(3) Dykes, F. W., Fletcher, R. D., et al., Ibid., 28, 1048 (1956). ( 4 ) Farrar, L. G., Th>mason, P. F., Kelley, M. T., Ibid., 30, 1511 (1958). (5) Handshuh, J. W., C. S. At. Energy Comm., Rept. HW-66441 (June 1960). ( 6 ) Korn, G. A., Korn, T. ?VI., “Electronic Analog Computers,” McGrawHill, S e w York, 1952.
(7) Meites, L., Moros, S. A., ANAL.CHEM. 31,23 (1959). (8) Scott, F. A., Peekema, R. M., U. S. At. Energy Comm., Rept. HW-58491 (December 1955). (9) Shults, W. D., U. S. At, Energy Comm., Rept. ORNL-2776 (September 1959). (10) Shults, W. D., U. S. At. Energy
Comm., Rept. ORNL-2921 (April 1960). (11) Stromatt, It. W., ANAL.CHEM.32,
134 (1960). RECEIVED for review January 8, 1963. Accepted April 17, 1963. The information contained in this article was developed during the course of work under contract AT(07-2)-1 with the U. S. Atomic Energy Commission.
EIectroIysis CeH for Continuous External Generation of Reagents in Rea ge nt- Ad diti on An a Iyze rs RICHARD B. KESLER Engineering and Technoiogy Section, The Institute of Paper Chemistry, Appleton, Wis.
b To extend application of the method of coulometry with externally generated reagents to use in reagentaddition analyzers, w’wk was undertaken to design, builld, and test an electrolysis cell for this specific purpose. The cell developed consumes generator-electrolyte solution a t a rate of 3 ml. per minute or less, and is about 118 mm. high and 22 mm. in diameter and of single-piece construction. It is capal2le of continuously producing standmsrd iodine solutions of about 0.08N concentration, and bromine solutions of about 0.2N concentration. Development of this cell permits economic use of the external generation technique for continuous applications through minimum consumption of chemicals, while simultaneously providing known reagent solutions in macro concentrations.
T
technique of coulometric titration with externally generated titrant was established by DeFord, Pitts, and Johns (f), who also developed a two-arm electrolysis cell for reagent generation. Later, in an extension of the original work, Pitts, DeFord, LMartin, and Schmall (4) developed a second, single-arm electrolysis cell for generation of halogens. Head and Marsh (2) used a slightly modified model of the two-arm cell of DeFord, Pitts, and Johns. While both of the e1e:trolysis cells designed by DeFord, Fitts, and their associates are eminently suited for carrying out titrations in the laboratory on a batch basis, certaili design features of each make them lea suitable for use on a continuous basis. The two-arm cell requires a total flow of generator electrolybe solution 01’ about 12 ml. per minute to sweep the electro’ysis products out through ihe desired delivery tip. By increasing €;enerator electrolyte concentration, lower liquid flow rates could probably be used while maintaining 100% current sfficiency a t the HE
HZ
1
alyzer. The cell features single-piece, strong, compact construction; low generator-electrolyte consumption; and attainment of high current values for reagent generation a t 10070 current efficiency.
”‘”\
OVERFLOW ARM
CELL DESIGN AND CONSTRUCTION
118 MM.
1
I
CATHODE
ELECTROLYTE &$-ELECTROLYTE
GAID CA$S
Figure 1.
Electrolysis cell
stated current level of 250 ma., but at these lower flow rates, gaseous electrolysis products collect around the cathode when generating halogens, resulting in increased resistance of the cell to both electrical current and fluid flow. The single-arm cell is, in many respects, a great improvement over the first, but obviously was not intended for continuous operation because the cathode chamber must be periodically refilled. This cell requires a generator electrolyte consumption in excess of 6 ml. per minute. Because the catholyte passes over the anode during cell operation, it is possible that its decreased hydrogen ion cencentration, caused by generation of hydroxyl ions a t the cathode, may limit an otherwise higher current value obtainable for generation of some reagents a t 10070 current efficiency. Both cells are of multipiece construction. While Pitts, DeFord, et al., suggested that the external generation technique should find considerable application in continuous-flow analysis, no cell had been developed for this application. This paper reports the development of an electrolysis cell especially designed for continuous operation, as would be required, for instance, in a reagentaddition type of continuous-flow an-
Figure 1 is a sketch of the cell. The anode, similar to that used by Pitts, DeFord, Martin, and Schmall, is made from ten 10-mm. diameter disks cut from 52-mesh platinum gauze, and two 10-mm. lengths of 20-gauge platinum wire. The disks are welded together by the flame-welding technique used by Pitts, DeFord, et al. The two 10mm. lengths of 20-gauge platinum wire are each flattened a t one end for about 2 mm. of their length, and are welded between the fifth and sixth disks a t diametrically opposed points as close as possible to the periphery of the pad. These wires are for electrical connection to the anode. The assembled anode pad is sealed inside a short length of borosilicate glass tubing, having an i.d. of about 10 mm., with the connecting leads sealed through the tubing wall. Just below the platinum anode, the glass tubing is sharply drawn down to an 0.d. of 4 mm., and an 8-mm. length of 4-mm. tubing is joined to the constricted opening. About 4 mm. above the anode pad, two 8-mm. lengths of 4-mm. tubing are joined through the larger tubing a t diametrically opposed points t o provide a means of feeding electrolyte solution to the cell. Immediately above these feed nipples, a tractrix-shaped structure of glass is sealed to the inner mall of the 10-mm. i.d. tubing and truncated to provide an orifice close to 3 mm. in diameter. This orifice leads to the cathode chamber, which is formed from an 85-mm. length oi 22-mm. o.d. qlass tubing drawn sharply down to about 10-mm. i.d. a t one end and joined to the anode section as shown. About 35 mm. from the top of the cathode section, a 15-mm. length of 10-mm. glass tubing is sealed through the wall of the 22-mm. tubing a t an angle of about 45’ downward, VOL. 35, NO. 5, JULY 1963
963
IIOV.
60 CY A.C.
T
Figure 2.
CI, CP, I
R H. RE.
R1.
Rz. R3.
Rj. S1.
sz. P. F. T.
ti
RI,
1.
D.C. current supply with integrator
Filter condenser, electrolytic, 150 mfd., 150 volts Rheostat, 150 watts Rectifier stack, selenium, full wave, 1 ampere Built-in resistor on pilot light assembly Filter resistor, 2 ohms, 5 watts Filter resistor, 10 ohms, 50 watts Power resistor, 15 ohms, 200 watts Toggle switch, double-pole, single-throw Current range switch, triple-pole, double-throw, nonshorting Neon pilot light Fuse, 1.1 -ampere, slow-blow Isolation transformer, 32-volt taD on secondary Power resistor, 1 5ohms, 200 wbtts Integrator
to provide an overflow outlet from the cathode section. The cathode is formed from a 130-mm. length of 20-gauge platinum wire, partially wound into a helix, with the other end sealed through the wall of the cathode chamber a t a point opposite the overflow arm and several millimeters below the level of the overflow opening. This ensures its constant immersion in catholyte. The open top of the cathode chamber is closed with a vented polyethylene cap. The protruding anode and cathode leads are soldered to grid caps, and the caps are fastened to the cell with a Khotinsky-type cement to secure a rigid, air-tight seal. Two feed points for electrolyte solution are used to prevent any turbulent flow in the vicinity of the anode, preventing, in turn, anode reaction products from mixing with the liquid above it and possibly entering the cathode chamber. The truncated, tractrixshaped structure leading into the cathode chamber ensures the escape of any gas bubbles from the anode compartment-for instance, when the cell is first filled through the feed nippIes. The sharp constriction of the glass tubing immediately below the anode provides a holdup volume of about 0.5 cc. CELL OPERATION
Although the cell is specifically designed for operation with metering pumps, fluid flow rates may be regulated by gravity flow. It is essential, however, that the generator electrolyte feed rate be maintained in slight excess of the rate a t which the anolyte is drawn off through the nipple below the anode. This ensures a positive flow of fluid upward into the cathode section, thus preventing catholyte and ailolyte from mixing. The excess fluid exits from the cell through the overflow arm in the cathode section. The remainder of the generator
964
s2
I
ANALYTICAL CHEMISTRY
CALIBRATION
FACTOR, MILLIEQUIVALENTS PER COUNT
Figure 3. Calibration of integrator
electrolyte flows through the anode grid in a direction normal to its major plane, and constituents in the solution react at the electrode to produce the desired reagent, which is drawn off through the nipple below the anode. Gaseous reaction products formed a t the cathode escape through the vented polyethylene cap closing the top of the cathode section. After an initial filling of the cell, occasionally some air will not be displaced from beneath the anode by liquid. Then, it is necessary to close off the overflow arm, the feed nipples, and the exit nipple by means of clamped rubber or plastic tubing and place the cell under vacuum from the top of the cathode chamber. This also serves to remove any tiny air pockets that may be lodged within the anode structure.
terminals. The integrator was initially calibrated by means of a silver coulometer of the porous cup type recommended by Richards and Anderegg (6). Calibration was checked occasionally by a reference standard d.c. milliammeter and a timer. Although the speed of rotation of the integrator motor is supposedly a linear function of the voltage drop across its terminals, this is not so until the drop approaches 3 volts, as shown by the calibration chart in Figure 3, and as discussed in more detail by Parsons, Seaman, and Amick.
PUMP. To feed generator electrolyte to the cell and to draw off the effluent through the anode, a multichannel peristaltic proportioning pump was used. This pump, manufactured by Technicon Controls, Inc., operates by driving rollers a t constant speed over flexible tubes pressed flat against a plate. Various flow rates are obtained by selecting tubes of the proper i.d. and using them either singly or in variEXPERIMENTAL ous combinations to obtain the desired flow. Apparatus. CURRENT SOURCE.T O Tygon tubing was used to pump supply d.c. current for the electrolysis solution to the cell, and a specially cell, a line-operated, nonregulated resistant tubing supplied by the pump source was used which incorporated a manufacturer and called Acidflex was current integrator. I t s circuit diaused to pump the cell effluent. The gram is shown in Figure 2. The Acidflex tubing was found to be pracrheostat, RH, is used to adjust the d.c. tically inert to reaction with iodine and output to the level desired. Rd and bromine under the experimental conRg are large power resistors with a temperature coefficient of j=0.00002~0. ditions. REAGENTS. The solution used for The windings are of Advance wire (43% generation of bromine was 1.5M in Ni-57% Cu) on a Steatite L3 core. H,S04 and 0.3M in KBr. That for Switch Sz has large silver contacts for iodine- generation was 0 . 1 in ~ H3B03, negligible contact resistance and places 0.6M in NazS04, and 0.2M in KI. either R4 alone or Rd and Rp in series Both were placed under about 25-inch across the terminals of the integrator, Hg vacuum for deaeration before use. I . The integrator is a low-inertia d.c. Generated iodine solution was titrated motor, of the type described by Parsons, with standard O.1N Na2SzO8solution. Seaman, and Amick (S), which drives Generated bromine solution was abtwo hands on a dial face evenly divided sorbed in various aliquots of standard into 100 divisions. The longer hand 0.05 or 0.06n’ .ls203 solution. The rotates 100 times faster than the shorter, amount of AspOa used was calculated and a full revolution of the slower hand to provide an excess, which Tvas subresults in 10,000 counts. sequently titrated with standard 0.1N In this work, the integrator was used KBr03 solution, using the discolorawith only resistor R4 across the motor
CURRENT SUPPLY
Table 1.
DuraEffluent tion flow
rate, m!./ min. 2.5
)*\
(2
I
1
I
/
hi-;
5 % h4L ERLENMEYES
Figure 4.
3.0
Experimentlil arrangement
tion of methyl orange indicator as a means of end point detc,ction. PROCEDURE. The efficiency of the cell for generation of iodine and bromine a t various current levels and fluid flow rates was determined by generating a specific amount of halogen, as measured by the integrator and comparing this with the amount found by titration. The arrangement wed is shown in Figure 4 in diagram form. Feed of generator electrolyte to the cell was maintained a t a rate of flow close to 0.5 ml. per minute in excess of the effluent flow, by selection of the proper pump tubing. When generating iodine, the effluent solution was simply collected in a 500-ml. Erlenmeyer flask and titrated upon completim of the run. When generating bromine, the effluent was collected in the same flask by placing a delivery tip of 4-mm. glass tubing well below thcr surface of the AszOs solution which was stirred by means of a magnetic stirrer. The initial total volume of the bromine absorbing solution was adjusted to 100 ml. and to a sulfuric acid concentration of 1.5M. Upon cessaiion of current flow to the cell, effluent solution was allowed to be pumped to the collecting vessel for 5 minutes to remove all generated halogen from the cell and pump lines. RESULTS AND DI5iCUSSION
A t two different flow rates of electrolyte solution through the anode, iodine was generated at increasingly higher current levels until the quantity of iodine found by titration differed significantly from that measured by calculation from the integrator reading. Titration results 0.5% below the calculated result were accepted as a significant departure from 100% current efficiency. Tn most caws, tiny gas bubbles, assumed to be oxygen, appeared on the anode when this difference in the two values arose. The appearance of oxygen, of course, indicates that a portion of the current is not being utilized for oxidation of iodide ions to iodine, but is being used for oxidation of the solvent. Usually, also, during the 5-minute flushing period following cessation of current flow, when gas bubbles had appeared on the anode, bits of solid iodine could be seen to drop from the
of
current flow, see. 1987 1014 807 589 600 568 573 35 _1. 388 487 686
Counts on
integrator 2500 3000 4000 4000 3000 3500 4000 2500 3000 4000 600 1
Approx.
2.0
3.0
4.0
553 415 1044 885 750 664 556 556 553 498 1516 1040 939 503 514 604 447 602 598 575 51 1 448 437 417 399 363
3500 3000 8500 8000 7500 7200 6500 7000 7300 6900 8000 7500 8500 5500 6501 5499 6501 9000 9000 6000 6500 6700 7500 7601 7701 7400
12
concn., Milliequivalents ~--______ of iodine current, equiv./ Calcd. Found Diff. Rel. error, yo ma. liter Approx.
1.278 1.506 2.000 2.000 1.500 1.750 1.996 1.248 1.497 1.996 2.995
Table II.
DuraEffluent tion of Counts flow rate, current on inteflow, m!./ grator min. sec.
Generation of Iodine
1.276 1.504 1.999 1.988 1.499 1.743 1.987 1.244 1.492 1.957 2.977
0.002 0.004 0.001 0.012 0.001 0.007 0.009 0,004 0,005 0.009 0.01s
-0.16 -0.13 -0.05 -0.61 -0.07 -0.40 -0.45 -0.32 -0.33 -0.45 -0.60
60 140 240 325 240 300 335 345 375 395 425
0.015 0.035 0.060 0.052 0.050 0.063 0.070 0.071 0.077 0.082 0.087
Generation of Bromine
Approx. Brz Approx. concn.,
hlilliequivalents of bromine current, Calcd. Found Diff. Rel.error, % ’ ma. 1,750 1.497 4.242 3.992 3.743 3.593 3.244 3.493 3.643 3.443 4.000 3.743 4.242 2.745 3,244 4.241 3.244 4.491 4.491 2.994 3.244 3.343 3.743 3.793 3.844 3.700
1.750 1.500 4.238 3.990 3.739 3.590 3.245 3.488 3.628 3.420 4.000 3.750 4.235 2.742 3.240 4.230 3.232 4.467 4.455 3.000 3.246 3,340 3.738 3,787 3.830 3.675
anode structure and dissolve in the flowing electrolyte solution. The results for iodine generation are shown in Table I. While higher generation currents can be used a t 100% efficiency a t higher rates of electrolyte flow through the anode pad, the resulting concentration of iodine in the anode effluent appears to reach a limiting value for the experimental conditions. Table I1 shows siniilar data for generation of bromine a t three rates of electrolyte flow through the anode. Again, it is shown that the concentration of halogen in the anode effluent reaches a limiting value for a given combination of generation current and anolyte flow rate, except at the 2.0-ml -per-minute flow rate. The experiments at this flow rate were repeated with essentially the
0 0.003 0,004 0.002 0.004 0.003 0.001 0.005 0.015 0.023 0 0.007 0.007 0.003 0.004 0.011 0.011 0.024 0.036 0.006 0.002 0.003 0.005 0 . OOF 0.014 0.025
0 +0.20 -0.10 -0.05 -0.11 -0.08 +0.03 -0.14 -0.41 -0.67 0 +o. 19 -0.17 -0.11 -0.12 -0.26 -0.34 -0.53
-0.so
$0.20 tO.06 -0.09 -0.13 -0.16 -0.36 -0.68
305 350 390 435 480 530 565 607 635 666 255 345 435 525 610 675 700 720 725 500 610 715 825 875 930 985
equiv./ liter 0.09.5 0.109 0.120 0.135 0.150 0.165 0.175 0.189 0.198 0.208 0.053 0.072 0.090 0.109 0.126 0.140 0.145 0.149 0.150 0.078 0.095 0.112 0.129 0.136 0.144 0.153
same results, but no factual explanation has been found for the production of more concentrated bromine solution. Work nearly completed shows that the cell operates efficiently for continuous periods exceeding 6 days a t thc generation rates shown in this paper, and in industrial environments. LITERATURE CITED
(1) DeFord, D. D., Pitts, J. N., Johns, C. J., Ax.4~.CHEX.23, 938 (1951). (2) Head, W. F., Marsh, M. M., J . Chem. Educ. 38,361 (1961). (3) Parsons, J. S., Seaman, W., Amick, R. &I., ANAL.CHEX.27, 1754 (1955). (4) Pitts, J. K., DeFord, D. D., Martin, T. W., Schmall, E. A., Ibid., 26, 625 (1954). (5) Richards, T. W., Anderegg, T. O., J . Am. Chem. SOC.37,675 (1915).
RECEIVED for review January 10, 1963. Accepted April 15, 1963. VOL. 35, NO. 8, JULY 1963
965
