Determination of Truce Amounts of Sulfur in Iron and Steel by Combustion-Coulometry W. R. BANDI, E. G. BUYOK, and W. A. STRAUB Applied Research Laboratory,
U. S.
Steel Corp., Monroeville, Pa.
b The combustion of steel and determination of the sulfur dioxide in the evolved gases have been investigated by a sensitive direct coulometric titration. It has been demonstrated that normal combustion boats and combustion tubes contain sulfur which i s only released by penetration of the ceramic material with hot metal. Accelerators also contain appreciable amounts of sulfur. The recovery of sulfur from a steel sample varies with accelerators, combustion time, sample loading conditions, and the amount of sulfur trioxide formed and adsorbed in the combustion tube. The effect of hot gases on the temperature and vaporization of iodine from a coulometric cell has also been investigated. With this information, an apparatus has been assembled and a procedure developed which will determine 1 p.p.m. sulfur in all types of alloy steels by a combustioncoulometric method.
T
has been a n increasing interest in determining 20 p.p.m. sulfur or less in steels and other metals. Methods for the determination of sulfur in steel include the precipitation of BaS04 (2, 5 , I I ) , the evolution of hydrogen sulfide followed by titration with potassium iodate (3, 1 1 ) , and also the combustion of the steel and the sulfur compounds followed by the iodometric, acidimetric, or potentiometric titration of the absorbed sulfur dioxide effluent (4, 11). Large sample weights are required for the determination of traces of sulfur by the barium sulfate gravimetric method, because BaS04 is slightly soluble and quantitative precipitation is difficult. With the larger sample weights the interference of iron, nitric acid, tungstic acid, and silicic acid is more difficult to overcome. Nydahl (10) used a chromatographic column to overcome such interference but this is too laborious for a routine determination. A radiochemical variation of the gravimetric method has been used by Fulton and Fryxell (7) and also by Rooney and Scott ( l a ) ,but it is lengthy and requires a radiochemical laboratory for neutron activation of the sample and HERE
sulfur as sulfur dioxide from t>hecombustion boat (6-9). 2-4 psi In spite of the high sulfur blanks, several workers have attempted t o Ahstos determine very small amounts of sulfur Needle NaOH - Mg (CIO,), by collecting and measuring the sulfur valve - H2S04 Impregnatd dioxide after combustion of the metal I sample. Burke and Davis (6) absorbed I Combustion ~l~.,, Wort the sulfur dioxide and determined the Tube Flowmeter - Controller - Solenoid sulfur spectrophotometrically. Abresch Combustion valve -Tube and Claassen (1) and Hibbs and Wilkins (8) collected the sulfur dioxide, transFigure 1. Oxygen purification train ferred it to a titration cell, and titrated for combustion of sample SO2 coulometrically. The repeatability of these values was 2 or 3 p.p.m. We have applied the direct coulometric titration as combustion of the sample radiometric counting of the precipitate. occurred. The evolution of hydrogen sulfide To apply coulometry directly, it was from an acid solution cannot be applied necessary to establish the effect of hot to the determination of small amounts gases flowing through the iodine conof sulfur in alloyed steel because organic centration cell. sulfur compounds are formed, inorganic To determine p.p.m. sulfur the blank sulfides are precipitated, and some characteristics of combustion boats, sulfides such as titanium sulfide that are combustion tubes, and accelerators (Cu present in alloy steels cannot be disand Fe) used in the sample combustion solved in hydrochloric acid. A variation were investigated. In addition, the of this method called “dry evolution,’’ recovery of sulfur as a function of the in which hydrogen gas is passed over a sample preheat and combustion times, heated metal sample and the resulting oxygen flow rate, gas flow rate through hydrogen sulfide is trapped and titrated, the titration cell, temperature of the may be applicable. titration cell, and type and amount of Small amounts of sulfur can be accelerator used in the combustion, and separated from steel by combustion of method of loading the sample and acthe sample in oxygen and trapping of celerator into the boat were inthe effluent sulfur dioxide. However, vestigated. the combustion method has not been With these investigations this comwidely used for the determination of munication demonstrates that it is posp.p.m. sulfur because the sulfur consible to determine p.p.m. sulfur in all tained in the ceramic materials and actypes of steel alloys by simultaneous celerators causes high blanks (6, 7 , 9). coulometric titration of the sulfur diIn addition, loss of sulfur through the oxide evolved from sample combustion. adsorption of sulfur oxides in the cooler sections of the gas train causes EXPERIMENTAL low results (7, 12). Fulton and Fryxell ( 7 ) and also Rooney and Scott (12) Apparatus. The apparatus ultishowed that sulfur recovery can be 90% mately developed in this work is or more under ideal conditions, but is shown in Figures’l and 2. Figure 1 between 80 and 90% under actual is a flow diagram of the oxygen laboratory operating conditions. purification system and flow controls. Reports of several authors on the Figure 2 shows the gas train from the exit end of the combustion furnace. effect of the boat in the sulfur deterOne path of the gas-stream splitter is minations are in conflict. It has been 0.5mm. diameter standard bore glass shown by radiochemical studies ( I d ) tubing; the other side, 2.0-mm. dithat combustion boats retain sulfur ameter. The titration cell contains a fronl the sample. Other investigations generator anode and cathode, a sensor have shown that penetration of a electrode, and a reference electrode. ceramic boat by hot metal causes the It has a total volume of about 50 ml. evolution of as much as 30 p.p.m. and is operated with about 25 ml. of Two-Stage Tonk 0, Regulator 30 psi
-
- OnaSbape Regulator - Quick Valv*
d
VOL 38, NO. 1 1 , OCTOBER 1966
1485
Figure 2.
Exit gor train with coulornetrictitrotor
A.
Tube furnace Topered-end lube C. Soisnoid-operated lbree-porl valve D. G.s-strem splitter E. Titrotion cell F. Mognelic stirrer G. Coulomctsr H. 1-my. strip-chart recorder 1. Go%-bubbling bowl* containing elhyiene giycol 6.
electrolyte. The positions of the pia& inum generator and the sensor electrodes are important in terms of the SO, gas stream and the reference iodine electrode, respectively, and must be experimentally determined. The electrolyte is an aqueous solution containing 0.04y0 potassium iodide and 0.4% acetic acid. The Dohrmann Model Cl00 coulometer unit consists of a detector, an amplifier, and a servocontrolled generator system that is built for use with the titration cell and can determine sulfur concentrations in parts per billion. The unit is equipped with controls for setting the iodine concentration, changing _ _ the sensitivity, etc. However, not all of this apparatus was used in the initial tests. and as the parameters were investigated the a p paratus was modified to the final form shown. (With induction heating of the sample, the correction for hot gases, SO, adsorption, and other interference might he different and design of the apparatus would have to he changed accordingly.) A furnace temperature of 1500' C. was used in all the experiments. The tapered-end zirconia combustion tube is 76 em. in length, 2.86 cm. i.d., and 3.50 cm. 0.d. An insert linear tube, 2.60 cm. o.d., having a porous plug on the exit end, is placed in the hot zone of the combustion tube. All recordings of peak areas (the pen response for the amount of iodine generated in the titration cell over the period of gas flow) were made at 1.6 or 6.4 ohms, which is or respectively, of the maximum sensitivity of the coulometer. Measurements of the peak area were made with a planimeter. Finally, a calibration curve was prepared and the developed method was evaluated by analyzing a number of lowsulfur %eels. 1486
ANALYTICAL CHEMISTRY
Figure 3. Pen response for SO2 ( A ) evolved from a combustion boat and ( 6 ) evolved from a combustion boat and copper accelerator Scnsilivi+y, 1.6 ohms; bias, 60
RESULTS AND DISCUSSION
Preliminary Experiments. In the initial experiments an empty combustion boat was treated according to a tentative combustion-titration procedure. This initial procedure included an oxygen flow rate of loon mlhninute, and a bias setting of 60 on the coulometer (the bias setting on the Dohrmann coulometer controls the initial or base line concentration of iodine in the titration cell). Curve A of Figure 3 is a recording of the iodine generated because of the evolution of sulfur dioxide from a combustion boat. Curve B of Figure 3 .shows the pen response for a combustion boat plus a copper accelerator strip of the type normally used in the sulfur combustion method. The increase in peak area of Curve B over Curve A represents the sulfur in the copper accelerator strip. Further, experiments using one or more copper strips established that the sulfur content of the copper accelerator strips was variable (between 0.003and 0.006'% sulfur based on the weight of copper). Curve A of Figure 4 is the peak area for sulfur dioxide evolved from a boat with copper accelerator and 1 gram of Matthey iron which had been estimated by other methods to contain 10 p.p.m. sulfur. The peak area of this recording is 23 square inches. Because some sulfur escaped the titration cell and some sulfur was not evolved from the sample, the sulfur recovery was shout 50D/,, and this area is equivalent to 30 p,p.m. sulfur. Curve B of Figure 4 is the peak area for 0.5 gram of Matthey iron burned in the same boat used to obtain the peak in Curve A ; there should therefore he no contribution of the boat to this peak. In this instance, the peak area is a p
proximately 4 square inches. Therefore, the peak area for 1 gram of Matthey iron under the same conditions should he approximately 8 square inches. Most of the sulfur in Curve A of Figure 4 is contained in the boat and is only liberated when the ceramic boat is penetrated by the molten metal. Establishing a Reproducible Base Line. Only approximate measurements of the peak area are possible in Figures 3 and 4 because of an irregularly tailing base line that did not return to zero until the gas flow was stopped. It was thought that the erratic base line was due to vaporization of the iodine from the cell as a result of an excessively fast flow of the hot combustion-product gases and oxygen through the titration cell. The cell was designed to accommodate a maximum flow rate of 200 ml./minute but the normal combustion method requires a flow rate of 1000 ml./minute. When this flow rate was reduced to 500 ml./minute, the base line was more even, hut the combustion characteristics of the sample were erratic and incomplete. To reduce the gas flow through the titration cell while maintaining a 1000-ml./minute flow rate through the combustion tube, a gas-stream splitter was inserted into the effluent gas stream (Figure 2). This stream splitter diverted a fraction of the gas into the titration cell and vented the remainder to the atmosphere. To further control the flow of gas that passed through the titration cell, a gas-bubbling bottle containing ethylene glycol was connected to the vented side of the stream splitter (Figure 2). By adjusting the level of the solution in the titration cell and the level of ethylene glycol in the gasbubbling bottle, a reprcducihle gas flow
I
--
IllIh
BEFORE ANNEAL
L
B
Figure 5.
TI ME,minutes
AFTER ANNEAL
Effect of hydrogen anneal on the sulfur content of iron
Sensitivity, 1.6 ohms; bias, 100; accelerator, zone-refined copper
02,
1 2 5 0 rnl./rninutej cell Row, 40 rnL/rninute;
W
n
4 I minute
TIME, minutes Figure 4. Pen response for SO2 (A) evolved from boat, accelerator, and low-sulfur iron sample and (6) evolved from iron burned in previously penetrated boat Sensitivity, 1.6 ohms; bias, 60
could be maintained. Even with a reproducible gas flow, results for the combustion of low sulfur did not give reproducible peak areas. During the initial rapid exothermic combustion period, oxygen was being consumed faster than it was being supplied, the pressure in the effluent-gas train was reduced, and the liquids from the titration cell and the gas-bubbling bottle were being sucked back toward the combustion tube. With the changing flow rate of effluent gas, the constant flow through the two paths of the stream-splitter could not be maintained. To maintain a uniform flow it was necessary t o increase the oxygen flow rate to 1250 ml./minute and reduce the sample size t o 0.5 gram for those samples that burn rapidly. Decreasing Boat and Accelerator Blanks. With the establishment of a stable base line, methods of decreasing the sulfur blank were studied further. Zone-refined copper was substituted for the copper accelerator strip, and the combustion boats were preignited for 24 hours or longer in a muffle furnace at 1000' C. The amount of sulfur remaining in the boats after preignition was established by combustion of high-purity (low-sulfur) iron in the boats according to the procedure. To produce iron essentially free of sulfur, steel containing about 0.001% sulfur was rolled to 0.014 inch. The sheet was cut into 1- by 12-inch strips, which were treated at 1300' C. for 7 hours in pure hydrogen flowing a t 4 cubic feet/hour. Samples were analyzed before and after annealing in hydrogen
by using preignited boats with zonerefined copper as a n accelerator. Figure 5 compares the pen responses. (It is important to remember that in this and all subsequent figures the streamsplitter has been placed in the exit gas train and the responses are therefore not comparable with those shown in Figures 3 and 4.) The peak area for the hydrogenannealed steel was very small; however, it was not reproducible from day to day and varied between 0.08 and 0.30 square inch at 1/32 of maximum sensitivity (2 to 7 p.p.m.). Also, the amount of Table 1.
sulfur did not increase or decrease proportionately for heavier or lighter samples of the annealed steel. Therefore, it was concluded that at least part of the blank was associated with the penetration of the boat by the molten metal even though the boats had undergone a lengthy preignition at 1000' C. to remove sulfur. Even so, these analyses indicated that the annealed steel was nearly free of sulfur and could be used to determine the daily boat blank and to accelerate the combustion of low-sulfur high-alloy steels. Recovery of Sulfur. With a preignited boat and with zone-refined copper as the accelerator, the peak area shown in Figure 6A was recorded for the combustion of 0.5 gram of National Bureau of Standards (NBS) Sample 55e (O.Oll~oS). With gas flowing through the cell a t 40 ml./ minute (a 40/1250 aliquot being analyzed) and 80% recovery of the sulfur from the sample, 1.76 fig. of sulfur was titrated. The peak area in Figure 6 is 2.35 square inches. If 1.76 fig. is substituted in the following equation,
Effect of Various Accelerators on Sulfur Determination (Peak Area) by Combustion of 0.5 Gram on NBS Standard 101e Sn, grams Cu, grams Fe, grams Peak
Test No.
On sample
Under sample
On sample
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18" 19" 20"
0.13 0.26
0.26
...
... ...
0.39
...
...
... ...
0.06 0.06 0.06 0.13 0.13 0.13 0.13
...
0.13
... ...
... ... ...
... ...
... ... ...
... ...
... ...
Under sample
On sample
Under sample
area (inches)*
...
...
... ... ... ...
... ...
...
... 0.13 ...
0.13 0.13
... ... ...
0.06 0.06 0.06 0.13
...
1.52,1.50 1.57 1.52,1.60 1.43,1.53 1.51 1.72 1.77 1.73,2.08 1.91 2.03 1.79 1.59 1.93 1.93 2.12 1.87 2.04 1.85 2.05,2.05 2.23,2.19
...
... ... 0.13
...
...
0.13
... ... ... ... ...
... ... ...
... ... ... ... ... ... ...
...
0.5 0.5 1 0.5
... ... ... ... ... ...
...
... ... ...
1 0.5 0.5
...
... 1 1 0.5 0.5 1 0.5 0.5 1
0.13 0.13 0.13 0.13 ... ... 0.13 ... " The accelerator was piled in one spot in Test 18 and was spread out evenly in Tests 19 and 20.
... ... ... ...
... ...
VOL 38, NO. 1 1 , OCTOBER 1 9 6 6
1487
TIME, minutes Figure 6. Peak areas ( A and B) for combustion of 0.5 gram of NBS 55e SemIHvIFy, 1.6 ohms, bbr, IOOi Os, 1 2 5 0 mllminutei cell flow, 40 mL/ mlnvte a~ceIer-alOr,zone-refmed ropperi area, 2.35 Inches'
b
(I
Figure 7. Correct sample loading 0.
b. c.
X
%'Order tivity sensi-
(minutes )
inches'mv. Voltagedivider resistance (ohms)
loe Ng.
60 seconds
96,500
volts
amp. second equiv.
The calculated peak area is 2.58 square inches. Maintaining Titration-Cell Equilibrium. I n calculating the peak area for Figure 6 A , a small preliminary peak, which is also shown in Figure 5, was ignored. This peak was caused by a change in equilibrium concentration of the iodine because the temperature or the rate of vaporization of the iodine changed when the hot comhustion gases entered the titration cell. It was eliminated as shown in Figure 6B when an alternative gas path was provided through two 3-port valves and a second combustion tube (Figure 1 and Figure 2). This alternative gas path admitted hot gas to the titration cell when admitting or removing samples from the primary combustion tube; therefore, an equilibrium temperature and constant iodine vaporization r8t.e could be maintained a t all times. Decreasing the Tube Blank. The use of an alternative gas path revealed still another source of error. When the oxygen was switched to the analytical path after it had been flowing through the alternative path for 10 or 20 minutes, a peak was obtained even though no sample had been introduced. This peak was a p parently due to the desorption of SOI from the combustion tube according to the reversible chemical reaction
so, + '/, 0, so* 1488
ANALYTICAL CHEMISTW
x
e) eqmv.
Apparently some SOI had been adsorbed hy the combustion tube during previous analyses of steels containing much higher amounts of sulfur. Rooney and Scott (22) have also reported this observation. To eliminate as much of this sulfur as possible, new combustion tubes should be installed when sulfur is to be determined hy coulometric titration, and these tubes should be held a t the operating temperature with oxygen flowing for several hours before any sulfur determinations are attempted. (The ceramic liner used in this determination should he treated similarly.) Further, to obtain a stable blank, the first two blank values obtained should be ignored. Eliminating Recorder Noise. I n Figures 5 and 6 some base line noise is evident. When attempts were made to record sulfur titrations of 5 p.p.m. or less at higher sensitivity settings, this noise interfered seriously. The installation of a constant-voltage transformer on the 8.0. line eliminated most of this noise. Preheat and Combustion Times. Optimum preheat and combustion periods varied depending on the sample composition. For example, for 0.5-gram samples of NBS Standard 55e and NBS Standard lOle (a stainless steel), which have approximately the same sulfur contents, the following preheat and combustion times were found to be best: Time, m e ComStandsrd Preheat bustion NBS ingot iron 55s 1 4 NBS stainless steel lOle 3 10
M e t a l piled In one place Correct method M e t a l mreod too thh
The 10-minute combustion period is necessary because of the slow evolution of the sulfur dioxide from the stainless steels and the presence of a secondary peak. Stainless steels and many highalloy steels are heat resistant and/or oxidation resistant, which explains the increased combustion period. However, because of a slow combustion, larger samples can he used. Copper (0.13 gram) was used as an accelerator for the analysis of both NBS 55e and NBS 101e. In addition, 0.5 gram of iron was also used with the NBS lOle Standard. Preheat and combustion times identical to those for stainless steels were also found satisfactory for maraging steels. Addition of Accelerator. Table I shows the results of a number of experiments to determine what accelerator conditions result in the best recovery of sulfur from NBS Standard lOle (stainless steel). One result (tests 1-4) is that tin causes low recoveries and therefore should not be used with the Combustion conditions and equipment described. This is completely opposite to the results reported by Hibbs and Wilkins (8) and Burke and Davis (6). Copper placed underneath the sample (tests 6 and 7) increased the sulfur recovery and also caused a more uniform evolution of SOI, thus facilitating the measurement of the peak area. Highest sulfur recoveries were obtained when both iron and copper were placed in the bottom of the boat, and when the sample and aeelerators were spread out evenly (tests 19 and 20). Correct loading of the boat is shown in Figure 7. These conditions are applicable to all ateels. Evaluation of Method. A calibration curve at a sensitivity setting of 1.6 ohms was prepared over the range
fable 11.
Sample No.
Results of the Combustion-Coulometric Determinah’on of Sulfur in Iron and Various Steels
Sulfur, p.p.m. Electrolytic iron 8.0 9.6 36, 38 14, 12
4
High-Carbon steel (no alloying elements) 5 6
7
8 9 10
27 24, 25 28,27 21, 21 31, 32 30, 26
Steel, 5% Nil 0.5% Cr, 0.5% Mo 11 12
26, 26 28, 26
of 5 to 55 pap.m. sulfur by adding various weights of NBS Standard 55e to various weights of low-sulfur iron so that the sample weight was 0.5 gram. A peak area of 0.08 square inch, representing the blank obtained from the combustion of the low-sulfur iron in a preignited boat with 0.13 gram of copper, was subtracted from all peakarea measurements. The remaining peak area was plotted against the amount of sulfur in the weight of NBS Standard 55e burned. The curve was a 23.4 y sulfur straight line with a slope of (inched2 * A similar plot of amount ‘sulfur‘ us. corrected peak area was obtained for
Sample No.
Sulfur,p.p.m. Maraging steel
13 14 15 16 17 18 19 20 21 22 23 24 25 26
14, 13, 12 10, 11 12 11, 12 14, 12 15, 15 18, 18 41, 32, 32 10, 11 21) 21 18, 18 11, 11 18, 18 14, 13
Steel, 57’ Nil 0.757’ Cr, 0.570 Mo 27
27, 27
NBS Standard 101e, with 0.5 gram of sulfur-free iron and 0.13 gram of copper added to each sample to accelerate the combustion. The similarity indicated that the same recovery could be obtained with high-alloy steels providing proper preheat and combustion times are used and the proper accelerators are added to the sample. A calibration curve for the range of 1 to 5.5 p.p.m. sulfur was obtained in the same manner as previously described except that the sensitivity was changed to 6.4 ohms and a bias setting of 120 was used. The slope of this calibration curve was 5.87 y sulfur (inches)2 ’
Sulfur results were obtained for a number of low-sulfur steels (Table 11). The deviation from the average of duplicate results is no more than 1 p.p.m. for all but two samples. The average deviation of the points on the calibration chart is also about 1 p.p.m. From theoretical calculations of the peak area based on 80% recovery, the estimated accuracy of results between 2 and 50 p.p.m. sulfur is &lo’%. LITERATURE CITED
(1) Abresch,
K., Claassen, I., “Die Coulometric Analyse,” Verlag Chemie, Weinheim, Germany, 1961. (2) “ASTM Chemical Analysis of Metals,” Philadelphia, Pa., 193, p. 25 (1965). (3) Ibid., p. 27. (4) Ibid., p. 94. Standards Institution, (5) “British BS1121, part lA,” British Standards Institution, London, 1957. ( 6 ) Burke, K. E., Davis, C. M., ANAL. CHEM. 34, 1747 (1962). ( 7 ) Fulton, J. W., Fryxell, R. E., Ibid., 31, 401 (1959). ( 8 ) Hibbs, L. E., Wilkins, D. H., Anal. Cham. Acta. 20,344 (1959). (9) Luke, c. L., ANAL. CnEM. 29, 1227 (1957). (10) Nydahl, F., Ibid., 26, 580 (1954). (11) Pigott, E. C., “Ferrous Analysis,” pp. 442-68, Wiley, New York, 1953. (12) Rooney, R. C., Scott, F., J. Iron Steel Inst. (London)195, 417 (1960).
RECEIVEDfor review May 18, 1966. Accepted August 7 , . 1966. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., 1966.
CouIometric Titration of +3 Iron with Electrogenerated Chlorocuprous Ion JAMES J. LINGANE Department of Chemistry, Harvard University, Cambridge, Mass.
b The factors which govern the accuracy and precision of the coulometric titration of $3 iron with electrogenerated chlorocuprous ion in a hydrochloric acid-sulfuric acid medium have been identifled. Using potentiometric end point detection, 5to 50-mg. quantities of iron were titrated with a mean error of +O.OOl mg., and an average deviation of ? O . O l B mg. By reversing the polarity of the platinum generator electrode, 4-2 iron can be titrated anodically with electrogenerated chlorine, and thus mixtures of + 3 and +2 iron can be analyzed. Relatively few other metallic elements interfere.
I
in the coulometric titration of $ 3 iron to the + 2 state with electrogenerated chlorocuprous ion only partly because it has BECAME INTERESTED
02 7 38
not been described heretofore, but chiefly because it appeared a prim* as an unfavorable reaction in a thermodynamic sense, and thus presented an interesting challenge. It is unfavorable thermodynamically because its equilibrium constant is only about 10+6, which is much smaller than one usually considers necessary for a precise titration. . However, the ferric-ferrous and cupric-cuprous couples both function reversibly in chloride media, so that reproducible and precise results are easily attained when the titration is monitored potentiometrically. Arthur and Donahue (1) were the first t o describe a reductometric coulometric titration of + 3 iron, and they employed electrogenerated 3 titanium. Lmgane and Kennedy (4) and Malmstadt and Roberts (6) also used electrogenerated + 3 titanium for this purpose.
+
+
The use of electrogenerated 5 uranium for the titration of + 3 iron was described by Edwards and Kern ($), while Schmid and Reilley (IO)employed the ferrous EDTA complex generated by the electroreduction of the ferric EDTA complex, and Dunham and Farrington (.@ utilized electrodeposited metallic copper. The characteristics of these methods, and their relative virtues, have been discussed in reference (6), which also describes previous coulometric titrations employing electrogenerated chlorocuprous ion. EXPERIMENTAL
As shown in Figure 1, the titration cell (capacity 100 comprised a platinum generator electrode and a silver auxiliary electrode, the latter being separated from the test solution VOL 38, NO. 1 1 , OCTOBER 1966
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