methods for palladium is given in Table IV; the sensitivity of the tin(II) p h o s p b t e method reported herein is relatively high. LITERANRE CITED
(1) Ayres, G.
H., Berg, E. W., ANAL.
C ~ M25,980 . (1953).
(2) Ayres, G. H., Meyer, A. S., Jr., Ibid.. 23,299 (1951). (3) Ayres, G. H., Tutliy, B. L., Ibid., 24, 949 (1952).
4) Cheng,K. L.,Zbid., 26, 1894x1954). !5).Mel10r7 J. “Comprehensive Treatise on Inorganic and Theoretioal Chemistry;’ vel. VII, p. 482, hogmans, Green, h n d o n , 1947. (6) Nielsch, W., 2. anal. Chem. 142, 30 (1954). (7) Pollard, W. B., Analyst 67, 184
w.,
(1942). (8) Rice, E. W., ANAL.CHEM.24, 1995 (1952). 9) Ryan, D. E., Analyst 76, 310 (1951). [lo) Sogani, N. C., Rhattacharyya, S. C., ANAL.CHEV 29, 397 (1957).
(11) Yoe, J. H., Kirkland, J. J., Zbid. 26, 1335 (1954).
RECEIVEDfor review July 28, 1958. Accepted February 24, 1959. Condensed from dissertation submitted by John H. Alnop I l l to the graduate school of The University of Texas in partial fulfillment of the requirements for the dcgree of doctor of philosophy, May 1957. Work s u p ported in part by U. S. -4tornic Energy Commission under terms of Contract No. AT-(40-1)-1037 with the University of Texan.
Modified Sargent-Malmstadt Automatic Titrator for Remote Control Use with Plutonium Solutions GLENN R. WATERBURY Universiiy of California, Lor Alamos Scientific Laboratory, Los Alomos, N. M. bSeveral modifications t o a d a p t the commercially available, differentiolpotentiometric automatic titrator for use with plutonium solutions, and to improve parts of it for specific purposes include replacing t h e gravity-flow buret with a motor-driven syringe buret, replacing the stirrer with a magnetic stirrer, rewiring the titrator for remote operation inside a plutonium dry box o r hood, and adding a microammeter to indicate potential changes. For trial titrations of cerium(lV) and chromiumlVl) with iron(ll), standard deviations of less than 0.01 relative 7 0 were obtoined by using large samples and weight burets with the modified titrator.
T
EB u8e of an automatic potentiometric . titrator . for the determination of plutonium seemed advantageous, if a commercial model mere available with certain features. The titrator should permit the use of corrosive oxidizing titrants such as cerium(1V) sulfate, accurate and rapid indication of the equivalence point for solutions of various concentrations and of different starting potentials, rapid indication of a slight excess of oxidizing or reducing agent prior to a titration, and remote operation from within a plutonium dry box t o prevent radioactive c o n t a m h e tion of the main electronic components of the titrator which would be kept outside the dry box. Because the backtitration technique, in which only a small excess of an oxidant or reductant is titrated, is often advantageous, t h r third feature is particularly important. Several automatic potentiometric titrators have been described (f-10). In general, they operate by recording the entire potentiometric titration curve
1138
ANALYTICAL CHEMISTRY
( 1 , 5, 4,8),or by stopping the addition of the titrant either at the inflection point (6) or at some preset value of the potential (4, 7 , 9 , 10). The titrant may he added at a constant rate ( 1 , 5) or added slowly or interrupted near the end point to avoid an excess (4,7, 8). No commercial model possesses all the required features. The advantages of end point detection with the differential automatic titrator (6) made the SargentMalmstadt instrument preferable. This paper discusses the modifications made to include desirable features and gives data for trial titrations. MODIFICATIONS OF TITRATOR
A complete description of the SargentMalmstadt automatic titration is given by Malmstadt and Fett (6) and in the operating manual for the instrummt. The changes made include: replacing the gravity-flow burrt with a motor-
Figure I .
drivel, Gilinont buret, replacing the stirrer with a magnetic stirrer, rewiring the titrator for remote operation, and adding a microammctw to indicate potmtial changes. Mixing-Delivery Assembly. The original assembly consisted of a stirring propeller, a delivery tip attached to a gravity-flow buret by rubber tubing, and a solenoid-actuated plunger operating on the rubber tube t o control the flow of titrant. The stirring motor, propeller, solenoid, and electrodes are mounted in a housing above the titration beaker. This buret system has %vera1 disadvantages: Special overhead extensions of the usual plutonium dry boxes are required for use with the tall gravity-flow buret, solutions such as eerium(1V) sulfate react with the robber tubing connector, and the components of the titrator mounted directly over the titration vessel increase corrosion mid contamination problems and reduec neccss to the equipment. For
Modifications a d d e d to Sargent-Malmstodt titrotor
these reasons, the nuxing-deliverj, assembly was replaced by a magnetic stirrer and a motor-driven Gilmont buret as shown in Figure 1. With this arrangement only the glass tip of the buret, the electrodes with their small holder, and a carbon dioxide entry tube were above the titration beaker. The platinum or platinum-rhodium ringtype indicating electrode and the calomel reference electrode supplied with the instrument were used without modification, The magnetic stirrer was mounted on :i verticd support rod and could be rotated in a horizontal arc under the electrodes. I n operation, the stirrer is svcung to the left, the titration beaker containing the sample and stirring bar is brought up t o surround the electrodes and buret tip, and the stirrer is rotated back under the beaker. Poner to the stirrer is controlled by a switch on the front of the motor-control box, shown to the right of the buret in Figure 1. Malmstadt and Fett (6) mentioned that a motor-driven syringe buret could be substituted for the gravity-flon buret supplied with the instrument. -1 Gilmont buret (Emil Greiner Co.) seemed suitable because the titrant contacts only glass or inert plastic, the dial IS easily read a t a distance of 2 to 3 feet, and the buret capacity of 1 ml. is sufficient for the back-til ration technique. The straight glass tip of the buret was ieplaced by a longer, gooseneck tip as shown in Figure 1, and limit switches nere mounted on the back of the buret housing. A simple gear train, on the right of the buret, was added to operate the limit switches. The knurled knob n a s slipped on the shaft and held by screwing the knob tightly against the gear. This small gear engages a 72-tooth gear which has a threaded central hole and turns or screws on a threaded shaft. The threaded shaft has two flattened surfaces and fits into similarly shaped holcs in the gear box frame to prevent rotation of the shaft. As the knob is turned, both gears turn, and the flattened shaft is screwed through the larger gear by the threads. Two projections near the end of the shaft engage the limit microsvc-itches and turn off the motor of the buret n hen it is emptied 01 completely filled. Thus no damage tci the buret drive mechanism results. A 3 r.p.m. reversible motor (hIerkleKorff Gear Co., Chicago, Ill.) connects to the knurled knob of the Gilmont buret by a sleeve with a set screw. As shown in Figure 1, the motor is enclosed in a box which can slide on ball bearings on a horizontal track. The sleeve on the motor shaft is bhown in the disconnected position in the figure. I n this position the buret may be operated manually for rapid filling. A l l b v o l t relay, fitted with a small plunger, mechanically stops the motor when the relay is not energized. Figure 2 shows that the relay is ciiergized and the motor is freed whenever current is supplied to the motor. This mechanical stop for the motor prcvents rotation of the buret shaft after shutoff of power. Remote Control Circuitry. The Sargent-Malmstadt automatic titra-
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VOL. 31, NO. 7,JULY 1959
11-39
Table I.
Ce Soln., Grams 31.2182 38.5282 38.8768 28.9917 42.6918 29.5862 42.1467 43.4111 36.8944 45.7635 I
Cr S o h , Grams 9.3398 8.3955 8,6561 7.9215 8.4231 10.1236 10.6310 10.9031 10.6412 10.4978
.
Titration of Ceriurn(lV) and Chrorniurn(V1) with Iron(ll)
Ce Soln. from Buret, Gram 0.0175 0.2439 0.2681 0.1498 0.6995 0.3697 0.0267
Fe Soln., Grams 2.5380 3.1499 3.1801 2.3675 3.5249 2.4336 3.4265 3.5270 2.9989 3.7767
0.0056
0.0176 0.7231
3.7989 3.4286 3.5777 3.2768 3.4454 4.1121 4.3276 4.4449 4.3362 4.2776
0.0677 0.2236 0.7576 0.7277 0.3001 0.0020 0.1131 0.2010 0.1789 0.1677
tor is readily adapted for remote control use in which the main electronic components are kept outside of the dry box. The buret, stirrer, meter, and other components inside the dry box become contaminated with plutonium and surgeon’s gloves are worn when handling this equipment. The changes consisted of installing control switches in the dry box and making the necessary cable connections. All switches are mounted in the motor-control box. As shown in Figures 1 and 2, the rear switch, SW 1, on the right side of the box reverses the direction of rotation of the motor and connects the appropriate limit microswitch to the buret motor circuit. The two positions of SW 1 are labeled Fill and Titrate to correspond to the appropriate operation. Switch SW 2, which is directly in front of SW 1, connects the buret motor through the limit microswitch system either directly to the incoming power, pin D, or to the automatic power shutoff control of the titrator, pin B. The two positions of SW 2 are automatic and on t o denote the source of power. On the front of the motor control box is a threeposition snitch, SW 3, which controls the stirrer and starts the automatic titrator. K h e n the sn-itch is in the up position, the stirring motor is connected directly to the incoming ower through pins C and D. I n the epressed position, nhich is momentary, pins B and D are shorted and the automatic titrator is activated. As the switch is released from the depressed position, it returns to the central position where the power to the buret motor, through pins B and C, and to the stirring motor, through pins A and C, is controlled by the automatic titrator. The original manual controls on the front panel of the instrument are not
cp
1 140
ANALYTICAL CHEMISTRY
Total Oxidant as Cr, Grams
Ce Soln./ Fe Soln., Wt. Ratio 12.3072 12.3090 12.3093 12,3090 12.3099 12.3093 12,3080 12.3098 12.3085 12.3088 12.3089 AV. Std. dev. 0.0008
9.3533 8.4402 8,8976 8.0670 8.4831 10.1240 10.6536 10.9433 10.6770 10.5313
Cr Soln./ Fe Soln., Wt. Ratio
2.4621 2.4617 2.4618 2.4618 2.4622 2.4620 2.4618 2.4620 2.4623 2.4620 -1v. 2.4620 Std. dev. 0.0002
changed, and thus alternate sets of controls are available, except for the power sm-itch which must be operated a t the titrator. However, the rubber gloves must be removed before using the original manual controls which are not contaminated. Connection between the various components is by cables as shown (Figure 2). A cable connector box mounted at the face of the dry bos provides the boundary between the contaminated and uncontaminated parts of the equipment. All cables are of the plug-in type and each part of the apparatus may be disconnected easily. Indicating Meter. When using the back-titration technique, some system which indicates a slight excess of oxidant or reductant is desirable. The automatic shutoff feature of the Sargent-Malmstadt titrator is not satisfactory because dropn-ise addition of the titrant will trigger the circuit before the end point is reached. By using a potentiometric recorder connected at the jack provided on the titrator, the second derivative voltages may be followed, but the equipment is expensive. To provide a n inexpensive indicating system which would be reliable for all redox titrations, a microammeter, TT’eston Type 301 (Keston Electrical Instrument Corp.) vas installed, as s h o m in Figures 1 and 2. The knobs on top of the meter box connect to the two variable resistors, each 1 megohm, which are used to adjust the zero setting and the sensitivity of the meter. Because the meter is used only to indicate potential changes and not to determine exact potentials, it need not be expensive. For any set of titrations, the sensitivity setting shouldnot be changed. Because the meter is connected into the sensing circuit of the titrator, changes
in the sensitivity setting alter the triggering or automatic shutoff of the titrator. OPERATION
Turn SW 1 to titrate, SW 2 to automatic, and the power switch to on. -4110~the instrument to tvarm up (10 minutes is usually sufficient), place the titration beaker containing the sample and a stirring bar in position on the stirrer. Turn SW 3 to the up position and adjust the variable control of the magnetic stirrer to obtain a rapid stirring rate. Slowly add the reductant or oxidant from a weight buret until the meter indicates a rapid potential change. Keep the excess to a minimum. When the meter reading becomes constant, read the dial of the buret and then depress SW 3 momentarily, allowing it to return to the central position. At the end point the titrator will automatically shut off. The buret may be filled automatically by inserting the tip into the titrant and turning S W 1 to fill and SW 2 to on. K h e n the buret is filled, the limit switch turns off the buret motor. Alternately, the buret motor may be disconnected from the buret (Figure 1) and the buret knob may then be rotated manually for more rapid filling. The buret dial should be calibrated to read in grams of solution delivered by weighing several volumes delivered and dividing the weight by the dial reading. The satisfactory adjustment of the rates of stirring and addition of titrant is discussed in the operating manual supplied with the instrument. The same principles apply to the modified instrument. However, because the rate of volume delivery of the titrant is fixed, without changing the buret motor, the rate of addition of titrant is varied by changing the concentration of the solution used. EXPERIMENTAL RESULTS
Reagents. Cerium(IV1 standard solution, 0.04 meq. per gram. Dissolve about 32 grams of ceric ammonium sulfate in 500 ml. of 1M sulfuric acid. After dilution t o 1 liter, standardize against sodium oxalate, using weight burets in the titrations for greater accuracy. Chrornium(1-I) standard solution, 0.2 meq. per gram. Keigh accurately 4.9 grams of dried potassium dichromate (h’ational Bureau of Standards Reagent) and dissolve in about 500 ml. of water in a weighed flask. Keigh the solution and flask, and calculate the equivalents of chromium(V1) per gram of solution. Iron(I1) solution, 0.5 meq. per gram. Dissolve about 200 grams of Mohr’s salt in about 1 liter of 1M sulfuric acid. Titrations. T h e modified instrument was tested by titrating iron(I1) solution with solutions of cerium(1V) and chromium(V1). B weighed amount of standard cerium(1V) or chromium(VI) solution containing about 1.5 meq. of oxidant was added to the titration beaker with 10 ml. of 50% sul-
furic acid. Iron(I.1) solution was added dropwse from a weight buret until the meter showed a large potential change. Then the excess iron(I1) was titrated automatically with standard cerium(1V) solution from the Gilmont buret. Cerium(1V) solution was always used for the automatic titrations to prevent changing solutions in the titrator and to maintain constant conditions in the end point determination. The average value for the weight of ceriuni(1V) solution equivalent to 1 gram of iron(I1) solution obtained for 10 titrations is 12.3089,with a standard deviation of 0.0008 or 0.006 relative yo (Table I). This standard deviation is about twice the weighing error for the iron(I1) solution. The average value for 10 determinations of the weight ratio of chromium(V1) solution t o iron(I1) solution is 2.4620, with a standard deviation of 0.0002 or 0.01 relative yo (Table I). These data show the over-all precision for the determinations using large samples with the back-titration technique, and that the per cent standard deviations are lorn because of the large sample sizes. To determine the precision of only the automatic part of the titration, tht. iron(I1) solution was diluted to a concentration of
about 0.03 meq. per gram, and direct titrations of small weighed portions were made with the titrator. The results of these titrations of 0.01 meq. or less (Table 11) show a precision of 0.5 relative yo. Assuming 0.01 meq. or less of cerium(1V) would be used in all titrations of plutonium, over 5 meq. of iron(I1) should be used in the backtitration to keep the standard deviation of the determination below 0.00005 meq. or 0.001 relative yo for the automatic titration part of the determination. By using large samples with the back-titration technique, highly precise results are obtained rapidly. ACKNOWLEDGMENT
The assistance and suggestions of Gilbert Apprill and James Deal of the instrument group of Los Alamos Scientific Laboratory were very helpful in modifying the titrator for remote operation. LITERATURE CITED
(1) Barredo, J. M. G., Taylor, J. K., Trans. Electrochem. SOC.92, 437 (1947). (2) Delahay, P., “New Instrumental Methods in Electrochemistry,” pp. 382-90,Interecience, New York, 1954.
Table
II.
Direct Titration of Iron(ll) with Cerium(1V)
Net Fe, Fe Soln., Buret Ce Added, Meq.1 Gram Reading Meq. Gram 133.4 0.006270 0.03144 0.1994 0.2155 143.1 0.006726 0.03121 89.0 0.004183 0.03152 0.1327 0.3295 219.2 0.010302 0.03127 129.8 0.006101 0.03124 0.1953 0.2736 184.1 0.008653 0.03163 Av. 0.03139 Std. dev. O.OOOl7 Std. dev., 700.5 (3) Kordatski, ITT., Wulff, P., 2. anal. Chem. 89, 241 (1932). (4) Lingane, J. J., ANAL.CHEM.20, i97 (1948). (5) Lingane, J. J., “Electroanalytical Chemistry,” pp. 128-35, Interscience, New York, 1953. (6) Malmstadt, H. V.,Fett, E. R., ANAL. CHEM.26, 1348 (1954). (7) Muller, R. H., Lingane, J. J., Zbid., 20, i95 (1948). (8) Robinson, H. A., Trans. Electrochem. SOC.92, 445 (1947). (9) Shenk, W.E., Fenaick, F., IKD. Em. CHEY.,ASAL.ED.7, 194 (1935). (10) Vogels, Henry, Bull. sci. acad. roy. Belg. (5),19, 452 (1933). RECEIVED for review November 3, 1958. Accepted March 5, 1959.
Determination of Platinum and Palladium in Ores and Concentrates New Fire Assay Method M. E. V. PLUMMER and F. E. BEAMISH
,
University o f Toronto, Toronto, Ont. Canada
b A method for determining microgram amounts of platinum and palladium in ores and concentrates involves a collection by an iron-copper-nickel alloy in a clay crucible from a fused mixture of the oxidized ore, sodium carbonate, borax, and graphite. The fusion is accomplished b y a standard air-gas furnace. The accuracy is comparable to that of the single available assay method applied under optimum conditions and nickel does not interfere. While the efficiency of recovery of the remaining platinum metals has not yet been definitely ascertained, these metals do not interfere with the recovery of platinum and palladium,
A
NEW approach to the analytical extraction of platinum and palladium from ores (7) involved a reduction
by the walls of a carbon crucible of iron, nickel, copper, platinum, and palladium in a sodium carbonate-borax glass medium. The metal alloy mas removed from the slag and dissolved in appropriate acids. The solution was passed through a cation exchange resin to remove base metals. It is of interest to compare the data in Table IV with those recorded by the senior author in publications on the efficiency of recovery of platinum and palladium by the classical fire assay (3, 4). Under optimum conditions, the initial lead buttons obtained from salted samples yielded a recovery of 98.8% of the platinum and 98.1% of the palladium originally added t o the synthetic samples. Under the recommended coiiditions the new method yielded a recovery of 99.3y0 platinum and 98.2% of pallad ium.
Because the principles of this method suggested useful metallurgical applications, an effort was made to overcome such objectionable characteristics as the large sample of ore required, the specially constructed and expensive carbon crucibles, the need for a high frequency furnace, the excessively large button, and certain procedural complications. The result has been the successful collection by iron-copper-nickel of platinum and palladium from relatively small samples of ore by fusion in a standard gas furnace. Some evidence indicates that copper is itself a good collector of platinum and palladium, and by increasing its proportions in the ore, a rclatively low temperature can be used for assay fusions. APPARATUS AND REAGENTS
The apparatus and reagents have VOL. 31, NO. 7, JULY 1959
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