is a t its equilibrium value or is set a t a value for which the faradaic current is negligible. The charge supplied to the working electrode is such that it brings E in a range in which there is an appreciable faradaic current. The cell is a t open circuit, for all practical purposes, after charging, and the faradaic current is entirely supplied by discharge of the double layer capacity. The potential tends to return to its initial value before charging as the double layer capacity is progressively discharged. Potential-time variations depend on the double layer capacity and the magnitude of the faradaic current. Since the faradaic current generally
depends on the concentrations of the substances involved in the electrode reaction, these concentrations can be determined, in principle, from the potential-time variations. This method was originally developed for the study of adsorption kinetics (6, 6) and electrode (I,3) processes. The expression “coulostatic method” was coined (6) for application to adsorption kinetics because the charge on the electrode remains constant during recording of potential-time curves. The double layer is discharged by the electrode reaction in analytical applications, and the expression “chargepulse method” would be more accurate. However, there hardly seems a need for two different expressions according to the applications of the method. The coulostatic method is related to the classical interrupter method for the study of electrode kinetics (7) in which potential-time variations are deterVOL 34, NO. 10, SEPTEMBER 1962
* 1267
