960
ANALYTICAL CHEMISTRY
This method has been used in the authors’ laboratories for the routine assay of tablets containing aspirin, phenacetin, and caffeine for about a year, with satisfactory results. LITERATURE CITED (1) Assoc. Offic. Agr. Chemists, Analysis,” p. 675, 1945.
“Official and Tentative Methods of
( 2 ) Green, Corbin, and Powers, paper presented before Scientific Section, American Pharmaceutical Association, August 1948. (3) Green, hl., and Green. N., J . Am. Pharm. Assoc., A36, 235 (1947).
(4) Holt, K. E., Ibid., 35, ? I (1946). (5) Mattocks, A. M., and Hernandez, H. R., BuU. .VafZ. Formulary Cornm., 18, 113 (1950). (6) Parke, T. V., Ribley, A . A I . , Kennedy, E. E., and Hilty, W.W., ANAL.CHEM.,2 3 , 9 5 3 (1951). (7) Washburn, W. H , and Kreuger, E. O., J . Am. Pharm. Assoc., 38, 623 (1949). ( 8 ) Wilson and Hilty l b i d . , 35, 97 (1946).
RECEIVED August 1 7 , 1950
Presented at Meeting-in-Miniature of the Metropolitan Long Island Subsection of the New York Section, AMERICAN CHEMICAL SOCIETY, March 17, 1950, Brooklyn, S . T.
Analysis of Aluminum and Aluminum Alloys Using Pin Samples Direct-Reading Spectrochemical Methods R . W. CALLON AND L. P. CHARETTE .4luminium Laboratories, L t d . , Arvida, Quebec, Canada The development of direct-reading spectrochemical methods using an ARL research model quantometer with high precision source for the quantitative analysis of aluminum and aluminum alloys was undertaken in order to obtain more rapid and accurate, as well as cheaper, methods than those using photographic methods. Analytical methods were developed utilizing pin samples 0.25 inch in diameter, which provide improved precision and accuracy, coefficients of variation of the order of 1% for elements greater than 5%, and coefficients of variation
T
HE principles and advantages of direct-reading spectrometers, utilizing photomultiplier tubes instead of a photographic emulsion as the recording medium for the analysis of metallic samples, have been discussed ( I d ,8). As it was felt that this type of instrument would be useful in some of the analytical laboratories of the Aluminum Co. of Canada, Ltd., and obher companies of the Aluminium, L a . , group, experimental tests were run in 1947 on the two spectrometers commercially available at that time. At the end of these tests it was decided that the ARL ,research model quantometer would be more suitable for the work. REQUIREMEhTS
Adaptability to Development Work. In such work it is a distinct advantage to be able to experiment rapidly with an almost unlimited number of spectral lines, as very little information has been Dublished on direct-reading- methods for the analysis of a l u m h m and its alloys. Flexibilitv. The instrument had to be sufficiently flexible to allow deveiopment and routine work to be carried- out during alternate shifts. Accuracy. The instrument had to provide the highest possible accuracy, so that elements of the order of 10% or higher might be determined satisfactorily. Efficiency in Routine Analysis. The instrument had to be capable of analyzing daily 1000 or more samples of commercia1 purity aluminum plus several hundred alloy samples. Reliability. The instrument had to be designed so as to facilitate diagnosis and repair in case of failure, as it had to carry a large portion of the analytical load. The ARL research model quantometer and high precision source unit ( 6 )which were installed in the analytical laboratory of the Arvida Works of the Aluminum Co. of Canada, Ltd., are
of 1 to 29’0 for elements in the range 1 to 49’0, with satisfactory precision for lower percentage elements. Satisfactory limits of detection were obtained for all trace elements which are of interest; in general, these limits are better than those obtained previously by photographic methods. By making minor modifications to the instrument, including the use of a two-position, water-cooled electrode stand, the rapidity with which analyses can be made is approximately doubled over the most efficient photographic methods previously used.
shown in Figure 1. In this particular laboratory, 1000 to 1300 commercially pure aluminum samples and 100 or more alloy samples are analyzed daily for seven to eleven elements. The recording of results is simplified in that, for a large percentage of the aluminum samples of commercial purity, only two t o four elements are actually recorded, the rest being controlled a t an insignificantly low level. The instrument was calibrated quickly for the analysis of commercial purity aluminum, so that it began to pay for itself within a few weeks of the completion of the installation. Bttention was then directed to the development of methods for the analysis of alloys, where the accuracy requirements are more critical. The following high percentage elements were of particular interest to this plant: silicon (4 t o 14%), copper (3 to 8%), magnesium (1 to lo%), nickel (1 to 3%), iron (0.5 to 4%), and manganese (1 to 2%). It was felt that, if a satisfactory compromise betxeen the experimental conditions could be reached for these percentages, satisfactory values would be obtained for elements present in lower percentage. CHOICE OF SAMPLE FORM
.it the outset it was realized that in order to limit the scope of the program a choice of sample form was necessary. In the past two pin samples 0.25 inch in diameter were used for all routine spectrochemical work, whereas most American workers in the aluminum field have preferred a disk sample versus a graphite rod.
Advantages of Two-Pin System. A lower limit of detection is obtained for most elements. An average result for two samples can be obtained by using one electrode from each, This technique has been used to improve
V O L U M E 2 3 , NO. 7, J U L Y 1 9 5 1
961
Table I.
ite rod: In Canada t h i cost of high purity graphite is be?w&n 1 and 2 cents per exposure, which amounts to around S3000 a year for the ArGids. Wb'orkslnborat.ory. Unless the graphite rods m e carefully seleoted, significant. errors can he introduced from variations in their thermal and/or e l e c t r i d conductivity. In Some eases a t least, it is easier to obtain samples free from porosity. Interfering effects from molecular spectra of carbon compounds are avoided. Advantages of Disk Type of Sample. Accurate alignment of the electrodes is easier. However, little difficulty has been experienced an this score when using pin samples in the two-position, water-cooled electrode stand discussed below. Repeat analyses oan he made more rapidly on disk-type samples when it is satisfactory to use the same machined surface. It has been stated that the graphite counter electrode reduces oxidation and metallurgical effects. Fabricated materials are more readily compared to standards when using a counter graphite system. Thia faator is not of particular importance in a reduction plant. Less difficulty is experienced with surface oxidnt,ion effects. ~
Accuracy, however, is usually the main consideration and, in general, the same over-all t~eouracyhas been claimed in the litem ture for both type.? of samples when using photographic methods. Similar data. are not available for direeereading spectrometers. Difficulties associated with the preparat.ion of standard samples will vary with the problem. After due consideration, the use of pin electrodes was eontinued, partly for the rezsons given above, partly because all the company's laboratories were equipped with accessories for t,his type of sample, and partly because of the availability of a suitable range of standards in that farm. Precautions. The most important precautions that must be ohserved when using pin electrodes are: The samples should be free from porosity. This e m be %ecomplished by using hook-type steel molds with a controlled rate of cooling along the length of the sample. Samples cast by suction in graphite, borosilicate glass, copper, or steel tubes have not been found, in general, t o be as satisfactory. Solid ejector-type molds are very useful in certain case8, a6 the diameter of the sample is more easily controlled. They must be free from fins, for accurate alignment, and of uniform diameter. Samples are controlled at 0.250 * 0.005 inch. They must he free from either real or apparent segregation along their length. A few elements such as lead, bismuth, and
A B
C D E F G H 1
Variables to Be Studied in Development of Alloy Procedures Internal standard line Spectral line 0. alloying element Bimary or secondary voltage of spark soiiroe Induotame in series with analytioal gap Duration of prespark period Duration of expmure Type end diameter of sample Electrode spacing Optical variables and portion of analytical gap utilized
Table 11. Reproducibility for Silicon (12970)in A l u m i n u m 12% Level
AI 2568 A. Si, A. 2516 2882 2988
cv* 1.5 1.8 0.9
AI 3066 A.
dfa
DV
100 100 200
1.7 2.0 1.0
df 100 100
3w
.413944 A. DV
df
2.0 2.4 1.2
1M) 100 100
Coeffiaient ai variation. defined BS ratio of standard deviation to amount presant, expressed 8s B .,eroentage. b Degreea of freedom a
Experimental conditions Inductance 720 ph. Primary voltage 160 volts Spark gap 2 mm.
Preswrk
E x p ~ u r e(approx.)
4 seoondr
25 seoonda
hours of machining. The surface must be smooth and clean-cut, the lathe tools being arranged so that no center tip remains. The electrodes should have a reuroducible chamfer of aonroximstelv '/u inch. The machined surface mu& not corn; into contict with t h e analyst's fingers, as this will cause sodium contamination and promote oxide formation. The electrode surfaces must he accurately aligned, with B constant separation. This DOint is discussed in dealing with electrode stands DEVELOPMENT
OF ALLOY METHODS
The parameters which were considered to he the most important in the development of a method when using the ARL high precision source unit are listed in Table I. Arbitrary restrictions were made for some of these variables from visual observations of typioal spark discharges. Factors considered were steadiness of discharge, tendency to spark t o the sides of the electrodes, tendency to flame outward, and evenness of etch. In general, the spark became less steady after a total discharge period of 30 to 35 seconds, thus limiting factors (E) and
(F).
Figure 1. ARL Research Model Q u a n t o m e t m and High Precision Sowree U n i t Installed in Arvida Works Laboratory
As the optimum electrode diameter will depend, to aome extent, on the electricd conditions and as samples 0.25 inch in diameter had been found most suitahle for photographic work, this diameter was chosen, thus eliminating (GI. At a later date some limited tests using different diameters were made for silicon at t h e 12% level; maximum reproducibility was obtained, fortunately, with the samples 0.25 inch in diameter. A s some leeway was desired for the over-$1 exposure level, the grating aperture was left at SO% of full aperture. With this setting the gap length utilized was slightly more than 1.9 mm. of the central portion of the discharge. The entrance slit was set at 50 microns and all receiver slits a t 100 microns. As other optical variables such a8 the focal length of the external lens were arhitrarily fixed, factor (I) was eliminated. Even with these limitations, a large number of tests were required in order to determine optimum conditions. Fortunately, some cornhinations of the other parameters were also discarded om the hasis of visible or audible unsteadiness of the spark discharge. By studying simultaneously a8 many lines of aluminum and an allaying element as possible and restricting other parameters ta
ANALYTICAL CHEMISTRY
962 :ii,ound three to four Ievc~ls,the s(:ope of t,he tAxperiment required for e w h element, was kept within reasonable bounds. The hest combinatioris of parameters, as dt.terniined from thcw precision tests, were thcm rhecked more th t c c u r x y \vas obtained \\.it11 the line pair AI 2668 A . 4 2882 A. with ric'st hwt results obtained with .\I 2.568 A.-Si X ) I l i A. In that work an inductanw of 360 PI!,, :i primary voltage of :tround 250 volts, and :LII :tnalytic.ai gap of 3 mm. were used. Whrn using pin samples thc results \vert> { i i f h e n t , as shown in Figurrl 2. For comparison purpose8, t,wo intliwtance values and three :iluminum lines are shon-n for both :tluniinum of commercial puritJ- and :I 12% silicon alloy. A linear rtblationship was not, obtxined for :in inductance of 360 ph., but wari obtained for 720 pli., the inductanw that eventually was 1'iiund to give the most precise resultF. It, does not' newssarily t'ollow, however, that a. lintw rthtionship is requirrd for thta high(,st precision. With regard tn prwision, the best rwults oht,ained from mmbinations of tlrr follo\ving silicon and ~lurninunil i n ~ s Si 2217, 2435, 2507, 2516, 2882, 2988, 4552 A. A1 2373, 2568, 2852, 3066, 3093, 3944 A . givrn in Tahle IT. A1 2568 .I. : t i i d 3066: A. nxsre hoth eatisL'iwtory as the internal st:ind:ird lint., d i t w a . Ai 2988 4.gave decaidedly superior results. Si 2516 A. gave approximately the saini: reproducibility in iaounts as Si 2988 A. but had H less favorable analytical slope. Si 2882 -4. gave relatiwly poor rwults. The best resu1t.s were obt ; t i i d with different sourw conditions, t,he spark gap heing 2 nim., inductance 720 hh., and primary volt'age 160 volts. \'ariation in the prespark period from 0 to 10 seconds had little effect on the coefficients of variation, but, wit'h tot,al sparking Iwriods of over 30 seconds the spark discharge tended to become irregular and the coefficients of variation to increase. Because luring the first few seconds the number of countv was relatively high and was above tht. rwoniniended rate of operation of the count,ers, a prespark of 4 scwnds and an pxposure of around 25 .+ec:ondswere chosen. In more recent work 700 routine casts of 12% silicon alloy were :tidyzed in duplicate on the quantometer in parallel with the chcamical laboratory, over :L period of 4 weeks. I n such parallel :tiidyses there are a number of Iources of error which are not in(.luded in precision tests suc,li :is thoRe reported in Table 11.
Voltage
The first five sources I J ~t'rror rtin I)(' niininiiztd to a point at tvhicth they are negligihlr in rompirison t o the o t h r m , assuming, of course, that maximum r~irc? is c~rrc~rcired at, all stages of the m a lytical procedure. Thr sixth error, variation in the percentage of othrr el(~nients, cau be important, in t,his alloy if sodium is present. .-in incrcxse in the sodium content causes an apparent increase in the percentage of silicon. -1st,he range of sodium value? encountered is usually narrow, and ii suitable correction can be applied if wid$.variations exist,, thiF leaves only errors in chemicd :in:ilysis :ind the inherent error of tlie quantonietrics analysis. The r e s u h obtained in parallel determinations of silicon are summarized in Table 111. In general, the duplicate analysis waa obtained about 4 hours after the original analysis, the instrument having heen standardized every hour.
:tw
Setting-up errors. T1it:sr include both the error in making wnsitivity and tab adjustments and errors in determining the (exact calibration curve a t levels other than those of the standards. Drift of the instrument bet,\veen standardizations. Bias due to incorrect values assigned to the standard samples. ude variation in tlie metallurgical 8truct.ure between and routine r:miplrs. ation in real o r :ipp:\wnt (tomposition in different sections of the st,andard. ling error, in bot11 clic~niicaland spectrochemical samples. neous element effects. 'or? in chemical :iii:iI
Table 111. Comparison of Chemical and Quantometric Values for Silicon in Alcan 160N Alloy (127, Si)
Coefficient of Variation, 70 0.9
Quaxitometer reproducibility Av. of duplicate quantometric values 2 8 . original 2.4 chemical value A>-. of duplicate quantoiiietric values 11s. checked chemical valuea 1.3 First quantometric value us. checked chemical 1.4 valuea No. of determinations 700 Internal standard line, A . AI 3066 a Chemical values checked whenever they differed by more t h a n from average quantometric values.
Bias, 70 Si
..
0.Wl 0.024
0.027
0.20%
Thus, the coefficient of variation included setting-up errors and instrument drift error as well :as the quantometric error. The coefficient of variation of 0.9%, as computed from the duplicate quantometric values, agreed very well with the results of the rapid reproducibility tests, shoxving - that the standardizaton 100 w errorP were negligible. (Coeffiricxnt of variation = __ X 1.128 %SI \There ii is the average difference between duplicate values.) The coefficient of variation of 2.4% between the chemical value and the average of the quantoinrtric lesults was only fair, but was reduced to 1.3q/0by checking chemical values that appeared out of line. Finally, the coefficient of variation was calculated, assuming that only the first qumtometric value was :tvailable. This
V O L U M E 23, NO. 7, J U L Y 1 9 5 1
963
gave a value of 1.4% of the amount present. As the last two values still included most of the normal chemical error of routine control work and a small bias, this showed that the results of rapid precision tests could br approached in long-term routine analysis. All these results for silicon were obtained using a regular rteel book-type mold without a controlled rate of cooling along the length of the electrode. As this type gives some porosity, and as the largest deviations in the quantometer results are obtained for samples shoxing porosity, it is expected that still lower coefficients of variation R-ill be obtained using samples from a newer type of mold which eliminates this source of error. Some points of interest were observed in the analysis of “high copper” alloys. As shown in Figurr 3, rather surprising shapes were obtained for the analytical curves, an inflection point being observed at copper contents above 12%. S o practical disadvantage resulted, however, asall commercial alloys have less than 10% copper. Figure 4 shons the variation in integrated radiant power for both aluminum and copper with increasing percentages of copper when time-controlled exposures are used. This explains at least qualitatively the shapes of the curves shoim in Figure 3.
Table IF’.
Keproducibilitj- f o r Copper (.5.Oqc and in Aliiminum
108 1.0 0.9 108 1.0 51.53 0.9 0.9 108 5218 0.8 Experimental conditions Inductance 720 fill. Primary voltage 160 volts Spark gap 2 nun.
2824
108 108
0.95 1.3
212
108
1.1
104
104
Prespark Exposure (approx.)
0.95 1.1 1.1
i.57~)
112 104 104
4 secuud+ 2 5 second5
Table \.. Reproducibility for Magnesium ( S S % and 9.5%) in .4luminuni 5 . 5 % Level -dl 2568 A . -41 306K-x. SIg, -1. cv df CY df 1.7 100 1.6 100 5184 1.2 100 1.1 100 5528 Experimental conditions Inductance 720 r h . Primary voltage 160 volts Spark gap 2 mm.
9.5% Level 4 1 3066 ~i.. cv .if 1.9 160 1.2 213
A1 2568 A. cv df 1.7 160 1.2 160
Prespark 4 sevonds Exposure (approx.) 25 s e i : o n h
H r i e again. mtended t w t b were ruii t o determine the best combination of the variables, which was found to coincide with that foi silicon. The beet results from the reproducibility trets using thew csonditions find thr following sprctral1inr.q
TIME, SECONDS
Cu A1
2192, 2247, 2824, 5106, 5153, 321%, 5782 A. 2568, 3066, 3093, 3944 A.
are sho\%nin Table 1V. CU 2824 A. giver more repiodurible result-q a b the percentagc. is increased, and it is probable that this line would be preferable when determining high percentages of ropprr. As the wnie wurce conditions were found suitable for both copper and silicon, thew conditions were tepted for magnesium using combinations of the follon-ing lines: INTEGRATED RADIANT POWER, INTERNAL STANDARD CONTROLLED COUNTS
Figure 3.
Analytical Curves for Copper in Aluminum
Cu S P l l b
-.-
INTEGRATED RADIANT POWER, TIME-CONTROLLED COUNTS
Figure 4.
Variation in Aluminum and Copper Integrated Radiant Power us. Copper Content
Mg 2780,2791,2803,2852,2929,3332,3337,
A1
3832, 3838, 4481, 5173, 5184, 5528 A 2568, 3060,3944 A.
As shown in Table V, satisfactory r r w l t i were obtained for some line pairs; $0 no other source conditions v,Tere tested. The radiant powrrs of both aluminum anti magnesium lines for samples having high percentages of magnesium varied considerably-, rising to a maximum in about 5 seconds and then dropping to 30 to 40% of the peak radiant power in about 10 eeconds. I n order t o maintain uniform exposure conditions, a 4second prespark was used, even though this resulted in a high counting rate during the initial part of the exposure. Although this might have resulted in some jamming of thr, electromechanical counters, no difficulty of this nature was experienced. This work was extended to iron, muanganese, and nickel using the same source ronditions and the follon ing spectral lines: Fe hfn
2396, 2599, 3735, 3820 A. 2576,2594,2606,2673,2795,2801,2933, 2949, 3460, 4031, 5341 A.
964
ANALYTICAL CHEMISTRY
Xi
2254, 2265, 2270, 2287, 2438, 2473, 2633, 3134, 3415, 3434, 3493, 3515, 3525, 3858, 3975, 3994, 4402, 4459, 4714, 5477 A. 2568, 3066, 3093, 3944 A.
.41
The limit of detection is defined here as the percentage a t which the integrated radiant poffer, expressed in counts, of the spectral line plus the total background-i.e., spectral background and dark current or electrical noise-is 10% higher than that of the total background alone. These limits of detection depend on source conditions, ratio of sensitivity t o total background obtained with a given mutiplier tube, the percentage range covered with a given number of counts, and the width of the receiver slit, in this case 100 microns. The positioning of receivers plays an important part in the final selection of lines and was one of the main reasons for picking the line which is listed f i r R t for each element for most analyses. With this receiver arrangement, it is very simple to change from commercially pure metal to alloy anal+. -4satisfactory limit of detection is obtained for each element when the high precision type of spark discharge is uscd. The limits of detection are, in general, conservative estimates, because a criterion of 10% above total background is higher than is really necessary, and standards were not available at very low percentages in a number of cases; conPequently, no values below O . O O O 1 ~ o have been reported. At various times reproducibility checks have been made for each of the minor elements. Typical coeffirients of variation are also shown in Table VII. The values given for some of the low percentages are abnormally high because the data were not recorded to a sufficient number of decimals. While there is considerable difference between the elements-relatively large deviations usually being caused by a poor choice of line-they are all satisfactory from the practical viewpoint.
.4s shown in Table VI, satisfactory coefficients of variation were obtained for certain line pairs. Better values might possibly have been obtained using other source conditions, but the need for uniformity ruled out any further investigations. Comparing the results obtained in Tables 11, IV, V, and VI it may be seen that, when using the best spectrum line for each element, AI 2568 A. was significantly better than AI 3066 A. for nickel, while the reverse was true for manganese. S o significant differences were obtained for the other elements. .4s there was negligible difference in the over-all reproducibility, the h a 1 choice of the internal standard line Tvas determined by a number of secondary factors which favored .\I 3066 .$.-for example, the results of reproducibility tests for commercially pure samples obtained with AI 3066 A. as internal standard line were better than those obtained with AI 2568 -4. Similarly, it was found that accidental misalignment of the spectromet,er optics and extraneous elements had leRs effect on the nnnlytical rrwlts when -41 3066 .4. was used. ANALYSIS OF LOWER PERCENTAGE ELEMENTS
iVhile the lines chosen from t,he above work are satisfactory with regard to reproducibility at higher percent,ages, tht) are not necessarily satisfactory for the determination of these elements at, low percentages. Table VI1 gives data on the limit of detection and precision for lines considered to he satisfactory for low perwntages of these and other elenimts of interest,
I Y STRUMENTAL MODIFICATIONS FOR ROUTINE ANALYSIS
Reproducibility for Iron (3.5q"c), Kickel (2.8%), and Ilanganese (1.3%) in Aluminum
Table 1 I.
-
Iron, 3.5y0
df 2868 150
.I.
:iOM
2396 A .
1iO
2.4 2.8
2599 A.
3735 A.
2.1
2.0 2.1
2.0
Coefficients of Variation-Sickel, 2 8YL 3820 A. df 3134 A 3b58 .4. 1.9
100
2.2
100
Jlvprriinental conditions Inductance 720 p h . Primary voltage 160 volts Spark gar) 2 miii
1.9 2.4
Presparh Exposure (apjirm
-
1.6 1.9
7
Manganese, 1.3% 2933 A . 3460 A.
df
2673 A.
120 120
1.8 1.4
1.6
1.2
1.6 1.1
4 seconds
)
2.5 seconds
___'J'ableV11.
Limit of Detection and Reproducibility for Low Perrentage Elements in Aluminum Coefficient of Limit,of D et cc tion, A, A
5%
Z:MY
0.0001
3130
0.0001
Bi
3068
Cr
4254
Element Re
C U
I'e
a
~~
cvu,
%
7; level 0.00~ 0.02 0.002 0.02
3.2 5 1 4 5.5
0,003
0.010 0.049
4.3 1.8
0 .OOOi
0,006 0 038
3 2.1
0 010 0.045 0 010
4
3274
0.0004
3248
0 0008
373.5
Variation -~
0 002
Element Na
.
3.1
0 10 0 45
2 1.3
hlg
5184 2852
0.003 0.0001
0 0.12 0 031 0 042
1, r, 6.6 4.1
11n
2933
0.003
2594
0.0004
0,006 0.047 0.006 0.049
4 1.8 3.2 1.:
A.
5890
Si
3858 3415
0.010 0,0008
0.04 0.006 0.035
5 4.5 3.0
Pb
4058
0,002
0,005
6
2516
0.0008
2882
0,0007
0.10 0.23 0,095 0.25
1.9 1.5 2.6 1.9
3176
0.005
0.013
7.2
3686
0.001
0,005 0.04
3
3349
0.0002
0.005 0.042
4.6 2 1
5 2.7
0 045
A,
Coefficient of ~ i ~of ~ ~ i Variation t Detection, cva, % % level % 0.0001 0.006 4
1. .5
4379
0,001
0.015
2
Zn
3343
0.00'2
0.005 0.05
7 1.5
Zr
3392
0,0001
0,005
1.5
Coeffioient of variation using 48 degrees of freedom. Expprimental conditions. Internal standard line A1 3066 A . for all elements except for BI 3068 A. (AI 2568.4.) Prespark 1 second Inductanre 720 ph. 8.5 0.5 seconds Exposure Primary voltage 160 volts 2 mm. Receiver slit 100 microns Spark pap f
As the instrument had to be capable of carrying a large analytical load, some changes were necessary in order t o achieve the desired rate of analysis. First, the exposure time for analysis of metal of commercial purity was cut to a p p r o x i m a t e l y 8.5 seronds with 1-second prespark. The coefficients of variation given in Table VI1 were all obtained using these exposure conditions. In order to prevent jamming of the counters, a maximum recording cycle of 300 counts instead of 600 was used. A second reset tab was added to each tape in order t o cut the reset time from a maximum of 10 to 5 seconds. When analyzing alloy samples, the second tab is removed from the internal standard line tape and appropriate alloying line recorders. This arrangement can be improved by adding a switching system, so that in one position the second tab will not actuate the reset mechanism. Filters xere installed on all receivers, so that a greater range of light intensities could be covered without a major change in the attenuator settings, which in
V O L U M E 23, NO. 7, J U L Y 1 9 5 1
965 The plan view of the electrode stand is shown in Figure 6. The base is an aluminum casting; the horisontal mm. which rotates on ball bearings, is of polystyrene, as are the handle and various insulating spacers. The vee-blocks are of brass. drilled. ~.. for cooling water. and have sniine C I ~ ~ D B
Figure 5.
Fipure 6.
Two-Position Electrode Stand
Plan T-iew of Two-Position Electrods S t a n d
'turn would cause R ?hang
of the imam of the snilrk g a n h the grating. T a t e r leads &reof transparent Tvgon tubing, and are grounded near the center of the movine assemhlv to obviate any possibility if&&io&shoek through the water. The resistance from each vee-block t o ground is about 1.5 megohms with tap water flowing through the leads. A Lucite hood (not shown) over mast of the stand prevents the operator from accidentally touching high-voltage components and protects the discharge from air ourrents. Both polystyrene and steel spacers have been used for setting the electrode gap; the former are preferred. Very accurate alignment is necessary with this type of stand, Ibecauae standards may be run in one position and routine mmples in the other. Maximum tolerances of *0.001 inch were specified for the positioning of the samples. Even so, some asymmetry was observed. Careful adjustment and equalization of the r&istance to ground in the two water connections did not completely eliminate this trouble. Another difficulty was unevenness in the spark etch. .4ir jets provided some improvement, hut of a variahle nature. The problem was satinfactorily solved h y cementing small glass or plastic plates in the cavities behind the electrodes. These apparently altered the electrical field 80 8 9 to equalize the discharge and thereby provide a uniform etch on the electrodes. This also improved the symmetry t o 8r satisfsctory level far analysis of commercially pure aluminum samples and some alloys. Only one side of the stand is used for alloys containing high percentage elements. This stand has been used to analyze around l,OOO,OOOs&mplos and has not shown any signs of m.ear, except for the sliding contacts which are readily replace%ble. As a check on the uniformity of the results obtained with this strtnd, standard samples were analyzed as.routine samples each day for several weeks. No significant differences, using around 200 degrees of freedom, were obtained between the averages of the coefficientsof variation calci?I&d far each side of the stand and thosp obtained when all the results were grouped together-Le., the bias was negligible in all d entirely from standard, commercially available parts. I t has very good stability, sensitivity, and flexibility, as demonstrated by its use in a number of well-known analytical reactions. The availability and simplicity of operation of the apparatus should enable many analysts to apply it to their problems, and the capacitance and conductance data reported here should lead to a better understanding of the theoretical aspects of the method.
S
KVERBL varieties of apparatus have been described re-
cently for use of high frequency for the indication of chemiVal end points. The literature on this subject has been adequately reviewed by Jensen and Parrack (4), West, Burkhalter, and Broussard ( 6 ) ,and Blaedel and Malmstadt (a). The types of apparatus reported to date introduce a cell into one or more components of the tank circuit of a vacuum tube oscillator and measure the effect of changes within the cell on one or more parameters of the oscillating circuit-for example, Jensen and Parrack (4)measured the effect on the plate current of the oscillating tube. Anderson, Bettis, and Revinson ( 1 ) measured the effect on the grid current of the tube, and West, Burkhalter, and Broussard ( 6 ) measured the shift in frequency of the oscillating circuit. The authors have tried modifications of some of these schemes, but found that they lacked sensitivity, stability, or the flexibility desired. The present paper reports the use of an impedance-measuring circuit for direct determination of changes in the components of admittance, susceptance, and conductance, in a cell in which a reaction is being carried out. The admittance, 1', is defined by the relat,ion.
I' = G
+ iH
C'
b
CN
a '
"
--
POWER SOURCE
I
c
DETECTOR
R
d
I
Figure 1. Basic Diagram of Twin-T
where G represents the conductance, B represents the susceptance, and j is the operator. The susceptanoe is obtained from the capacitance, CO,by the relation
