ANALYTICAL CHEMISTRY
86 thalene (218" C.), it is relatively easy to separate a decalintetralin-naphthalene mixture into an overhead containing decalin and tetralin and bottoms containing tetralin and naphthalene, provided that there is sufficient tetralin in the sample to serve as chaser for the decalin. The method can be made applicable to
really a 3-component system, it behaves as a 2-component system with resp-ct to specific dispersion, because the two isomeric decalins sholv es9entially the same value for specific dispersion. Darmois ( 7 ) ]vas the first to point out, t,hat all saturated hydrocarbons, cyclic and noncyclic, and independent of molecular weight, have essentially the same specific dispersion and that the specific dispersion of hydrocarbons is a function of unsaturation. This fundamental fact has been Tvidely utilized in the analysis of petroleum hydrocarbons (8, 9, f0,19, BO, 21). ACKNOWLEDGMENT
The authors express their thanks to Dorothea Wagner for assistance in the experimental work. LITERATURE CITED
samples of low tetralin content by adding a known amount of tetralin to the original sample in sufficient quantity to serve as intermediate fraction between the overhead and bottoms. It was shown that the refluxing of tetralin and of a 50/50 tetralinnaphthalene mixture for the length of time required for the distillation did not change the specific dispersions significantly. Although the overhead (cis-decalin-trans-decalin-tetralin)is
(1) Adkins and Reid, J . Am. Chem. SOC.,63,741 (1941). (2) Anisimov and Poloeov, Khim. Tverdogo Topliva, 7, 982 (1936). (3) Bishop and Wallace, IND.ENG.CHEM.,ANAL. ED., 15, 563 (1943). (4) Block and Thomas, J . Am. Chem. SOC.,66, 1589 (1944). (5) Bruckner, Gas- u. Wasserfach, 5, 573 (1932). (6) Corson and Brady, IND.ENG.CHEM.,ANAL.ED.,14, 531 (1942). (7) Darmois. Compt. rend., 171, 952 (1920): 172, 1102 (1921). (8) Dixmier, Chimie & industrie, Special No., 338 (Sept. 1926). 29, 319 (1937). (9) Fuchs and Anderson, IND.EN&CHEM., (10) Grosse and Wackher, IND.ENG.CHEM.,ANAL. ED., 11, 614 (1939). (11) Hughes, Wilson, and Bosanquet, J . SOC.Chem. I d . , 55, 86T (1936). (12) Linstead, Millidge, Thomas, and Walpole, J . Chem. Soc., 1937, 1146. (13) Mair and Streiff, J . Research Natl. Bur. Standards, 27, 343 (1941). (14) Rossini and Mair, RefineT Natural Gasoline Mfr., 20, 494 (1941). (15) Schroeter, U. S.Patent 1,763,410 (June 10, 1930). (16) Scott, J . Research Natl. Bur. Standards, 25, 459 (1940). (17) Seyer and Mann, J . Am. Chem. SOC.,67, 328 (1945). (18) Seyer and Walker, Ibid., 60, 2125 (1938). (19) Ward and Fulweiler, IND.ENG.CHEM.,ASAL. ED.,6,396 (1934). (20) Ward and Kurts, Ibid., 10, 559 (1938). (21) Waterman and Perquin, J . Inst. Pdroleum Tech., 13, 413 (1927). (22) TVillstRtter and Seite, Ber., 56, 1388 (1923).
Spectrographic Analysis of Zinc-Base Alloys R. W. SMITH AND J. E. HOAGBIN, AC Spark P l u g Dicision, General Motors Corporation, F l i n t , Mich. A rapid spectrocheniical method has been developed for the routine control analysis of zinc-base die-casting alloys. By using self-electrodes and a controlled spark source w-ith proper choices of capacitance and inductance, quantitative determinations can be made of the tin, lead, cadmium, and iron impurities, as well as of aluminum, copper, and magnesium.
S
IXCE any analytical procedure is prescribed by the job to be
done, a brief description of zinc alloy melting and casting as practiced at the AC Spark Plug Division of General Motors Corporation will give a background for the methods and equipment adapted for analytical work. There are a t present two separate departments, each of which operates its own melting and die-casting equipment. The method of handling the molten metal is different in the two departments, however, because of differences in the construction of their die-casting machines. I n one department the metal charge is weighed up and melted in moderately sized iron pots, then cast into ingots and transported to the individual die-casting machines where i t is remelted as required in the small pots integral with each machine. I n the other department the metal charge is weighed up and melted in
Automatic operation of the spectrograph and spark source and the use of a simple calculating board allow a single operator to handle a considerable work load. The precision and accuracy to be expected from the method are indicated. .4description is given of experiments conducted to discover possible causes for the observed shifting of analytical working curves.
large iron pots, then conveyed in large ladles as required to be poured into the holding pots a t each die-casting machine. The difficulties involved in production control of composition by wet chemical methods may readily be seen. I n the department where ingots are cast, they must be segregated and held for 24 hours, the time normally required for a chemical analysis of aluminum and copper alone. Any rejected metal must then be classified and stored until it can be used up by realloying with new metal. I n the other department !There the molten metal is ladled directly into the die-casting machines, such a 24-hour delay would mean that an entire day's production of finished parts must be stored until a laboratory release is obtained. Furthermore, since wet chemical determinations of lead, tin, and cadmium are not feasible on a routine basis for the minute percentages involved,
VOLUME
9, NO. 2, F E B R U A R Y 1 9 4 7
I Figure 1.
87
SPARK
High-Voltage Controlled Spark Circuit for Analysis of Zinc Die-Casting Alloy
Large inductance used in lead, tin, cadmium, and iron determinations
Figure 2. Automatic Control Circuit for Timing Complete Operation Sequence Required for Recording Spectra of Test Samples
their presence in undesired amounts would be detectcd only by tests on the finished casting after many expensive operations had been performed on it. Failure to control the presence of these harmful impurities can cost the large producer thousands of dollars in an incredibly short time. Elimination of the possibility of undue harmful amounts of lead, tin, and cadmium in finished castings has in itself undoubtedly paid for the spectrographic installation several times over during the time this control has been in operation. The zinc-base die-cast metal is melted to the following specifications : Al, 3.5 to 4.370 Cu, 0.75 to 1.25% Mg, 0.03 t o 0.08% Fe, 0.10% maximum
Pb, 0.007~0maximum Sn, 0.00570 maximum Cd, 0.005% maximum
The analytical procedure desired was therefore one which would, in as short a time as possible and a t most in a n hour, permit determination of each of these elements with satisfactory accuracy. It should also be flexible enough to provide for checking additional or special samples for iron or for other impurities within a few minutes. I n addition, it should involve as simple apparatus and as little manipulation as possible, since it is desired to use only previously untrained help on the job, preferably only one operator per shift. LAYOUT AND EQUIPMENT
In order. to simplify the equipment and controls i t was decided to attempt to use a controlled spark (2) source for both major constituents and trace impurities, although it seemed probable that the circuit constants might have to be different for the t.wo types of determinations. It was early found that the most favorable conditions for the determination of aluminum and magnesium in zinc consisted of a combination of high capacitance and low added inductance. These conditions were not a t all suitable for the detection of impuritiei, however, as the sensitivity was poor and spectral background was too high in the neighborhood of the persistent lines. By using moderate capacity and very high values of added inductance it has been found possible t o reduce background to a negligible amount and yet maintain sufficient sensitivity to construct analytical curms down to 0.004~0lead, 0.00370 tin, and 0.00370cadmium. The circuit of the interrupted spark used is shown in Figure 1. The circuit constants are as follolvs: Transformer. 3 kva., 35,000 volts between high-tension terminal and ground Condenser. 0.005-mfd. nitrogen-filled capacitor Inductance. 0.17 mh. for A1 and Mg determinations, 7 mh. for Cu, Fe, Pb, Sn, and Cd determinations Primary series resistance. 24 ohms Analysis gap series resistance. 1 ohm Analysis gap shunt resistance. 0.8 megohm Interrupter gaps. 0.075 inch (1.9 mm.) Analysis gap. 0.115 inch (2.9 mm.) The 1-ohm resistor in series with the analysis gap was chosen arbitrarily as a value appreciably greater than any possible variable contact or conductor resistances in the circuit and one which would therefore introduce a definite and constant amount of damping. The 0.8-megohm shunt resistor across the analysis gap is used simply to make certain that the interrupter gap brcaks down first. Operation of the various controls during the making of an exposure is accomplished automatically by the use of solenoids and motors actuated by a mechanical timer. The control circuit is shown in Figure 2.
A Multiflex seven-circuit timer is arranged to move the shutter, move the lens, rack che plate, and switch the inductance, all in the proper sequence for a complete exposure. On the optical bench are mounted a shutter and quartz condensine- lens.. each movable bv a senarate solenoid into and out of the optical ax&. 16 addition, a solenoid-actuated high-voltage switch connects or disconnects the large 7-mh. added inductance. A small electric motor with reducing gear racks the plate down by means of a gear mounted on the hand crankshaft of the usual Bausch & Lomb plate racking mechanism. This motor is energized and de-energized by two sets Qf the timer contacts. Figure 3 is a block diagram showing the sequence of operations in a single exposure and the time involved for each.
T/Mf SL-CONB5
Figure 3.
Sequence of Operations Performed in Recording Sample Spectra
Before an exposure is made, the shutter and lens are out of the light path and the high-voltage switch is closed to short out the added high inductance. The plate holder is in position to record the first spectrum. When the main switch is closed
ANALYTICAL CHEMISTRY ing curve established by' the m a l y t i d results obtained with standard samples. The charmteristie curve is plotted and the percentages are read directly off the working curve strips in the usual wrt)
ZBlCAl
PREPARATION O F STANDARDS
a?*.,".,.." ".i+l. ,LA Onc of the main difficulties entmuLrunru YLlr spectrographic analysis of zinc-base die-cast alloys is the inability so far to produce standard samples of sufficient useful sizc and uniformity of composition to have life. To date the largest sample the authors have been ahlo to cast with sufficient uniformity of structure and 7lpIE IN SECONDS composition to he called a standard is a rod 0.93 cm. .. Figure 4. Prespark Curves for A l u m i n u m and Magne:sium inch) in diameter by 15 em. (G inches) long, although they . Analytical Pairs prefcr to use 8. rod 0.78 am. (I/,# inch) in diameter. Howlogarithm bass 1.5 ever, by the time sufficientmillings m e removed from such a rod for a referee analysis for aluminum and copper, thero will he very little standard sample left. m e snumer m m e a m e i y moves mzo m e ugnz Pam mu u i w a p r a circuit is.energized. At the end of the prespark time the shutter is satis. ~ ~"1 the hot l topon ~ the cast ~ rad isi not ~ and the firbt exposure made' .At the end Of the ktexfactory, as it is not very uniform and is not always representstivc posure the spark is switched off. Immediately the plate racking of the composition of the rod. If three rads are cast from a ladle.motor is energized just long enough to rack the down a ful of metal from a carefully stirred and skimmed pot, the determined distance. At the same time the lens and high voltage switch solenoids are actuated so as to move the lens into the light chemical and spectmgraphic results will often he identical for the path and switch the 7-mh. inductance into the spark circuit. The individual rods hut differentfor the individual hot This diflatter is then switched on and the impurity exposure made. ference, however, seldom amounts t o more than 5 % of the amount At the conclusion of thesecond exposure, the spark is switched present. For the purpose of routine production control this difoff and the plate-racking motor is again energized long enough t o move the plate into position for the excitation ofthe next sample. ference did not seem to justify the scrapping of a sampling proFinally, the main timer circuit itself opens and the timer autostanding, M~~~of the preliminary cedurc of mhny matically resets, ready for the next sequence of operations. was done using this type of sampling. The present procedure is to cast a =/a inch rod, turn off the outer surface, and then turn thc The expasure times shown &re 8. compromise betwoen several rod down to '/loinch diameter, using these latter turnings for the different factors. The exposure tirne for the impurity determi. chemical analysis. This provides a standard which can he used a is o~less set by the avaihble power in the spark SOurce Of times. large and the time required to bring out spectrum lines in that source No redly satisfactory answer to thc problem of the preparaup to a resdrahle intensity The primary current not be tion of magnesium, lead, tin, and cadmium standmds, has been increased mteriaUy without getting undue burning and pitting required for a found. Because Of the large amount Of of the sparking surface; and the ratherlongexposuresshom were referee analysis for these low percentage oonstitutents, i t is not ~h~ prespark time also represents a those decided possible to cast small rods and have thcm analyzed. Upon conF~~~~4 is a DresDark DuTyefor aluminum oromise. 50,&1#
~
. .
I
and magnesium in zinc with exposures taken every few seconds after startine the soark with the shutter men. The line intensity ratios d&ertse.rapidly during the first i5 to 20 seconds and i t .. . . waa thought desirable to use a 20-second prespark, particularly since a long impurity exposure was required in any ease. Tho layout of the equipment is more or leiis standard.
.
,
The plate-processing equipment consists ,f Dietert develodns 'he spectrograph'is ument, mounted on a d shield for the mark ,geconductor extends :ap cabinet, which is ~~~
~
~
~
~~~~~
I
~.~~~~ I
~
~~~
is mountedon the oDtica1 bench immediate1 trograph slit. The densitometer is au A.R.L. Dietert of I induction voltage regulator is used with it, uuli line vuiwgs reg"lation obtained leaves much to be desired. It is believed that an Z%>Y"lj :1 . 1 electronic voltage regulator would be much more satisf--+--view of the sudden and iuxe fluctuations in line voltap3 encountered in the authors' plat The calculating hoard (Figure 5) is of a simple, ho me-nlade type which have been found vc'ry satisfactory. hnod ~~~~~
in
~~~~~
fittlvl with ~~
roll
I
~~
~
~
~~~
~
~~
s d of the Lucik T-square, on which is engraved a linear scale for plotting d v a n o m e t e r deflections. The lower surface of the cal-
and capable of b&g shifted to compensate for any shift m a work-
Figure 5.
Calculating Board for Rapid Evaluation of Microphotometer Readings
V O L U M E 19, NO. 2, F E B R U A R Y 1 9 4 7
89
tion on the molten metal. This optimism was not justified, however, as glass tube sampling required as groat care in stirring and skimming the metal as any other method. I n addition, the rods formed i n the glass tubes were not SD uniform in composition along their length as were the chill-cast rods. This is possibly duo to the slower cooling of the metal in the glass tubes and to the tcndcncy of the molten alloy to stratify. A few experiments were also run, using the flat surface technique of sparking. The samples were specially cast in the Dictcrt disk-type mold and a carbon rod was used as counterdeotrade, after turning i t down t o the same geometry as the chillcast self-electrodes The spectrum intensity obtained with this arrangement was so reduced, compared with that obtained with self-electrodes, that greatly increased exposure times would have been required for determination of any impurity. No further work has been done with flat surface sparking, although i t a p peared that satisfactorily consistent and reliahle results could he obtained throughit use. I n Figure 6 is shown a loading jig which has been found useful and reliable.
sulting chemists and mctdlurgiats the authors were advised that with solid electrodes the best procedure was to makc up master alloys containing significant amounts of magnesium, lead, tin, and cadmium and have samoles from thesc chcmically analyzed. Then, if these were carefully remelted and dilutcd with high purity zinc, aluminum, and coppcr, a se:ries of standards would he nht.ainorl rrrhnao t a r l mmnnsi+i ...._I- r r l r n l o_".__ ,v~..ions would he sufficiently CIOSC to the truth. All standards far magnesium, lead, ,tin, and cadmium have heen made in this way-the mastor alloys being made up t o have about 0.1% of each constituent and tho dilutions carried out from there. These have worked out satisfactorily and the few independent outside chceks have given no reason to doubt their validity as stand^^^'~~
l l l l . . " -
" I
.
~~
--...
The electrode clamping block, with its handle, is dropped over the gapping fixture and the electrodes are slipped under the clamping fingers and pressed tightly against the gapping pin. The hlook is then lifted off and inserted into the optieal bench electrode holders, where it is automatically aligned by means of the locating pins shown protruding from the clamping block above and helow the electrodes. By using two clamping blocks, a pair of loaded,,nd gapped eleotrodes is always ready for quick insertion il
LYSIS
SAMPLING AND PREPARATION O F
ability to maintain proper sampling technique in the foundry. The zinc alloy being melted in iron pots, an aluminum-iron eamplex floats to the top in a sludge which must be carefully skimmed off. I n addition, molten zinc die-cast metal is very subject t o stratification and must he thoroughly stirred before sampling. Unless this is conscientiously done, the sample taken will not he reoresentative of the Dot or of the die c a s h e s evcntusllv made from the metal. All the first work was done with self-electrodes machincd from chillhast,rods, because this type of sample had always been used for wet chemical analysis and the die-cast departments were familiar with the technique of c a t i n g such samples. Individual steel molds were used which produced rods 6 / 1 6 inch in diameter and 5 inches long with a moderately large hot top. To prepare a sample for sparking, the hot top is knocked off and the rod halved. Then one end of aach half is turned down in a smll bench lathe to a a/ls inch diameter sparking surfaoe with a 45" chamfer. The surface finish is not critical, although it should be fairly smooth and as free from tool marks as possible. Thc &ctual si$e of the sparking surfacc is not particularly important, althoughifit isless than0.125iuchsparkingwithhighinductance tends to pit the surfaoe. The spark tends to anchor itself to one spot, with resultant fluctuation in spectrum intensity and line intensity ratios. I n the coiime of the work attcmpts havo heen msdc to improve results by using other methods of sampling. Considerable work was daue with the glass tube and rubbcr bulb mcthod, as il, secmed to be a simple and clean-cut means of procuring a solid sample unaffected by the presence af sludgc or othcr surfaec contaamina~
'The usual procedure In speotrographio analysis IS t o r e m or record only one odihration pattern on each plate. T o read more than ,one may, except in the case of many lined spectra, introduce considerable complication. I n fact, i t may not he possible without a special setup t o record calibration patterns a t the particular wave lengths in which one is interested. Therefore, if several lines extending over a wide range of wave lengths are t o he phatometered, i t is necessary either to plot a Characteristic curve for the plate st each wave length or else establish that the characteristic curves for the p h t e are substantially the same over the wave-length range in question. The dement lines comprising the analytical pairs cover a very aide range of wave lengths, as may he seen from Tahle I. I n ordcr to justify the use of a single wave-length calibration pattern, a study was made of the variation of contrast with light wave lengths of the two types of plates tried out;Eastman spectrum analysis No. I and Eastman process. Figure 7 shows clearly the great increase in contrast of the SA No. I over the process plate. By "contrast" is meant here the tangent of the angle hetween the straight portion of the H & D curve and tho horizontal axis. It also shows that the process emulsion has fairly constant contrast between 2600 and 3300 b.,while the SA No. I exhibits considerrable variation in this wave-length range. This, together with the fact that the lower sensitivity of the SA No. I plate t o the weak impurity lines prevents taking advantage of its higher . ,
90
ANALYTICAL CHEMISTRY of the curve above 2% aluminum constitutes a real departure from linearity. The working curve derived from the Cd 2288 line is shown rather than that of the Cd 3403 line, since it extends below 0.003% cadmium and indjcates the degree of sensitivity of this spark method. Since the aluminum determination was regarded as the most critical and the one for which an accuracy eomparable with that obtained by chemical methodswasdesired, lines and circuit constants which gave the best aluminum determinations have been chosen. For the best results with copper, a different choice of lines and working conditions would be dictated, but i t was felt that the extra complications involved were not justified. I n the course of routine operation the authors made some tests to indicate the precision and accuracy Kith which their method was actually performing. I n Figures 10 and 11 is shown the result of some reproducibility checks on an aluminum and on a cop-
Table 11. Precision of Analysis of Minor Constituents in Zinc Die-Casting Alloy Precision, % ' Determination (Average Deviation) (0.03%Mkmdard)
LOG RLLAT/VC /NTLU.WfY
Pb (0.007% standard) Sn (0.008% standard) Cd (0.005~o standard)
2.1 8.6
9.3 6.8
Figure 8. Analytical Working Curves for Analysis of Aluminum and Copper in Zinc Logarithm base 1.5
contrast, influenced the authors to adopt the process plate for the analytical work. They have found it a very satisfactory and uniform plate. Under normal operating conditions it is not necessary to put a calibration pattern on every plate. Film processing procedure is sufficiently uniform to require drawing a characteristic curve only once for each box of plates. The lo'garithmic sector is used for this, as it is convenient and does not require the insertion of special electrodes. The fact that the spark circuit constants could be so chosen that the analytical pairs for both aluminum and copper are approximately equal in intensity a t the middle of the composition range, provides an additional safety factor, in case slight variations in the characteristic curve occur. The shorter wave-lengthlines A1 2367 and Cu 2369 are convenient to use when a more extended analysis range is desired. Preliminary investigation has shown that the extension of the working wave-length range occasioned by their use will n$ require a calibration pattern other than the present one a t 2502 A. ANALYTICAL RESULTS
The analytical working curves obtained for the various elements are shown in Figures 8 and 9. The curve for aluminum in Figure 8 has been established from points derived from 37 chemically analyzed standards and the authors feel that the flatness
Figure 9. Analytical Working Curves for Analysis of Magnesium, Cadmium, Lead, and Tin in Zinc Logarithm base 1.
V O L U M E 19, NO. 2, F E B R U A R Y 1 9 4 7
91 background intensity ratio by using relatively large values of series inductance and resistance. The results were not so good as those obtained through the use of the high-voltage alternating current arc, but were satisfactory for the present problem. A typical set of transmission readings obtained during the analysis of a standard sample is presented in Table 111. This sample had the following composition:
r
d
'O
,I1;
I
Aluminum Copper 3Iagnesium Lead Tin Cadmium
4.10 0.95 0,035 0 008 0.004 0.002
I n the determination of iron, the analysis is not too satisfactory. The authors' experience has been that 0 - /< -/O 0 -5 any sample containing over % D,cYrn/ON 0.0670 or 0.077, iron is likely Figure 11. Precision of Copper Figure 10. Precision of Aluminum to be segregated and is not a Determinations Determinations desirable sample for spectroExpressed as per cent deviation from mean. Expresmed a a per cent deviation from mean. graphic analysis. In early Average deviation 2.470 Average deviation 1.2 9% work they often received pot samples analyzing as high as 0.57, iron which were rejected, only to find that die castings made from the same pot of per standard. The effect of setting up the method, principally to metal analyzed only 0.02 to 0.04% iron. It is now their firm begive good results for aluminum, is readily seen by noting the avlief that nearly every case of high-iron samples is due to improper erage deviation for copper, which is much higher than for alumisampling and should be immediately checked by asking for a new num. This difference is again seen in the corresponding curves sample. Present practice requires the spectrographer to examine for accuracy in Figures 12 and 13. The accuracy data are derived the wet plate before drying, for evidences of high iron as shown by from analyses on some 220 samples run on about 100 different the equal intensities of a certain analytical pair. If high iron is plates over a period of 2 months. This being a check on routine production control, the chemical analysis for each rod is actually an analysis of borings from the hot top on that rod, the long accepted method of sampling. To minimize the possibility of error due to improper sampling, the samples used were all taken by a Table 111. Line Transmission Readings Obtained in member of the process metallurgical depart.ment rather than by a Routine Analysis of Zinc Die-Cast Standard Sample production employee. The copper deviations of Figure 13 are (Clear plate reading 100 Figures In parentheses give percentage of each element actually present ) again greater than for aluminum and in this instance are not a t all symmetrical about the zero point. It is difficult t o explain Percentage Percentage Line Line Transmission Background Transmission this, since the original copper working curve was established 49 3018 Zn with a large number of standards. It might, possibly lie in the 70 2670 Zn 39 2816 A1 ( 4 . 1 0 ) fact t,hat n e v electrolytic equipment and personnel had made their 55 2961 Cu ( 0 . 9 5 ) appearance in the chemical control laboratory since the working 60' 2791 M g (0.035) 79 2833 Pb (0.008) curves were first made Gp, although the aluminum determinations 84 2839 Sn ( 0 . 0 0 4 ) 78 2288 Cd ( 0 002) showed no signs of a systematic deviation. Limited data have also been accumulated which will give an indication of the precision to be expected with the analyses of Table IV. Variation in Apparent .Analysis of Zinc-Base Die-Cast Sample magnesium, lead, tin, and cadmium. These were obtained by repeatedly running a single standard on a single plate. As no ac(Containing 3.98% aluminum run on 34 different plates without correction f o r working curve shift.) curacy data are available for these determinations, the results are Percentage Deviation Apparent Percentage Number of Times simply listed in Table 11. Range of Aluminum in Range Apparent Intensity Ratio The relatively poor precision of the impurity determinations is - 9 . 5 to - 7 . 6 3 . 7 6 to 3 . 8 0 - 7 . 5 to - 5 . 6 3 . 8 1 to 3 . 8 5 due possibly to segregation and to residual spectrum background, - 5 . 5 to - 3 . 6 3 . 8 6 to 3 . 9 0 - 3 . 5 to - 1 . 6 as at. higher percentages (circa 0.02y0)the precision is consider3.91 t o 3 . 9 5 -1.5 to +1.4 3.96 to4.00 ably better. The residual background, while not measurable is, +1.5to +3.4 4.01 to 4.05 + 3 . 5 to f 5 . 4 4.06 to4.10 of course, part of the line density which is measured. However, f 5 . 5 to +7.4 4.11 to4.15 + 7 . 5 to f 9 . 4 as background correctionis time-consuming and, at best,,of doubt4 . 1 6 to 4 . 2 0 ful validity ( 3 ) every effort was made to increase the line-to-
ANALYTICAL CHEMISTRY
92 found, resampling is immediately requested and no further work is done on the original sample. This immediate and impersonal check has resulted in a remarkable improvement in sample quality, apparently due t o the swift retribution which the sample taker now invites by careless work. So far it has been necessary to run a standard on every plate in order to analyze aluminum with sufficiently high accuracy. While the working curves do not shift a great deal, they vary sufficiently from day to day to require correction by running R standard on each plate. Some idea of the magnitude of these shifts may be obtained from Table IV, which gives the apparent aluminum analysis of a standard zinc die-cast sample containing 3.987, aluminum which was run on 34 plates on as many consecut,iveworking days.
’O
8 t
1
8-
t 6-
2
- /
/
c
% DLV/H?70,v
Figure 13.
m 3
4
5
6
7
Accuracy of Copper Determinations
Expressed an per cent deviation from chemical analyais. Average deviation 4.8%
The possible effects of selective absorption in the region of 2900
A., due to the strong water vapor absorption band in that neigh-
/3: DEb,/H7/0/Ks
Figure 12.
Accuracy of Aluminum Determinations
Expressed as per cent deviation from chemical analysis. Average deviation 3.0%
Since running standards adds considerably to the time and plate area requirement for an analysis, some work has been done from time to time to try to identify the causes of these shifts. The whole problem, however, is still baffling. A resume of some of these experiments may be of interest to others who have been puzzled by these so far mysterious phenomena. Churchill ( 1 ) has stated that working curve shifts are due, a t least in part, to atmospheric changes affecting the emulsion and excitation conditions, deterioration of the excitation apparatus, or both. The authors have not, however, been able to obtain any correlations. T o establish whether or not fluctuations in line voltage would cause accompanying variations in the relative intensities of aluminum-zinc analysis pairs, the primary current in the high-voltage spark transformer vias deliberately changed from 25% below to 257, above the normal primary current of 6 amperes. No significant change in relative intensity and only a slight change in spectrum intensity were observed. .4 record was kept over several weeks this past summer in an effort to correlate temperature, humidity, and barometric pressure with the index shifts observed from day to day. KO correlation was observed.
borhood, was tested by inserting a quartz water cell in the path of the light from the spark. X o change in relat’ive intensities could be detected. The effect of variable temperatures of the sample electrode rods was tested by alternately cooling the rods in solid carbon dioxide, heating them in an oven a t 100’ C., and taking an exposure after each heating and cooling treatment. S o consistent variation in relative intensities could be observed. The possible effect of wear of the rotary gap electrodes was checked by deliberately altering gap Settings from 10% below t o 10% above the normal gap setting of 0.075 inch. S o effect on relative intensity was noted, although a slight increase in spectrum intensity occurred. The effect of slit width was checked by varying the slit setting from 10% below to 107, above the normal slit widt’h of 50 microns. This caused a significant shift under only tlvo conditions: (1) a measurable background is superimposed on one or both lines of the analytical pair, (2) the two lines of the analytical pair are not of the same character-i.e., one is definitely sharper than the other. These conditions do not apply to aluminum, copper, and magnesium determinations. The effect of spectrograph focus was checked by deliberately changing the prism settings slightly. The effect is similar to that. obtained by changing the slit and did not affect the aluminum, copper, and magnesium analysis pairs. An investigation of the effect of electrode optical misalignment showed that it is most important in analyses made using a lens, and care must be taken to preserve the proper alignment of electrodes, lens, and slit. However, these adjustments are relatively easily made and checked and, with care, little trouble should be expected from this source. While none of the deliberate maladjustments introduced above produced variations outside of expected experimental error, it is always possible that the proper combination of several disturbing factors acting together might introduce a serious error. This work is being continued. LITERATURE CITED
(1) Churchill, J. R.,IND. EKG.CHEM., ANAL.ED.,16,653 (1944). (2) Sawyer, R.A., and Vincent, H. B., J . Optical SOC.Am., 31, 140. (1941). ( 3 ) Strock, L. W., “Spectrum Analysis with the Carbon Arc Cathode Layer”, pp. 39,40, London, Adam Hilger, 1938.
