Application of the Pararosaniline-Formaldehyde Spectrophotometric Method to the Determination of Sulfur in Blister and Refined Copper SlLVlO BARABAS and JOSEPH KAMINSKI' Canadian Copper Refiners lfd., Montreal East,
b The sensitive pararosaniline-formaldehyde chromogenic system has been applied to the determination of sulfur as sulfur dioxide in blister and refined copper. Several factors affecting the combustion, furnace temperature, use of fluxes, and combustion accelerators and retarders have been investigated. Conditions established are quite different from those recommended by other researchers in the analysis of high melting metals and alloys. The effect of temperature on color development of the sulfonic acid derivative of prosaniline and its stability has been established for the first time. Interferences from common impurities present in copper such as selenium, tellurium, arsenic, and antimony in concentrations up to 10 times that of sulfur have been investigated. A procedure has been devised allowing accurate determination of sulfur in copper in the range 2 to 500 p.p.m. The standard deviation at the 10p.p.m. level is less than i1 , and a t the 100-p.p.m. level less than 4.
*
S
electrolytic copper has been determined in thio laboratory during the past 10 years by the direct combustion-iodate titration method analogous to the ASTM method E30-56T for sulfur in steel and cast iron ( I ) . The concentration range covered by the ASTRl method is from 0.005 to 0.40$Z0sulfur. Since the sulfur content of the electrolytic copper is well below the lowest concentration level given for steels, and since the size of the sample is limited by the size of the combustion boats to about 2 grams, considerably weaker potassium iodate solutions had to be used in the analysis of copper than in the analysis of steels. In this laboratory, a solution 0.0007N KIO, was used for titrating sulfur dioxide evolved from a 2-gram sample. It takes only 1 ml. of this solution to titrate the equivalent of 10 p.p.ni. sulfur. Since the end point of such a dilute solution is quite elusive, variations in sulfur concentration of the ULFUR IN
1 Present address, Canadian Industries Ltd., hIcMasterville, Quebec, Canada.
1702
ANALYTICAL CHEMISTRY
Quebec, Canada
order of several parts per million, although physically significant, can hardly be detected with any acceptable degree of accuracy. Thus the lack of precision of the combustion-iodate titration method is attributable mainly to the subjective evaluation of a highly deceptive end point. Because of this shortcoming, inherent in the titration of any dilute solution, efforts have been made by a number of researchers to devise methods allowing spectrophotometric evaluation of sulfur. iz method that has had considerable success is that developed by West and Gaeke ( 7 ) . It involves the absorption of sulfur dioxide in a solution of sodium tetrachloromercurate followed by addition of p-rosaniline hydrochloride and formaldehyde t o form a purple-colored sulfonic acid derivative of pararosaniline (4). Larsen, Ross, and Ingber (3) applied the method t o the analysis of sulfur in uranium trioxide, sodium zirconium fluoride, and hydrofluoric acid while Burke and Davis (2) applied the same technique to the analysis of sulfur in nickel and steel and their alloys. Unfortunately, Burke and Davis, in analyzing the only copper-base sample (Cast Bronze, NBS #52-4) obtained a sulfur assay twice as high as indicated by the Sational Bureau of Standards. Invariably, and unmistakably, high results were also obtained in this laboratory when the same procedure was tried on a number of copper samples for lvhich accurate assays obtained by the long gravimetric method were available. Since the melting temperature of copper and its alloys is approximately 400" C. below that of steel and nickel alloys, and since the common impurities present in copper are quite different from those found in nickel and steel, we have undertaken to establish the exact furnace conditions, oxygen flow rate, use of fluxes, and effect of impurities as applicable t o the determination of sulfur in electrolytic, blister, and anode copper by the p-rosaniline method. .4t the time of reviewing this paper our attention was drawn to a recent work by Stange and Lehmann (6) dealing with the sulfur determination in
copper alloys a t the very high concentration level of 0.15%. Formaldehyde and fuchsine were used as the chromogenic reagents and the standardization was accomplished by means of steels of known sulfur content. EXPERIMENTAL
Apparatus. FURNACE ASSEMBLY. Combustions were carried out in a Dietert High Temperature Varitemp Furnace Model No. 3420 (H. W. Dietert Co., Detroit, Mich.). The four Globar heating elements connected in series when new, can be reconnected in parallel or seriesparallel as they age and the desired temperature can no longer be reached. The combustion tube, "Sulcum-Burrell," has an overall length of 27 inches, u-ith a restricted end 8 inches long X 3/16-inchi.d. X 1/2-inch 0.d. and the larger section 19 inches long X '/*-inch i.d. X 11/8-inch0.d. SAMPLE CONTAIXERS.A white variety of Lecotherm combustion boats, long style, 33/4 inches long X inch wide X '/le inch high (Canlab# 24-414) is sulfur-free and requires no blanking. A grey variety of Lecotherm boats should be ignited in a stream of oxygen for 10 minutes a t 1150" C. and btored in a desiccator. SAMPLE FORM.Samples in the form of drillings, rods, or grindings can all be r u n for sulfur. However, because of the possibility of surface contamination, samples of the smallest area, such as rods, are preferred. They are the easiest to clean, too, by abrasion with a file or by acid treatment. SPECTROPHOTOMETER. Beckman B Model, with matched 1-em. cells. Reagents. SODIUM TETRACHLOROMERCURATE, 0.2M. Dissolve 23.4 grams of sodium chloride in about 200 ml. of water and transfer the solution into a beaker containing 54.4 grams of mercuric chloride. Mix to dissolve and dilute to 1 liter with water. PARAROSANILISE HYDROCHLORIC ACID SOLUTION,0.04%. Weigh 1 gram of pararosaniline hydrochloride into a 100ml. beaker. Add 2 ml. of water and triturate with a glass rod to a smooth paste. Add gradually 85 ml. of water while stirring, then allow to stand for 24 hours. Stir again and filter through a 30-ml. medium porosity fritted glass crucible into a 100-ml. volumetric
flask. Make up to the mark with water. Pipet 4 ml. of the above solution into another 100-ml. volumetric flask and add 6 ml. of hydrochloric acid. Mix well, and dilute to the mark with water. FORMALDEHYDE SOLUTION,0.2%. Pipet 5 ml. of the 40% v./v. (37.2y0 w./v.) formaldehyde solution into a 1000-ml. flask and make up to the mark with water. STANDARD SODIUM SIJLFITE SOLUTION, 100 P.P.M. OF SULFUR.Dissolve 0.396 gram of anhydrous sodium sullite (approximate assay 99.4%) in water and make up to 1 liter. CALIBRATION
Prepare, in accordance with the conditions set below, two citlibration curves, one to be used for the lower sulfur contents found in electrolytic copper, the other for the highcr sulfur contents found in blister copper. (a) To the first serie3 of eight 100-ml. volumetric flasks containing 12 ml. of sodium tetrachloromercurate solution add the equivalent of C , 5, 10, 15, 20, 30, 40, and 50 pg. of sulfur. Now add from a buret 5 ml. of pararosaniline hydrochloric acid solution and 5 ml. of formaldehyde solution. Dilute to the mark with water and mix well. (b) To the second sttries of eight 250ml. volumetric flasks containing 30 ml. of sodium tetrachloromercurate solution add the equivalent of 0, 20, 40, 60, 80, 100, 120, and 180 fig. of sulfur. Add from a buret 12.5 ml. of pararosaniline hydrochloric mid solution and 12.5 ml. of formaldehyde solution. Dilute t o the mark with water and mix vcell. Place the flasks in a water bath a t 25" i 0.5" C. for 30 minutes and then read the absorbance ai 560 mp vs. water in the reference cell. Deduct the absorbaice of the reagent blank from that of the synthetic solutions. Plot two graphs showing absorbance due t o the suljur-pararosaniline compound in abscissa YS. micrograms of sulfur in the ordinate. PROCEDIJRE
Pickle in 1: I nitric acid for 3 t o 5 seconds a portion of elf ctrolytic or anode copper containing up ;o 40 pg. of sulfur and a portion of blister copper containing approximately 100 fig. of sulfur. Rinse the sample first in water, then in acetone, and finally air-dry. Where air may be contaminated by sulfur dioxide, or oiher sulfur compounds such as mercaptans, it should be purified before being iised for clrying purposes. In such a case, the sample is placed in a 50-ml. beaker tightly closed 11ith a tFT-o-hole rubbeis stopper provided nith inlet and outlei, tubes. Suction ia applied a t one end (of the beaker and air drawn a t the other end after passing, first, through an "acid tower" filled to one-third its vapacity with 1:1 glacial acetic acid t o remove sulfur dioxide, and second, through a "dry tower" filled with magnesium perchlorate to remove moisture.
Figure 1. Absorbance of sulfur-pararosaniline compound a s function of temperature 50 ,ug. of sulfur added to all sample solutions
Weigh the dry sample and place it in a combustion boat. Introduce the boat into the hot zone of the combustion tube with the furnace temperature set at 1150" f 10" C. and oxygen flow a t 1 liter per minute. Sulfur dioxide gas is scrubbed into a 50-ml. cylinder containing 12 ml. of sodium tetrachloromercurate. The total combuqtion time is 5 minutes signaled by an alarm clock. Treat similarly an empty combustion boat for the reagent blank. Xow remove the bubbler from the scrubber solution after rinsing it with water. In the case of electrolytic and anode copper, transfer the scrubber solution into a 100-ml. volumetric flask and add from a buret 5 ml. of pararosaniline solution and 5 ml. of formaldehyde solution. In the case of blister copper, transfer the scrubber solution into a 250-ml. flask and add 12.5 ml. of both pararosaniline and formaldehyde solution. Make up to the mark with water and mix. Place the flask in a water bath a t 25" == I 0.5" C. for 30 minutes and read the absorbances of the reagent blank and sample solutions a t 560 mp. DISCUSSION
Color Development. Because of the existence on the market of a variety of pararosaniline substances of varying dye potency, the conditions of the color development established by other researchers could not be applied without verification. Pate, Lodge. and Wartburg (5) listed 18 pararosaniline compounds from four suppliers with identical or vaguely differing designations, each having a different label assay. The rosaniline hydrochloride used in this laboratory (Eastman P 1378) was not among the 18 listed. The sulfonic acid derivative of the p-rosaniline hydrochloride solution showed the maximum ahorbance a t 560 mp. Beer's law was strictly obeyed for the concentration range from 5 to
120 f i g . of sulfur. The ma~111111111 absorbance was reached when the color was alloned to develop for 30 minutes In the course of this investigation it was also established that temperature bas considerably greater effect on color development than realized by any previous researcheri. West and Gaeke (79, who did pioneering studies related t o the formation of the sulfur-pararosaniline complex, had measured absorbances of standard solutions containing varying amounts of sulfur against the reagent blank after the color was developed in a thermostat a t six selected temperatures in the range 11" to 30" C. No appreciable variation in the absorbances of the colored compounds was observed from one temperature to another. This experience, hon-ever, although basically confirmed in the course of our investigation, nas somewhat misleading. This is due t o the fact that for the limited temperature range considered, the absorbances of the blank and the sample solution had increased with temperature by approximately the same amount, the difference between the two remaining about constant by mere coincidence. As a result, the absorbance attributable to the colored sulfur compound was assumed unaffected by the temperature variations. However, if the absorbances of the reagent blank and the sample solutions were measured separately for each temperature, and moreover, the temperature range extended to higher values, then the unequivocal relationship betn-een the intensity of the color and the temperature a t which the color was developed would have been established. The relevant data obtained in this laboratory are plotted in Figure 1. The graph shows clearly that the absorbance of the reagent blank increases appreciably with temperatures up to SO" C. nhen it levelc off. The VOL. 35, NO. 1 1 , OCTOBER 1963
1703
absorbance of the sample solution, on the other hand, first increases nith temperature reaching a maximum a t about 40" C., then rapidly decreases a$ apparently the sulfur - pararosaniline compound decompo5es a t higher ten)peratures, wltil finally a t 100" C. the absorbance of the sample solution approximates that of the reagent blank. aUl solutions v ere heated for 30 minutcs in a water bath set a t the tc.mperatures shown on the graph, followed (where required) by rapid cooling to 25" C. in
Table 1.
Sulfur in NBS Cast Bronze No. 52C (Certified Value, 0.00275)
SI %_____..___
First day 0.0026 0.0028 0.0026 0.0025 Av. 0.0026
Table II.
Second day 0.0024 0.0024 0.0026 0,0026 Av. o.0025
Precision of Sulfur Determination
s, %
AiXZ
Wire bar First day
copper 0.0116 0.0120 0.0116 0.0124
0.0010
0.0008
0.0009 0.0010
OTb lU
Av. 0.0009 Second day ~0.0009 ,0009
0.0118 ,0121 0.0120 0.0112 0 0.0118 ~
0.0009
0.0010 Av. 0 ~ Std. dev. 0.0001
0.00037
Table 111. Effect of Some Common Impurities Present in Copper on Sulfur Analysis
All solutions contained 25 pg. of sulfur Impurity Sulfur added, g. found, g. Selenium (Se+4) 0 125 126 25 24 125 250 2.5 Av.
Tellurium (Te+')
0
2.i
125 250 Arsenic (As+s)
Antimony (Sb+3)
10
25 125 250 0 25
125 250
2.i
26 26 24 Av. 25 24 23 25 24 Av. 24 25 25 26
Av.
1704
%
ANALYTICAL CHEMISTRY
E
ice n ater. Absorbances mere measured iminediately thereafter. The decompo-ition of the colored sulfur eonipomd a as complete after heating the solution in the steam bath for only 10 minutes. The solution, however, remained perfectly clear. In the temperature range 20" to 30" C., and for the sulfur concentration considered, the absorbance of the rwgent blank increases with temperature more, proportionally, than the absorbance of the sample solution. The difference between the two absorbances under the experimental conditions remained constant by mere coincidence. In view of the great variety of pararosaniline compounds of differing properties available on the market, and in view also of the desirability of extending the application of the procedure to as wide a sulfur concentration range as possible, it is felt that the knowledge of the mechanism of the color development as a function of temperature is essential. Factors Affecting Combustion. Setting free all sulfur present in the sample as sulfur dioxide is a prerequisite of an accurate sulfur analysis. The efficiency of combustion and the resulting recovery of sulfur depend on the exact control of the following factors: oxygen flow rate, furnace temperature, time of combustion, use of fluxes, and combustion accelerators or retarders. We have found that the same oxygen flow rate of 1 liter per minute used in accordance n-ith our combustion-iodate titration method i q also the most satisfactory in the comblistion - spectrophotometric method. The minimum safe furnace temperature assuring complete fusion of copper and release of sulfur dioxide in a 5-minute period is 1150" C., or about 70" higher than the melting temperature of copper. A t higher temperatures, molten coppcr tends to leak through the boat causing breakages of the combnstion tube. Fluxes, such as iron powder, added nhen combustion is carried out in induction furnaces are not needed when the radiant-heating Dietert Varitemp Furnace is usrd. Addition of accelerators, such as tinned copper strips, ciistomary in the analysis of high melting nickel and steel alloys, is not required in the analysis of copper which melts promptly. Thus, in eliminating fluxes and accelerators causing high sulfur blanks, better accuracy can be achieved. The use of comhustion retarders such as aliindum poFYder was also considered in this laboratory with a view to slowing down the rate of sulfur dioxide evohition. For example, the addition of retardeis was important in estimating sulfur dioxide by titration. IIov ever, in this instance, the sodium tetrachloromercurate solution was an exrellent absorbent of sulfur dioxide under the conditions given.
Accuracy of the Procedure. As mentioned in the introduction, Burke and Davis ( 2 ) in applying their procedure devised for the determination of sulfur in nickel and steel to the only copper-base metal obtained a result twice as high as given by the NBS. The presumable reason for obtaining a high result, suggested from our own experience, is that the combustion was carried out a t a temperature approximately 400" C. above the melting temperature for copper. At such a high temperature molten copper penetrates completely the combustion boat, releasing sulfur contained in the boat itself that otherwise is not liberated in the process of blanking the empty boat a t the same temperature. In analyzing the NBS Cast Bronze No. 52C for sulfur, the result obtained in this laboratory agreed closely with the NBS certified value. The data obtained from quadruplicate runs performed on two consecutive days are listed in Table I. The results obtained in this laboratory may appear to be slightly on the high side. However, it has to be pointed out that the fact that NBS does not report the fourth decimal figure is indicative of the inadequacy of the combustion-iodat,e titration procedure used by the laboratories cooperating in the XBS program The great accuracy of the pararosaniline procedure for higher sulfur contents was substantiated Ly analyzing three anode and blister copper samples containing, on the basis of the most careful gravimetric determination of sulfur as barium sulfate, 40, 115, and 370 p.p.m. sulfur. The results obtained by the spectrophotometric method agreed nitliin *2 p.p.m. for the first sample, and within *4 and *IO p.p.m. for the second and third samples, respectively.
Table IV.
Typical Analyses of Various Copper Materials
Material Wire bar
Lot s, % 10024-1 0.0009
0.0010
10026-2 0.0011 0 * 0012 11009-1 0.0008 11012-1 High purity research
copper
Blister copper
10662 15068
Anode copper
14475 15842
0.0007 0.0009
0,0009 0.0002 0.0002 0.0002 0.0001 0.0503
0.0510 0.0385 0.037.i
0.0065 0.0060 0.0046 0.0048
Since the results above 100 p.p.m. are customarily reported as percentage to the third decimal figure, the agreement between the two procedures can be stated as being within *O.OOl% sulfur. Precision. A sample of wire bar copper and a sample of anode copper were each run in quadruplicate on two consecutive days. The results are shown in Table 11. While the standard deviation for both wire bar and anode copper calculated on eight runs is extremely low, it is still more gratifying to note that the averages of quadruplicate runs obtained on two different days are practically identical. Interferences. Selenium, tellurium, arsenic, and antimony would all interfere in the sulfur analysis of copper by the combustion-iodate titration method if introduced into the reaction vessel. This is why no attempt was ever made to determine sulfur in anode or blister copper by the combustion-iodate method. These materials were always analyzed for sulfur by the long gravimetric
method as barium sulfate. It is, therefore, obvious that considerable saving in time would be realized if the pararosaniline method were applicable to the analysis of sulfur in anode and blister copper. To establish the possible interferences, increasing amounts of selenium, tellurium, arsenic, and antimony were added to synthetic solutions containing 25 pg. of sulfur and the color was developed in accordance with the standard procedure. The results obtained are shown in Table 111. The data indicate clearly that none of the four impurities considered, present in concentrations up to 10 times that of sulfur, has any effect whatsoever on the precision and accuracy of sulfur analysis. RESULTS
The versatility of the procedure as to the various copper products and the wide range of applicability as to the actual sulfur content are self-evident from Table IV.
The duplicate assays agree closely for the whole wide range of sulfur concentrations. ACKNOWLEDGMENT
The authors are indebted to Laszlo Acs for carrying out some of the experiments. LITERATURE CITED
(1) Am. SOC. Testing Materials, Phil-
adelphia, Pa., “ASTM Methods for Chemical Analysis of Metals,” (1956). (2) Burke, I(. E., Davis, C. M., ANAL. CHEM.34,1747 (1962). (3) Larsen, R. P., ROES,L. E., Ingber, N. M., Ibid., 31, 1596 (1959). (4) Nauman, R. V., West, P. W., Tron, F., Gaeke, G. C., Ibid., 32,1307 (1960). (5) Pate, J. B., Lodge, J. P., Wartburg, A. F., Ibid., 34, 1660 (1962). (6) Stange, H., Lehmann, G., Chem.
Tech. 13,595 (1961). (7) West, P. W., Gaeke, G. C., ANAL. CHEM.28,1816 (1956).
RECEIVED for review December 26, 1962. Accepted July 1, 1963.
FIame Photometric Determination of Manganese in Copper and Steel Alloys DOLORES A. JOHNSON and PFTER F. LOTT St. John’s University, Jamaica 32, N. Y. b Manganese
is cxtracted from copper and steel alloys with 2-thenoyltrifluoroacetone (TTA] for a rapid flame spectrophotometric determination of mangane:;e. Complexing agents are employed to prevent the extraction of other ions by TTA. Triplicate samples of alloys containing from 0.02 to 1.00~0 manganese (0.05 to 0.50 mg. of manganese) were analyzed within 45 minutes with a standard deviation of f0.01. The effect of foreign ions and reaction conditions for the determination is reported.
A
APPLICATIONS Of 2thenoyltrifluoroacetone (TTA) have been reviewed by De and Khopkar (3). Although this reagent has been employed primarily far the separation of metals, it has also kieen employed as a reagent for the flE.me photometric determination of aluminum (7) and yttrium (1). Cheng (2) reported in a study on masking tha%in the presence of tartrate a t pH 6 to 10, only manganese and cobalt formed a precipitate with TTA. Flame photometric methods for manganese have been reported (6, 6, 9). Dippel and Bricker (6) reported serious NALYTICAL
inhibition of the manganese emission from constituents normally present in manganese bronze, nickel-copper alloys, aluminum alloys, and stainless steels, and employed the method of standard addition to overcome this inhibition. Dean and Cain (4) employed a selective extraction procedure for manganese with diethyldithiocarbamate which required several extractions to remove manganese. Very little work has been performed to investigate the properties of solids formed in TTA reactions. This investigation reports a study of the reaction of manganese with TTA and the application of this reaction for a rapid determination of manganese by means of flame spectrophotometry. EXPERIMENTAL
Reagents. Standard manganese chloride solutions (0.0670M and 0.0500 mg. of manganese per ml.) were prepared by dissolving 13.00 grams of MnC12.4Hz0 and 0.1805 gram of MnCIz.4H10, respectively, in water and diluting to 1 liter. These solutions were standardized by titration a t pH 10 against standardized EDTA using Calmagite (G. Frederick Smith Chemical Co., Columbus, Ohio) in ethanol as the indicator. Other solutions were made by appropriate dilution.
BThenoyltrifluoroacetone solution 0.055M for gravimetric investigations, was prepared by dissolving 6.2 grams of the technical reagent in 125 ml. of acetone and diluting to 500 ml. with water. 2-Thenoyltrifluoroacetone solution, 0.055M, for the extraction of manganese was prepared by dissolving 6.2 grams of the reagent in 166 ml. of 4-methyl-2Dentanone and diluting .. to 500 ml. with ioctanone. Phosphate buffer, pH 8, was prepared by mixing 468 ml. of 0.1M NaOH, 500 ml. of 0.1M KH2POd, and 32 ml. of water. Ammonia buffer, pH 8, was prepared by dissolving 7 0 grams of NH&1 in 500 adding sufficient ml. of 15M “,OH, hydrochloric acid to adjust to pH 8 as indicated on a pH meter, and diluting to 1 liter with water. Mixed buffer, pH 8, was prepared by mixing 200 ml. of the ammonia buffer and 100 ml. of phosphate buffer. Apparatus. A Beckman Model D U flame spectrophotometer with SERA unit, Beckman Model A. C. power supply, and RCA 1P28 photomultiplier tube was used. Data were recorded on a 5-mv. Bristol recorder with a 1-second pen response. A Millivac-Poly-Functionist (Millivac Instrument Corp., Schenectady, N. Y.) was employed to adjust the zero position of the pen on the recorder. VOL 35, NO. 11, OCTOBER 1963
1705
