21/2- to 3-inch long cylindrical ingot. S o n e of these were particularly successful for ordinary aluminum alloys even when the thickness (length) of the ingot was made much smaller than 21; inches. If t h e radially-cooled mol& are used without the insulating bottom plug, the combination of radial and axial cooling in the bottom port,ion of the ingot produces a metallurgical structure with a less homogeneous spectrochemical response than that obtained with the plug in place. K e thus conclude from our experiments that radial cooling iz best for a large variety of alloys. In most instances to obtain an acceptable homogeneity, it is neeessarl- to cast the standard a t a temperature well above the temperature a t which solidification occurh. The metal cast into the mold will then also have, momentarily, a temperature vie11 above the solidification point, We believe this proc~edurea l l o w the liquid to become qiiiesczent after the high turbulence l)rodirccd by the pouring of the liquid metal into the mold. The excess of teml)crature recpired to produce this sitnation we have called superheat. In addition, the hypothesis of t h r nced for quiescaence i i borne out bj- the obseri-ation that the metal must be 1)ourcd into the mold very quickly, a1il)arently in order that quiescence o c t ~ i r sbefore ~olitlificationbegins.
XP indicated earlier, one requirement of a s.uitaile standard is that it has a metallurgical structure equivalent to that of the sample to be anall-zed. I t is not required that the b t r x t u r e of >ample and standard be identical, but only that the spectrochemical analysis obtained on the sample be the same as that derived from chemical analysis. The standards cast by the procedures recommended here are applicable, without correction to their chemical in the standard Ai8ThI mold ( I ) or molds equivalent to it.
in casting conditions do not affect all elements in the same way. I t is necessary, almost without exception, to use a casting temperature a t least 100’ F. above the solidification point of the alloy composition. If additions of elements, such as manganese, chromium, or titanium are made, higher temperatures are required. At higher temperatures, a greater cooling rate should be used. The system of three molds, together with variations possible in the cooling rates for each, and the different metal casting temperatures which can be used, provide a wide and easily varied range of caqting conditions.
CONCLUSIONS
a result of this work, the general validity of some of our hypotheses has been estahlished and others have been found to be true in borne hut not in all cases. 7-nidirectional radial cooling is definitelJ- h t . IT-e still l ~ l i e v ein the deiirahility of qiiiewence of the liquid but nica.siirable sillwheat is not always necessary. .llthough rapid cooling is generally desirable, verj- good standards have been made using slow to intermediate cooling rates in the radial aircooled mold or the radial water-cooled mold. Onlj- i n a few situation> was it found iircwhary to uqe the full cooling r of the caljaciity (20 gallon:: ~ ) e minute) radial \vater-sl)ray c~oolccl niold. IT-e ha1.e ohqen.ed generally that variation>
LITERATURE CITED
(1) “Chemical Analysis of Metals, ASTM Standards, 1964.” ASTM Designation E227-63T. American Society for Testing and Materials, Philadelphia, Pa. ( 2 ) Dean, R . B., Ilixon, W. J., ANAL. CHEM.23, 636 (1951). ( 3 ) Moritz, G. M., U. S. Patent 2,983,972, May 16, 1961. ( 4 ) “Met,hods for Emission Spectrochemical Analysis,” 3rd Ed., 1960. ASTM Designation E-2 Slfi-15. American Society for Testing and Mat,erials, Philadelphia, Pa. RECEIVED for review January 13, 1964. Accepted June 2-. Hon.ever, rol)per, a (miinion trace elemcnt found in the twhniral grades of all these clementr, and iisiial1~-also in the purer grzides, has a doping cffert on the thcrmoclec>tricmaterial. Therefore a method \vas noeticd for a direct copper tlctcrniiriation in tellririuni and in related thei,iiioelrc.tric cwml)oiinds. Siinic~roiis analyticd methods for traces of ( ~ p p e rh a w tieen reviewed
(16).
In teliuriurn, coljper is almost, exclusively determined by spectrographic techniques ( 2 , 18, 8 7 ) , but recently it has been determined polarographically ( I S ) or by various methods aft’er ion exrhange separation (16). Instruinentation available and the need for determining a single elenlent, in a variable matrix indicated an application of si)ectroi,hotoinet,ric methods (4,1 4 , 211. Large quantities of bismuth: present in thermorlectric compounds, exclude t,he use of dithizone and dithiocarbamate reagents. The isotollic dilution method Present address, llincs Branrh, Tlepartmerit of llines and Tecshnical Surveys, Ottawa, Canada. VOL. 3 6 , NO. 10, SEPTEMBER 1964
1961
(2O), may be an exception, as considerable quantities of bismuth can be tolerated when dithizone is used in the radiochemical procedure. lohexanone-oxalyldihydrazone are used mainly in organic or soil samples. The latter reagent is not extractable into organic solvents, and lead, present in crude tellurium, would interfere. The most sensit,ive spectrophotometric reagent, for copper, the 1,5-diphenylcarbohydrazide (Zl+), is also usually applied in biological samples with low levels of other metals and requires very close cont,rol of pH. Alpha-benzoinoxime has been used for determination of copper in bismut,h (22)down to 0.005%. hlso used are the phenant,hroline group reagents, in particular 2,9dimethyl-1,lO-phenanthroline (dmp), (23). Several reviews ( 5 , 7 , I O ) , and specific applications (6, 1 7 , 25) have been published. Dmp has been used in the present work to determine copper in tellurium and in related seniiconduct.or materials of the bismuth telluride type. At the time of reviewing this paper, three spectrophotometric methods for the determination of copper in tellurium were reported. In one of them dmp was used as the chroniogenic reagent ( 9 ) , while 2,9-diniet~hy1-4,7-diphenyl-l,lOphenanthroline ( l e ) , and sodium diethyldithiocarbamat,e (26) were used in the other two. The latter method cannot be applied to bismuth telluride materials, while t,he phenanthroline group reagents probably could if selenium did not, interfere. EXPERIMENTAL
Apparatus. A Hilger and \Tatts s lie ct'rop h o t o me t e r Cvis pe k wit,h matched 1- and 4-em. glass cells was used. T h e power unit was connected in series to a 5-kva Stabiline voltage regulator (The Superior Electric Co., Bristol, Conn.). h Beckman Model 76 pH-meter was used. Reagents. All common reagents met ACS specifications. Special reagents included 2,9-dimethyl-1,IOphenanthroline hemihydrate (G. F. Smith Co.) as O.lyosolution in ethyl alcohol, 99.999yc special high purity tellurium (-2merican Smelting and Refining Co.! or T h e Dow Chemical Co.), and high purit,y tellurium w i t h no detectable copper (Frigistors Ltrl.). The water was distilled and deionized. Standards. A 400-1i.p.m. Cu standard stock solution \vas prepared from 99.907, Cu metal (Fisher Scientific Co.) by dissolving in HNOa and adjusting to volume with distilled water. 1his holution was further diluted to yield working standards of 10.0 and 2.0 [l.~l.lll.of cu. 1000-11.p.m. Se standard solution !\-a< ipreparctl in n similar x a y from Y9.99c/;. elemcntal Sc> with a Cu c~ontent
+
7
,
A\
1962
ANALYTICAL CHEMISTRY
below 0.1 p.p.m. (Canadian Copper Refiners Lt,d., Montreal). Compound Buffer. Two hundred grams of citric acid monohydrate was dissolved in about 600 ml. of water. Hydroxylamine hydrochloride (practical grade, 120 grams) was dissolved in about 200 ml. of water and filtered. 130th so1ut)ionswere mixed and adjusted to a volume of about 1600 ml., and 200 ml. of concentrated IVH,OH solution was added. This mas cooled and adjusted with water to a volume of 2 liters, using a graduated cylinder. The pH of the solution was about 5.6. For critical applications the buffer should be cleaned by the addition of 1 ml. of the dmp reagent solution to 250-ml. portions of the buffer solution, and extracted three times with 10 ml. of chloroform. Procedures for the Determination of Copper. TELLURIUM. Weigh out 0.500 f 0.0005 gram of finely ground technical tellurium or 1.000 k 0.001 gram of pure tellurium in a 250-ml. beaker. Add 5 ml. of water, and 5 ml. of concentrat,ed nitric acid, cover, leave to react, heat gently, then increase the heat until expulsion of brown fumes. Rinse the match glass and walls. Evaporate on a hot plate (not' over 150" C.) until no fumes of nit'ric acid are apparent, but do not bake. Cool. Add from a buret, 7 . 5 ml. of concentrated hydrochloric acid, cover, and swirl occasionslly until the white residue disappears. The yellowish green solut'ion must be perfectly clear. Add in about three portions, 50 ml. of the cleaned compound buffer, while swirling the content,s in the partly covered beaker. The neutralization increases the temperature and some acid fumes appear. Cover and set aside for 5 to 10 minutes in a tray of distilled water which is kept cold in running water. Then add from a buret 6.0 ml. of concentrated ammonium hydroxide, swirling constantly. Cool for a feu- minutes. Transfer into a 128-1111. separatory funnel. Adjust to a volume of approximately 80 ml. Add 5 ml. of chloroform, extract for 1 minute, separate, and discard. Add 5 ml. of the dmp reagent and shake. After 2 minutes extract fcr 1 minute with chloroform. Cse 5 ml. fcr pure, and IO ml. for technical tellurium. Separate well and drain quantitatively through a small glass-wool plug in a funnel into a volumetric flask. Use a IO-ml. flask with 2.0 ml. of ethyl alcohol for pure tellurium, and a 25-ml. flask with 5 ml. for technical tellurium, respectively. Rinse with about 1 ml. of chloroform from the top, using a wash bottle. Extract again u-ith 1.5 ml. of chloroform in the case of pure, and with 5 ml. in the case of technical, tellurium. Separate and drain, rinsing the small funnel into the flask. Adjust to volume a t 20" f 2 " C. with chloroform. Measure at 457 in@ against water as the reference fluid. Correct for blank on reagents. containing the same quantity as sample of a reference tellurium
with a copper content below 0.1 p,p.m. and carried through the whole procedure. SEMIQ~ANTITATITE TESTFOR COPPER IN TELLURIUM. Weigh out 1.0 gram of the sample to be analyzed, and of the reference sample used in the blank determination. Pipet into the blank solution the number of micrograms of copper equal to the allowed limit, in parts per million. Then follow the normal procedure until addition of the chloroform for the extraction of the complex. In this case pipet' the 5.0 nil. of chloroform and extract. Compare visually the coloration of the sample extract to that of the blank, showing the limiting coloration for the given concentration of copper in parts per million. THERJIOELECTRIC COMPOUNDS. Keigh out 0.1 0.0001 gram of sample in a 100-ml. beaker, and add 2 ml. of water and 2 ml. of concentrated nitric acid. Dissolve, evaporate to dryness, and then repeat the evaporation with 2.0 ml. of 48% hydrobromic acid for a total content of selenium up to 4 mg. or with 2.0 ml. of a 1 : l O mixture of bromine and 48Y0 hydrobromic acid if selenium content is higher. Dissolve in 2.0 ml. of concentrated hydrochloric acid, and then add 25 ml. of buffer and 2.0 ml. of concentrated ammonium hydroxide. Transfer into a 60-ml. separatory funnel and adjust to a final volume of 40 ml. KO preextraction is necessary. Only 2.0 nil. of the dnip reagent are needed. Otherwise follow the same procedure as for pure tellurium. CALIBEATIOX.Weigh out reference tellurium or thermoelectric material containing the least possible amount of copper. Add the appropriate volumes of a copper standard from a microburet into the dissolved samples, and then follow the regular procedure. Lnless otherwise stated, 10-ml. flasks and 4-cm. cells have been used for all investigations, and 1.0 gram of tellurium with no detectable copper has been present in each measurement.
.*
DISCUSSION AND RESULTS
The basis for the evaluation of the r e s u h was the molar absorpt'ivity of 7950 ( 7 , as),corresponding to an absorbance of 0.050 per 1 pg. of copper (10ml. flask, 4-cm. cell). This value is further referred to as "quoted value.'! Dissolution of the Sample. The hydrobromic acid-bromine mixture may be used for the direct attack of tellurium or thermoelectric materials, but the nitric acid attack is preferred. The evaporation of the nitric acid solution requires, however, additional time. Care is necessary in evaporation of the bromide solution since copper i s lost by volatilization a t temperat,ures above 200" c. (1). The customary alternative procedure-direct addition of citric acid tvhile dissolving large quantities of tel-
lurium by means of nitric acid-does not yield perfectly clear solutions. After the evaporation, the residue of 1 grain of tellurium will dissolve in 5 nil. of concentrated hydrochloric acid but 7 . 5 ml. is more expedient. I n thermoelectric materials considerable quantities of selenium interfere, and its elimination by means of volatilization from hydrobromic acid medium is necessary. Complexing of Tellurium. T h e citrate ion is the most efficient complexing agent for tellurium. The quantities required are considerably larger t h a n specified in most d m p procedures. d minimum of 4.5 grams of citrate ion is necessary for the complexing of u p to 1 gram of tellurium, a t a pH of 5 to 6, and in t.he presence of the specified amounts of ammonium chloride formed upon neutralization. T h e complexing ion :must be present a t the dilution, since tellurium will hydrolyze very easily. Compound Buffer. T h e high concentration of the hydroxylamine hydrochloride is used to assure the reduction of copper(I1) upon dilution of t,he acid tellurous tetrachloride s 0 1 ~ t,ion. The use of su'ch high concentration has been reported (9) for cases where the complexing 1)riorto neut'ralization is necessary. This reducing reagent is used in the quantitative separation of selenium from tellurium in citrate medium (3). It will not precipitate, within the time necessary for the analysis, either tellurium, or ul) to 2000 pg, of selenium usually present with tellurium (Table I ) . However, the same quantity of reagents in a volume of 25 ml. did not yield satisfactory results, owing to too fast reduction of tellurium. The solution must not be left standing after the neutralization with ammonium hydroxide longer than necessary for cooling. The cleaned buffer may acquire a yellowish green coloration (possibly from decomposition products of the reagents) upon storing over extended periods of time, but this coloration does not influence the recovery of copper (Table 11). Pre-extraction. T h e pre-extraction of the final aqueous; solution with chloroform alone was introduced t o remove a n y coloration or foreign mat,ter, found occasionally after evapora.tion of tellurium solutions to dryness. .ilso, there is indication of the possible influence of the high concentration of salt,s ,311 absorbance. If the pre-extraction with chloroform is not included, and the dmp is omitted froin alcohol in the otherwise standard procedure, the absorbance is high and varies considerably instead of approaching zero (Table 111). The effect of possible residual dinp in the cleaned buffer Rolution has to be
Table I. Se added, !43 1000 2000 3000 4000 a
Influence of Selenium o n Copper Extraction" Appearance of Absorbance Deviation from aqueous phase per 1 quoted value, yo relative upon adjusting pH of c u 0 0508 +1 6 Clear until extracted 0 0514 +2 8 Clear until extracted 0 0568 +13 2 Slightly reddish 0 068 +36 0 Red Se precipitated immediately
6 0 pg of Cu added.
considered. Part of the copper to be determined could then be lost during pre-extraction. The dmp, used to clean the buffer, is removed therefore by a three-fold extraction with chloroform. The average molar absorptivity for relatively large quantities of copper in t,ellurium lies within the quoted values, proving that no copper is lost, in the pre-extraction. This is further confirmed by comparable molar absorptivity values for semiconductor materials, where pre-extraction is not required. Time of Buffer Action. T h e time necessary for the reduction has been investigated in the early stages of the work when no pre-extraction has been used. It is not' critical. T h e results in Table IV show a relative standard deviation of 1.2.3%,, b u t are all consistently higher than the quoted value for t h e molar absorptivity. Reference Fluid and Reagent Blank. T h e absorbances of the relev a n t reference fluids are presented in Table V. JVat.er was used throughout as the reference fluid. Reagent blank has to be considered only for the analysis of the authentic samples, since the calibrat'ion does not introduce any variation. The reproducibility of the reagent blanks without tellurium is more difficult to control, and considerable error may result from this source. Therefore it is recommended to use for the reagent blank a reference tellurium sample with a copper content below 0.1 p.p.m. However, most commercially available tellurium of bhe highest purity had a certified, spectrographically determined value of copper "below 1 p.p.m." This limit was stated by two specialized sources as the best, obtainable, and may not be sufficiently low to be insignificant. In the case of semiconductor mat,erials, reference material without any copper is even more difficult to obtain, but the additional evaporation improves the reproducibility of the reagent blanks, prepared by the evaporation of t'he volatile reagents. Blank amount's in both methods to about 1 p g . of copper. Precision and Accuracy. TELLURIUM. T h e influence of tellurium itself has been investigated for quantities het,ween 0 and 1 gram of tellurium
Table II. Influence of Storage on Compound Buffer" Deviation from quoted of - Absorbance value, c buffer, Of Per 1 fig. 10. months blank of Cuh relative 0 0499 -0 2 0 032 -0 6 0 023 0 0497 5 0 039 0 0509 $1 8 0 0505 +1 0 6'/z 0 022 0 025 8'/9 0 0512 1 2 4 a 5 0 ml of compound buffer only, various batches. 15.0 pg. of Cu added directly into buffer, 2.0 ml. of dmp solution. 6 Mean of two measurements, corrected for blank. 0
2'/2
Table 111.
Influence of Pre-extraction on Absorbance" Before analysis pre-extracted with 5 ml. of chloroform, times Absorbance 0,037 0.008 0.017 0.002 0,005 0.003 0.002 0.001 0.001
0
1 2
a
Y o reagent present in alcohol.
Table IV. Influence of Time of Buffer Action on Copper Extraction" Time between addiDeviation, tion % relative and From neutral- Absorbance quoted ization per 1 a. From meanc value of c u min. Ob 0 0534 +1 5 +6 8 2 5 5
005Q
0 0526
+02 1 0
+54 1 5 2 +2 4 +9 0 1-2 8
0 0512 -2 7 15 0 0545 +3 6 30 0 0514 -2 3 8 0 pg of Cu added. n u pre-extraction with chloroforni * Very warm because not cooled. c >lean = 0 0526 i 0 0012 10
Q
VOL. 36, NO, 10, SEPTEMBER 1964
1963
Table V.
Absorbance of Reference Fluids
Fluid
Absorbanceu Reference
80% v./v. chloroform
20% v./v. ethyl alcohol Chloroform Water 4-cm. cells.
+0.002 -0.003
5
Table VI. influence of Tellurium on Recovery of Copper“
Tellurium present, g. 0
Absorbance corrected for blank 0.061b f0.001 0.403 0,398 0,403 0.404 0.407 0.404 0,410 0.405 0.398
0 0 0.002 0.010 0.050 0.100 0,250 0.500 1,000 4 8.0 pg. of Cu added. Blank, mean of three, no Cu added. Mean absorbance = 0.404 Rel. std. dev. = k1.07c
Table VII.
present. The results are presented in Table VI. Tellurium does not influence the recovery of copper, each individual absorbance being within %yoconfidence limits, but a constant weight for all calibrations and determinations is preferred to duplicate closely the conditions. The relative standard deviation of the slope of the regression line is f3.2yO for low levels, and fO.6oj, for high levels of copper in tellurium, respectively. The molar absorptivities are 7900 f 250, and 8000 f 50, respectively, agreeing within k0.6% relative with the values quoted in references ( 7 , 23). I n Table VI1 results of the determination of copper in tellurium are summarized. THERMOELECTRIC hIATERI.4LS. From the Ringbom plot (19) for spiked authentic samples, a 1.0% photometric error corresponds to a relative error in concentration of 3.2Oj,. This corresponds well with the precision attainable ( 4 ) . Accuracy of the measurement at a single level and a t various levels of copper is presented in Table VIII.
Determination of Copper in Tellurium
Calibration Samples Absorbance Cu Corrected Certified cu added, for Per 1 crg. Cu content, found, p.p.m. fig. zero Cu of Cu Te Source p.p.m. 0.6 2 . Oa 0,096 0,048 High purity American
