gests a competition for the molybdenum by the citric acid to form a colorless complex and b y the mercaptoacetate reagent to form the colored complex. This aspect was not studied further because the tungsten interference was more pronounced in the presence of the higher mercaptoacetate concentration. I n fact, tungsten interfered less with 1000 p.p.m. citric acid and 0.4% mercaptoacetate than rT-ith 5000 p.p.ni. citric acid and 1.6% mercaptoacetate (Tables I and 11). Other Ions. T h e interferences due t o z i r c o ~ u m ,niobium, tantalum, a n d titanium were also briefly studied because these components of high temperature alloys, in addition t o tungsten, are those most a p t t o remain with t h e molybdenum after t h e abenzoinoxime separation. Siobium, tungsten, and tantalum are known t o contaminate the molybdenum precipi-
tate of a-benzoinoxime ( 2 ) . Zirconium and titanium were thought capable of doing so because of their tendency to hydrolyze. As much as 5000 y of tantalum, niobium, or zirconium causes negligible or no interference using the recommended procedure (Table 11). Only 200 y of titanium could be tolerated under these conditions. There is no evidence that a larger amount of citric acid could eliminate the interference of a greater amount of titanium. On the other hand, oxalic acid appears to be a better complexing agent for titanium than citric acid. The interference of these ions appears to be due, for the most part, to the formation of a precipitate upon neutralization of solutions containing them. Even in the presence of citric acid, a precipitate might be formed. A simple method of evaluating the degree of interference was found in treating an aliquot of the
sample in the same way as that used for the development of the molybdenum color, but using no mercaptoacetate reagent. I n the few cases where this interference was not completely eliminated, blank corrections, determined for each sample, were used. The maximum blank was equivalent to 3 y of molybdenum. LITERATURE CITED
(1) Greenberg, Paul, ANAL. CHEM. 29, 896-8 (1957).
(2) Hillebrand, W.F., Lundell, G. E. F., Brisht. H. K.. Hoffman. J. I.. “.4&likd Inorganic Analysis,” p: 310, Wiley, New York, 1953. (3) &ox-les, X. B., Bur. Standards J . Research 9, 1 (1932). (4) Kill, Fritz, 111, Yoe, J. J., ANAL. CHEM.25, 1363-6 (1953). RECEIVEDfor review July 17, 1957. rlccepted January 31, 1958.
Anomalous Copper Results with the Use of Porcelain Crucibles HARRY ZEITLIN, MICHAEL M. FRODYMA, and GEORGE IKEDA Department o f Chemistry, University o f Hawaii, Honolulu 14, Hawaii
b A blue ash and anomalous results for copper in tuna meat, obtained following dry-ashing of samples in porcelain crucibles, instigated a reinvestigation into use of this type of crucible in the procedure. Samples were dry-ashed in porcelain, silica, and platinum crucibles and copper was determined by the carbamate reagent according to ‘the AOAC method. Erratic results were obtained only when porcelain crucibles were used. Blue ash and erratic results are attributed to the action of a basic flux on copper present within the crucible. Satisfactory results for copper content of tuna meat were obtained by wet-ashing of samples, followed by determination of copper by a Venenate-carbamate method.
I
of work connected with the cause of an off-color condition in yellowfin tuna that often appears on precooking prior to canning, it was necessary to ash samples of cooked off-color tuna meat in porcelain crucibles. The ash obtained was copper blue. When portions of the ash were tested with diethyldithiocarbamate reagent, a golden-brown color was produced, confirming the presence of copper. A quantitative study of the copper content of normal and offN THE COURSE
1284
ANALYTICAL CHEMISTRY
color tuna meat was undertaken, to ascertain whether this trace metal might be a color producer contributing to off-color in tuna flesh in the form of copper proteins or as an ionic copper catalyst. Inasmuch as the study was stimulated by the appearance of a blue ash, i t appeared logical to continue in the investigation of the dry-ashing procedure ( I ) recommended by the Association of Official Agricultural Chemists for the determination of copper. I n excess of 70 determinations for copper in tuna meat from several sources were thus carried out in porcelain crucibles by various n-orkers adhering scrupulously to identical time schedules, reagents, and methods. -4 blue ash was frequently but not invariably obtained. The results were variable to a n extreme degree, with poor precision, and were, on the average, much higher than mould be expected on the basis of previous studies on copper in marine fish (6). S o conclusions, moreover, could be drawn on the role of copper as a color producer in tuna flesh. Consideration of all the factors involved as possible causes for the puzzling results led the authors to suspect the porcelain crucibles. T o check this possibility, it was decided to repeat and expand the previous work. A choice section of a loin of cooked tuna meat
v a s selected as the sole source of samples for the analyses. The dry-ashing was carried out in various types of crucibles coninionly available, such as porcelain (old and new), silica, and platinum; and the determinations were repeated by a sensitive wet-ashing procedure. I n this work the term “old crucible” refers to one previously used in the analysis of meat samples, whereas a “new crucible” is one removed from an unopened carton and never before used. The crucibles are of a type commonly used in the United States. APPARATUS A N D REAGENTS
All glassware used was treated with cleaning solution, followed by three washings with copper-free distilled water. Porcelain and silica crucibles n ere washed with detergent and boiled for 2 hours in concentrated nitrichydrochloric acids (1 t o 1). The cooled crucibles were washed with t a p water, distilled m,ter, and finally three times with copper-free distilled water. The dried crucibles were then heated to constant weight in a muffle furnace a t TOO” to 800” C. Platinum crucibles ryere scrubbed with scouring sand, washed with detergent, and treated with warm concentrated hydrochloric acid. They n-ere then washed with t a p water, distilled water, and three times with copper-free distilled water. They were finally heated to constant weight in a muffle furnace a t 700” to 800” C.
The water \vas doubly distilled water prepared in a n all-glass still and stored in glass-stoppered bottles. All chemi. reagent grade. Liquid cals were c . ~ or reagents were checked for copper contamination. MEAT SAMPLES
The tuna meat, carefully freed of skin and all blood meat, was passed through a meat grinder three times. The ground meat 1% as thoroughly mixed, placed in a glass-covered jar, and stored in a freezer until used. Prior to analysis, the meat was alloir ed to attain room temperature. PROCEDURE
T n o-gram samples of meat were accurately neighed into crucibles and dried overnight a t 100' C. T h e y n e r e then placed in a muffle furnace a n d slo~vly ashed a t dull red heat until t h e ash was completely free of carbonaceous matter. A t t h e completion of t h e ashing t h e crucibles were brought t o constant weight b y reheating a n d relveighing. The ash (whose iveight varied from 20 to 27 mg., depending upon the weight of the sample) was treated with a few milliliters of concentrated hydrochloric acid on a hot plate. The solutions were transferred quantitatively with the aid of hot copper-free distilled water to 100-ml. volumetric flasks and made up to volume. Aliquots of the solutions were analyzed for copper by the carbamate method ( I ) . The absorbances of the extracts were measured with a Beckman DV spectrophotometer a t 500 mp, using 1-cm. absorption cells. A blank of the reagents used was incorporated in each series of runs and the absorbance blank values were subtracted from the values obtained from the extracts of the sample solutions. A calibration m r v e nas constructed by analyzing standard solutions prepared from clear uneffloresced crystals of copper sulfate pentahydrate containing 10, 20, 30, 40, and 50 y of copper. The calibration curve was used to convert absorbanre readings, corrected for the blank, into microgranis of copper. Calculations of the per cent of copper were based on fresh sample neights. Wet-Ash Method. Weighed samples of meat (15 t o 50 grams) in beakers were placed in a vacuum oven at 100' C. maintained at a pressure of 100 m m . of mercury for 5 hours. T h e beakers were allowed to cool in desiccators, weighed, and reheated t o constant \\eight. T h e samples were n-et-ashed with nitric-perchloric acids according t o t h e procedure of Sandell (6) for the destruction of organic matter. Aliquots of the colorless solution. made u p to volume in 250-nil. volumetric flasks, were analyzed for copper b y the Yersenate-carbamate method described b y Cheng and Bray ( 4 ) . The copper, in micrograms, in each sample !vas determined as described above. Calculations of the per cent of copper nere based on d r y sample weights. Dry-Ash Method.
Tables I and I1 contain the data for the results obtained by the dry- and wet-ashing methods, respectively.
Table I. Determination of Copper in Tuna M e a t b y Dry-Ashing in Various Types of Crucibles
Fresh 7 c Cu, Sample Copper, Fresh Wt. W t . )G. x 10-4 Y
DISCUSSION
The significance of the results rests, primarily, upon the conclusions based on examination of the data in Table 1. The results of the dry-ashing in porcelain crucibles (samples 1 to 15) nere erratic and generally high, irrespective of use of old or new crucibles. I n contrast, the copper content of the 2gram samples of meat in silica and platinum crucibles (samples 16 to 25) was negligible. The absorbance readings for these samples. with the blank subtracted, were so low that the results in all cases were reported as trace. At no time during the dry-ashing in silica and platinum crucibles was a blue ash observed. I n the case of the porcelain crucibles, old and new, the dry-ashing of samples from the same source resulted, in many analyses, in the production of a blue ash. As a reflection of these observations, the data obtained with the use of porcelain crucibles nere erratic (Table I) and are considered misleading. The results obtained with silica and platinum crucibles indicated that. because of the low copper content, 2gram samples of meat were too small to be used satisfactorily for the analysis. Accordingly, relatively large amounts of sample (15 to 50 grams) were employed in the wet-ashing procedure. The results (Table 11) are more precise and are considered to represent the actual copper content of tuna meat. The authors are of the opinion that copper is incorporated in w r y i n g amounts in the raw materials or ingredients making up the porcelain crucible. The meat, presumably, acts as a fluu during the dry-ashing, drawing out the copper and imparting a blue color to the ash. The amount of copper available in this fashion 15-ould depend upon the crucible, the fluxing agent, and the conditions of ashing. The latter point is considered most vital. Several meat samples were ashed in porcelain crucibles a t 800" C. for 60 hours and the contents examined periodically. The amount of blue ash, negligible or slight at the onset, increased visibly n i t h time. S o results are reported for samples not analyzed for copper. Scrapings of the glaze and body of several porcelain crucibles together with ash samples were also sent to an independent laboratory for semiquantitatire spectrographic analysis. A high content of copper was reported for all samples. T o substantiate these conclusions, 30-mg. quantities of pure sodiuni chloride, potassium hydrogen sulfate, and a mixture of potassium hydroxide and sodium chloride were fused in porce-
Crucible9 OP 1 OP 2 OP 3 OP 1 OP OP OP OP OP NP
6 7 8 9 10
SP TP SP SP
11 12 13 14 15
SP
S
s
P P
P P P P P
P
2 1963 2 0864 2 2474 2 2318 2 1524 2 2714 1 7254 1 9020 2 5856 2 0476 2 0592 2 0284 2 0350 2 0755 2 1258 1 9703 2 0152 2 0350 1 9437 2 3126 2 0890 2 0376 2 2386 2 0800 2 0246
5
16 17 18 19 20 21
22 23 24 25
10 0 18 0 13 0
14 0 2 0 32 0 10 0 15 0 20 0 11 0 5 0 8 0 12 0 15 0
4 56 8 63 5 79 3 14 5 71 0 880 18 54 5 26 5 80 9 i7 5 34 2 47 3 93 5 78 7 06
Trace Trace Trace Trace Trace Trace Trace Trace Trace Trace
Trace Trace Trace Trace Trace Trace TraEe Trace Trace Trace
i o
a OP = old porcelain. ?;P = new porcelain. P = platinum. S = silica.
Table 11. Determination of Copper in Tuna M e a t b y Wet-Ashing
Drv
1 2 3 4
5 G
i
3 44 3 70 3 82 3 88 6 3 40 8 3 90 0 3 93 Av. 3 72 Standard deviation 2 1 X 10-5 4 3 3 3 4 3 4
0i 92 92 92 59 28 07
14 14 15 15 15 12 16
0 5 0 25
lain crucibles a t 700" to 800" C. for 1 hour and each flux was tested for copper by the carbamate method. An empty crucible was heated and treated similarly as a blank. No copper was detected in the blank, the neutral, and the acid flux. At the conclusion of the heating, the basic flux was decidedly blue and contained a considerable amount of copper. This is conclusive evidence for the presence of this metallic element, presumably in the porcelain glaze, and the effect on it by the action of a basic flux during a dry ignition. These Observations and conclusions may shed more light on the erratic results obtained recently by Borchardt and Butler (3) for copper in paper samples by dry-ashing in porcelain crucibles. The variable results were interpreted by these workers as due to either a posVOL. 30, NO. 7, JULY 1958
1285
sible volatilization of copper a t an elevated temperature or a n irreversible absorption of copper on the porcelain surface. It would appear, in light of the work reported herein, that a more satisfactory explanation should be based, primarily, on the presence of small and varying amounts of copper n ithin the porcelain cruciblc. As dry-ashing procedurt for the determination of copper in the latest edition of the AOAC methods ( 2 ) employs porcelain crucibles and thereby recommends their use, extreme caution should be exercised by proponents of this procedure in the use of these crucibles.
ACKNOWLEDGMENT
The authors wish to express their appreciation to A . L. Tester for allocation of funds supporting a portion of this study, to Donald McKernan and members of the staff of the Pacific Oceanic Fisheries Investigations for their assistance, and John J. Naughton for helpful comments and criticisms. They are also indebted to Kazuji Terada for technical assistance provided in the early stages of the study and to W. Y. Young of the Shipyard Laboratory, Pearl Harbor Naval Shipyard, Oahu, Hawaii, for spectrographic analvses.
LITERATURE CITED
(1) Assoc. Offic. Agr. Chemists, “Official and >,Tentative Methods of Analvsis, 6th ed., p . 122, 1945. (2) Zbid., 8th ed., p. 402, 1955. 1 3 ) Borchardt. L. G.. Butler. J. P.. ANAL.CHEX 29,’414 (19.57). ( 4 ) Cheng, K. L., Bray, It. H., Zbid., 2 5 , 655 (1953). (5) Sandell, E. B., ”Colorimetric Determination of Traces of Metals.” 2nd ed., p. 320, Interscience, i V e ~ York, 1950. (6) Tressler, D. K., Lemon, J. l l : , “Marine Products of Commerce. ’ 2nd ed., pp. 294, 299, Reinhold, S e i v York, 1951. >
,
RECEIVED for review August 22, 195i. Accepted Frbruary 21, 1958.
Automatic Titrator Based on Constant Current Potentiometric Titrations IRVING SHAIN and CALVIN
0.HUBER’
Chemistry Deportment, University of Wisconsin, Madison, Wis.
b An automatic titrator is described which is intended for use with constant current potentiometric titrations. These titrations ordinarily produce some sort of peaked titration curve. By restricting the applicability of the titrator to systems which produce this type of titration curve, it has been possible to build an instrument which is simple and inexpensive, yet sensitive and positive acting.
A
TITRATORS based on various kinds of electrotitrations can be classified into three main types: first, those involving automatic plotting of the titration curve; second. those involving termination of titrant flow a t a preset potential; and third, those involving termination of titrant flow on the basis of the shape of the titration curve (2, 5 ) . Automatic titrators of the third type are the most convenient and the easiest to operate. No detailed information (exact end point potential, exact rate of flow of titrant, and the like) need be known for the successful automatic operation of the titrator. This type of device was described b y Malmstadt and Fett (6), whose derivative circuit has been applied to several types of titrations. The automatic titrator described here is also of the third type, and was designed specifically for use with constant current potentiometric titrations. This means of end point detection
UTOXSTIC
1 Present address, Rockford College, Rockford, Ill.
1286
0
ANALYTICAL CHEMISTRY
has been described by Reilley, Cooke, and Furman ( 7 ) . Two indicator electrodes are polarized b y a small constant current, and the potential developed between the electrodes is measured during the course of the titration. The resulting titration curve frequently has a peaked shape (Figure 2,a). By restricting the applicability of the titrator to systems which produce some sort of a peaked titration curve, it has been possible to design an instrument which is simple and inexpensive, yet n-vhich is positive acting and sensitive. DESCRIPTION
OF TITRATOR
This automatic titrator (Figure 1) consists of an amplifier circuit (VI) and a trigger circuit (V2 and relays). The amplifier circuit is arranged so that only the potential change immediately after the end point reaches the trigger circuit to stop the flow of titrant. Amplifier Circuit. T h e potential developed between the electrodes in t h e titration vessel is amplified in triode V I A . Figure 2, a and b, illustrates t h e shape of a typical titration curve a t t h e grid and plate of V I A . This amplified signal is coupled t o t h e grid of V1B through C1. T h e grid of V l B , however, is clamped to ground by the silicon diode, X. The silicon diode has a very high resistance in one direction and a low resistance in the other. It is placed in the circuit so that any tendency of the grid of V1B to go negative is grounded. Conversely, if the signal goes positive (as after the end point), a very high resistance t o ground appears. Thus, the titration curve a t
the grid of B1B appears as in Figure 2,c. This signal is amplified in triode V l B , and the titration curve a t the plate of V1B appears as in Figure 2,d. Thus, during a titrat,ion the potential a t the plate of V1B remains constant until immediately after the end point. At that time a large decrease in potential appears a t the plate of T U ? . This sudden change in potential is used t o activate the trigger circuit which, in turn, stop2 the flow of titrant. Trigger Circuit and Relays. The duo-triode, P 2 , is arranged in a Schniitt-type, bi-stable trigger circuit ( 8 ) . In this circuit only one section of the duo-triode can conduct at any one time. Potentiometer R11 is set so that V 2 9 coilducts current before the end point. The large negative pulse a t t8he grid of 1-26 imniediately after the end point cause3 1’2.4 to cut off. T’2B then conduct>,and relay 1 closes. Relay 2, a lucking-type relay, also closes and operate5 tht. buret solenoid, stopping the flov of titrant. Because relay 2 is now locked in this position, the flow of titrant remains stopped until the next titration i.; Jtarted 117pressing the push hutton w i t c h , 82. Pilot lights are provided: *YE1 indicates the power on, and S E 2 indicates a completed titration. Cell Current. The cell current n.as supplied by a 4.j-wlt battery with a 5- or 20-megohin resistor in series with the cell. This allon-ed choice of either 9- or 2.2-,ua. cell current. The cell current also may be taken from the po\Ver supply. Power Supply. The titrator requires 110 volts alternating current for operation of the relay and solenoid circuit. T h e tulles require a total of about 6 ma. a t 300 voltsdirect current. An electronically regulated supply was
