1344 '
ANALYTICAL CHEMISTRY
Table IV. Sample NBS 123a 10270
AIS1 Armco Std. 30
Reproducibility of Method for Columbium, Tungsten, and Tantalum % No. of Present 0.75 0.11 8.02 0.76 0.15 0.19 0.60 0.02 0.35
Element Cb W
0.82
Cb
0.12
Ta
.,.
Ta
Cb W Ta Cb W Ta
w
Values 8 8 7 5 5 6 9 9 9 12 12 12
Range 0.73-0.75 0.06-0.10 0.00-0.01 0.71-0.76 0.15-0.15 0.20-0.23 0.57-0.62 0 00-0.03 0:31-0.36
Av.
0.74 0.074 0.008 0.74 0.15 0.22 0.59 0.01 0.34
u
0,009 0.014
..
0.018 O:t)12 0,015 0:OlS
0.81-0.86 0 . 8 2 0,013 0.34-0.39 0 . 3 6 0.018 0.11-0.12 0 . 1 2 0.005
may be obtained by the pyrogallic acid method by using a larger initial sample. The accuracy of a method is difficult to determine, but the values obtained on standard samples indicate that the columbium and tantalum values are in reasonably good agreement with values obtained by other methods. The tungsten values appear to be slightly low (probably because of incomplete recovery in the hydrolysis separation) but should be satisfactory for most work where the tungsten content is less than 1% and where the total of the columbium and tantalum contents is more than the tungsten content. ACKNOWLEDGMENT
The authors wish to acknowledge the permission of the Armco Steel Corp. to publish this paper. iicknowledgment is also due the other members of the Research and Rustless Division Laboratories for technical assistance.
REPRODUCIBILITY AND ACCURACY OF METHODS
h statistical study of the recommended procedure was made by applying it to a series of Type 347 stainless steel samples. The values obtained from this study are shown in Table IV. These values were obtained over a period of 2 months by one analyst. The average standard deviations (Iu) were 0.014% for columbium, 0.016% for tungsten, and 0.011% for tantalum. This indicates that the reproducibility of the method (2u) is &0.03% for columbium and for tungsten, and +0.02% for tantalum. Recent experiments indicate that with tantalum contents below O . l % , a reproduribilitv of better than 3=0.02%
LITERATURE CITED
(1) Bogatski, G., 2. anal. Chem., 114, 170 (1938). (2) Heyne, G., 2. angew. Chern., 44, 237 (1931). (3) Johnson, C. M., Iron A g e , 157, 66 (1946). (4) Knudson, H. W., Meloche, V. W., and Juday, Chancey, IND. ENG.CHEM.,ANAL. ED.,12, 715 (1940). (5) Thanheiser, G., Mitt. Kaiser-Wilhelm-Inst. Eisenfwsch. DQsseldorf, 22, 258 (1940). ( 6 ) Weissler, h.,IND.ENG.CHEM.,-4x.t~.ED.,17, 695 (1945). RECEIVED for review April 23, 1963. Accepted July 2, 1953. Presented before the Pittsburgh Conference on Analytical Chemiatry and .4pplied Spectroscopy, March 2, 1953.
Automatic Coulometric Titration with Photometric Detection of Equivalence EDW4RU K. WISE', PAUL W. GILLES, AND CHARLES A. REYNOLDS, J R . Department of Chemistry, University of Kansas, Lawrence, Kan.
To eliminate the troubles associated with anomaIous behavior of sensing electrodes and to reduce the number of electrodes in a titration vessel have been dual purposes of a successful attempt to develop apparatus and procedures for automatic photometric detections of equivalence in coulometric titrations. An all-electronic self-contained instrument, operating from the 115-volt alternating current supply, to produce constant current over a continuously variable range and to terminate the titration by photometric detection has been designed, constructed, tested, and operated. Coulometric titrations producing three types of color change at equivalence were investigated. In each titration the response of a photocell to the color change opened the generating and timing circuits. Samples of from 0.2 to 1.0 meq. w-ere titrated with average errors of from 0.06 to 0.15%.
T
HE detection of the equivalence point in coulometric titra-
tions has received considerable attention. Both potentiometric (2-4, 6) and amperometric (1, 6,7 , 8, 10, 13, 1 7 ) methods have been investigated, and a detailed study of electrometric processes has been published (16). With such methods, the problem of obtaining consistent electrode behavior has arisen, and has occasionally necessitated meticulous care in the handling of the electrodes. It has sometimes been necessary to store the electrodes under controlled conditions between titrations, and to pretreat the electrodes before a titration. In automatic coulo1 Present address, Department of Chemistry, University of Arizona, Tucson, Ariz.
metric determinations. electrical coupling between the generat ing circuit and the indicating circuit may prove troublesome. The present investigation was undertaken to eliminate the troubles associated with sensing electrodes, and also to reduce the number of electrodes in a titration vessel, by employing photometric detection of equivalence in coulometric titrations. This type of detection is limited to systems which give a suitable optical indication of equivalence, such as by the formation of a colored species, like iodine, following the attainment of equivalence, or to systems to which a chemical indicator may be added to produce a change in the absorbance (optical density) of the solution to provide the desired indication of equivalence. Other
V O L U M E 25, NO. 9, S E P T E M B E R 1 9 5 3
1345 Table I.
Stability of Constant Current during I-Hour Operation
Mean Current, Ma.
MaXim"m Deviation, %
Deviation, %
24.963
0.0040 0.0040 0.0040 0.0042 0.0063
0.0028 0.0014 0.0015 0.0018 0.0031
s0.022 101.971 142.992
17Z.036
Mean
APPARATUS
Figure 1. Automatic Coulometric Titration Instrument changes in the absorbance of the solution, such as may he brought about by dilution or by the formation of bubbles, a precipitate, or other complicating absorbing species, should be avoided. This paper describes the design, construction, and testing of an all-electronic self-contained instrument, operating from the 115volt alternating current line, that produces constant current over a continuously variable range, and terminates a titration by a photocell response to a color change in the solution. This paper also presents the initial performance of the instrument in three wulometric titrations producing different types of color change at equivalence.
Each of the published circuits for obtaining a cpnstant current was found to contain some objectionahle feature, such as the incorporation of a vibration-sensitive galvanometer ( 1 0 , the use of batteriesfor areferenoevaltage (14,16),or asourceofpowerother than the usual IlO-volt, 60-cycle, alternating current (4). I n the course of this work an all-electronic self-contained instrument (Figure 1) to aITord constant current over a continuously v a r able rmge and to terminate the titration by photometric deteetion was designed and constructed according to the circuit shown in Figure 2. The constant-current section (Te,Tza,and TU of Figure 2) w m a modification of the dependable constant-voltage circuit designed by the Radio Carp. of America. far its Type 5651 voltage reference tube (12). The modification provides constant current, including automatic compensation for a change in the resistance of the titration cell, such a8 might be produced by a change in the conductance of the electrolyte, or by the farmation of bubbles on the surface of the generating elPctrodes.
COMPONENTS
Figure 2.
Circuit Diagram of Coulometric Titration Instrument
ANALYTICAL CHEMISTRY
1346 The electrolysis current f i o w through T6>through the resistances associated wit,h SWr, through the generating electrodes, through the meter and shunts associated with the upper half of SW,, and through the resistances associated xvith the lower half of SWS. The voltage. drop across the resistances associated with the lower half of S W Sis compared by the second half of Tla with a constant voltage drop across Tll. If the electrolysis current should tend to change, the resultant change in voltage across the resistances associated with the lower half of SWs would be noted bv the second half of Tlo; this signal ~ o u l dbe amplified by the fikt half of Tla, and would alter the conductivity of Tg in the direction required to return the electrolysis current to its original value. The equivalence-delectioii section (7'. and Ta of Figure 2 ) incorporates a photocell relay circuit which initiates and terminates the electrolytic generating current according to the response of a photoelectric cell, and simultaneously switches on and off an elertric t,iming clock. This sertion was designed as a phototube modification of the excellent trigger circuit published by lliiller and Lingane ( 9 ) . The instrument, as designed and ronstruc ted, provides flexibility of autom:itic coulometric titration using photometric detection of equivalence in the range of current from 15 to 200 ma. Its performance in supplying a constant electrolysis current was tested by measuring the voltage drop across a 1-ohm standard resistance with :I Leeds and Northrup IC-2 potentiometer. -4100ohm resistance was connected to the output terminals as a substitute for a titration cell. hfter a xvzrm-up period of 30 minutes the subsequent maximum deviation of the electrolysis rurrent during the next 5-minute period was only 0.01%. The current output was measured continually and wts recorded a t 5-minute intervals for 1-hour periods at carh of five wlues of current. The results are shomi in Table I . That, a constmaritcurrent was maintained 1)y the instrument despite resistance change in the titration cell IT-asindicated by experiment,s in which an increase in the cell-substitute resist,ance of 50% (from 100 to 150 ohms) caused a decrease in the electrolysis current, of O.W5% in a current of 1 i 5 ma., and of 0.004% in currents from 140 to 25 ma. The 110-volt alternating current input to the instrument was supplied by a constant-voltage transformer to attenuate fluctuations in line voltage. EXPERIMENTAL
Coulometric tit'rations producing three types of csolor changes a t equivalence were investigated. In the first series of experiments arsenic was oxidized with t.lcctrolyt~icallygenerated iodine. and the color of unreacted iodine following attainment of chemical equivalence was detected by thc photocell. In the second experiment an acid-base titration \vas performed using the onecolor indicator phenolpht,halein. \vhosta red color was detected by the photocell. In the third experiment the same acid-base titration was performed using t,he two-color iridicat#orthymol blue, its change from yellow to blue being detwted by tho photocell.
-1very simple optical system n x s employed in this esploratory investigation. A &volt automobile stoplight bulb. operated from the 110-volt lines by means of it constant-voltage stepdonn transformer, vas used as the light sourre. I n the first experiment a mask on the side of the 25O-ml. titration beaker liniited the light to an area 1 em. high and 0.5 em. wide. This light \vas focused by the curved sides of the bealier to form a vertical line a t the phototube. S o auxiliary lenses or filters were used in the first experiment. I n the second and third esperinients, using acid-base indicators, a single condensing lens \vas used to concentrate the light on the small titr:ition vessel, and optical filters were used to enhance the sensitivity of the phototube to the color change. The generating electrodes used in all expcrilnents \yere platinum foil, 4 X 15 mm., sealed in the end of 4-mm. glass tubing. In the first experiment the generating electrode in the titration beaker was placed PO that the generated reagent lvould be immediately swept into the light path hy the continuous stirring action. In the second and third esperinients the relatively small size of the tit,ration vessels made the position of the generating electrode unimportant, so long as it was not iii the light path. Magnetic stirring was used in all experiments. The timer used was Model 5-100 of the Standard Elec%ricTime Co. .4Model S-10, which arrived after the work was completed, is easier to read, and its w e ifi recommended. Detection of Color of Reagent. I n these experiments invol\hg
the coulometric titration of arsrnite samples with electrolytically generated iodine, the generating cathode was enclosed in a conipartment consisting of a glass tube 1.5 em. in diameter and 10 cm. long with a fine porosity fritted-glass disk fused onto its bottom. This compartment was filled n-ith 1 N sulfuric arid; a platinum electrode was inserted, and the assembly was placed in the titration vessel alon with a platinum foil generating anode. Potassium iodide electrofyte, 140 ml., was added to the titration vessel and the generation of iodine was initiated. The generation was quickly terminated by the photometric detection section of the instrument as a small concentration of free iodine was formed in the electrolyte. The timing clock was then set to zero, and coulometric titration of the sample was automatically initiated by the addition of a 10-ml. sample of arsenite solution. T h e titration was automatically terminated b the photometric tietection section when the same very smalf concentration of free iodine was again present in the solution. The.product of the time of t'itration in seconds, as shown b y the electric timing clock, and the electrolysis current in amperes, as determined by measuring the voltage drop across a standard 1-ohm resistance with a precision potentiometer, was the number of couloriibe of electricity used in generating iodine, and was electrochemicallequivalent to the amount, of arsenic oxidized. The quantity of :trscnic may be calculated by the following equation:
11illiequivalent s =
current (in milliamperes) X seconds 96,500
I~E.WEKTS AXD RESL-LTS.. I 0.4000 iV standard arsenite ROIUtion was prepared from Sational Bureau of Standards arsenic trioside to be used as a stock solution for the preparation of it more dilute working solution. Dilutions were made using cdibrated glassware a t 20" C. I'reliniinary esperiments disclosed the desirability of controlling the pll of the solution t o be titrated. The kinetics of the rwction between arsenic and iodine is affected by the pH, and a decrease in the reaction rate favors proper anticipation of the equivalence point. I n the first series of titrations :t generating current of approxiniately 1 i 5 ma. was used, and a 10-ml. portion of a 0.1000 S arsenite solution was the sample. The electrolyte was 0.5 III in potassium iodide and 0.25 Jf in sodium dihydrogen phosphate (present as a buffer), and was adjusted to a p H of 7.0 with sodium hydroside. In the second series of titrations :t generating current of approximately 100 ma. was used, and a 10-ml. portion of 0.04000 -V :rrsenite solution was the sample. The electrolyt,e was 0.4 .l/ in potassium iodide and 0.1 ,If in sodium dihydrogen phosphate, :rnd n-as adjusted to a pH of 6.6 with sodium hydroside. I n t,he third series of titrations a generating current of approximately 50 ma. \vas used! and a 10-nil. port,ion of a 0.02000 S
Table 11. Titration of Arsenic I .0 3Iillicquivalent c0.04 - 0 , 10 -0.10 00 - 0 13
so
-0.13 -0.0ti
-0.07 0 . 4 .\lilliequivalent
,382 1 3S2,li 381 7 ,'is1 8 361 8 381 8 381. S
982 (i 382 3
0,3997 0.4001
0.3993 0,399.5
-1) 11s To.n2
-n.iR
n,3994
-n -0 -n
0 .m
f0.02 = 0 , 00
n
3994
0,3993 0.4001
o
,
12 13
I;,
-0.18
0.2 3Iilliequivalent - 0 , 1.i
- 0 . ii ii -0 I O + 0 . 10 - 0 . 1.5 t-0.2.5
-n.
V O L U M E 2 5 , NO. 9, S E P T E M B E R 1 9 5 3
1347
arsenite solution mas the sample. T h e electrolyte was 0.4 . I I in potassium iodide and).l !lf in sodium~dihydrogen phosphate, and was adjusted to a pH of 6.4 with sodium hydroxide.
:ipprminr:Ltoly 50 ma. w i s w e d . The results are presented in T;hlc 111. The average ermr w'ils 0.04% and the m x i m u m error "BS 0.11%.
The results of these titrations are presented in Table 11. The average error was 0.08% a t 175 ma., 0.10% a t 100 ma., and 0.15% a t 50 ma. The accuracy and precision obtained are considered to be favorable when it is considered t h a t no optical system or filters were used to enhance the sensitivity of detection of the end point. One-Color Indicator. I n an acid-base coulometric titration the electrolyte used is one whose ions do not discharge a t the generating electrodes, so t h a t hydrogen ions are produced a t the anode by the discharge of oxygen, and hydroxyl ions are produced a t the cathode by the discharge of hydrogen. The electrodes must therefore not only be isolated, but the analyte and catholyte must not be permitted to mix. The titration cell used for acid-base titrations is shown in Figure 3.
Two-Color Indicator. T h r same apparatus and procedures >\-ereused for titmtions employing thymol blue as a two-color indicator as far those employing the one-color indicator, with the followine exceptions.
A vollow filter, Wratten S o . 77, \vas used instead of the green
were initially pkced i d t h e titration ves'sel i n s t e d af'20 ml. a i
tralyti diffused through the frftt,cd-glass plate in70 the 20 m l . ~ & distilled water in the iit,ration vessel to permit the initial electrolysis \which established the ealoy to be detected as the end point by the photometric det,eetion section of the instrument. A sharper end point was prodtxed by use of distilled water than >\-as obtained when 0.5 M electrolyte w i s used in the titration vessel. REAGENTSAND RESULTS-A solution containing 0.04186 meq. of potassium acid phth:datc per gram of solution x a 8 preoared from the NBS salt. Thc 0.5 sodium sulfate eleetrolvte k e d in the center tube of thc titration assembly did not contiin any indicator. An electrolysis current of 56 ma. was used. The results are presented in Talde IV. T h e average en.or was 0.07%, and the maximum errar USLS 0.16%. DISCUSSION
F i g u r e 3.
T i t r a t i o n Cell Assembly
T a b l e 111. Acid-Base T i t r a t i o n w i t h One-Color I n d i c a t o r Titration
To.
Reid Sample,
Carrent,
Ma.
Seconds
Acid Pound. Mea.
Error,
The eleetrolybe flows down through the center tube and up into each of the side vessels through fritted-glass plates. The UIIwanted electrolyte is continuously discharged from the vessel on the ripht, and titrations are performed in the vessel on the left.
t&t;.ation p"eriad. A deep green optical filter W a t t e n No. 61, was inserted in t,hc light path to increase the sedsitivity of detection of the end point. For each titration the 30-ml. vessel a t the left (Fieure 3 ) iims drained by use of a small flexible tube connected tb dn aspirator, was rinsed v i t h electrolyte, and then had 20 ml. of electrolyte added to it. Reagent generation was then initiated, and this was quickly terminated by the photometric detection section of the instrument when the phenolphthalein in the electrolyte developed a faint color. The electric timer was reset to zern and coulometric titration was automaticdly initiated by the addition of approximately 5 ml. of a sample of potassium acid phthalate from a weight buret. T h e coulometric t,itration WBS automatically terminated when the phenolphthalein returned to its initial faint color. The milliequivalents of acid thus titrated %-erecalculated using the formula given above for the redox titration. REAGENTS AND RESULTS.A dried sample of NBS potassium acid phthalate was used to prepare a solution containing 0.04991 meq. of acid per gram of solution. The electrolyte used was 0.5 IM in sodium sulfate and contmned the equivalent of 2 drops of 0.1% phenolphthalein per 20 ml. An electrolysis current of
The major difficulty encountered in this investigation m s concerned with the absorbance produced by the bubbles of gas formed on the generating electrode during acid-base titration. These bubbles made the determination of equivalence with a s y c tem of indicator and optical filter which transmitted less light before than after equivalence practically impossible, bemuse the additional density of the bubbles invariably caused overtitration. I n the reverse operation-is., titrating with 8. system of indicator 2nd optical filter transmitting more light before than after equivalence-the density duc t a the buhbles usually caused a premat,ure termination of tho titration, hut as soon as the bubbles left the solution the photometric deleetion section of the instrument reinitiated titration to t.hc true cnd point. I n the latter case t.he nccuracy of the titration was unaffocted because of thc synchronized switching of tho clcetrolysis current and the timing clock. This offeet of the density due to bubbles may be greatly suppressed, if not completely eliminated, by the use of two photocells operating in electrical opposition, each viewing half of the light coming from the titration cell, and being equipped with suitnllle filters.
T a b l e IV.
Acid-Base T i t r a t i o n w i t h Two-Color I n d i c a t o r
Titrstion
loid Sample,
Yo.
Men.
c.n.ent,
\In.
Seconds
Acid Bound, Xea.
Errov, %,
This investigation has sho\x-n t h a t sutomatic coulometric titrations employing photometric deteotion of equivalence can he performed rapidly, aecuratcly, and reproducibly with a relatively simple optical system. It should be possible to utilize the eonstantcurrent source reported here coupled with a differential photocell circuit to run rapid volumetric analyses of many typcs in hoth &queous and nonsqueous media. ACKNOWLEDGMENT
The authors are ploased to acknowledge support in this in-
ANALYTICAL CHEMISTRY
1348 vestigation by the Office of Ordnance Research of the Army Ordnance Corps. LITERATURE CITED
(1) Brown, R. A., and Swift, E. H., J . Am. Chem. SOC.,71, 271i-19 (1949). (2) Carson, W. N., and KO. Roy, AHAL. CHEW, 23, 1019-22 (1951). (3) Cooke, W. D., and Furman, N. H., Ibid., 22, 896-9 (1950). (4) DeFord, D. D., Johns, C. J., and Pitts, J. N., Ibid., 23, 941-4 (1951). ( 6 ) Farrington, P. S.,and Swift, E. H., Ibid., 22, 889-91 (1950). (6) Furman, N. H., Cooke, W. D., and Reilley, C. K., Ibid., 23, 945-6 11951). (7) Furman, N. H.,Reilley, C. N., and Cooke, W. D., Ibid., 23, 1665-7 (1951). ( 8 ) Meier, D. J., Rlyers, R. J., and Swift, E. H., J . Am. Chem. SOC.,71, 2340-4 (1949).
(9) RIuller, R. H., and Lingane, J. J., ANAL. CHBM.,20, 795-7 (1948). (10) Myers, R. J., and Swift, E. H., J . Am. Chem. Soe., 70, 1047-52 (1948). (11) Patton, H. W., ANAL.CHEW,23, 3 9 3 4 (1951). (12) Radio Corp. of .imerica, “Tube Handbook,” HB-3, Vol. 2, Tvoe 5651. (13) Rams&, W. J., Farrington, P. S., and Swift, E. H., ANAL. CHEU.,22, 332-5 (1950). (14) Reilley, C. K . , Adams, R. K., and Furman, N. H., Ibid.. 24. 1044-5 (1952). (15) Reilley, C. N., Cooke, Re. D., and Furman, N. H., Ibid., 23, 1030-2 (1951). (16) Ibid.. DD. 1226-9. (17j Woost&, W. S., Farrington, P. S., and Swift, E. H., Ibid., 21, 1457-60 (1949). RECEIVED for review February 2 6 , 1953. Accepted June 25, 1953. Presented before the Division of Analytical Chemistry at the 123rd.Meeting of the AMERICAS CHEsrrcAL SOCIETY. Los Angeles, Calif.
Observing the End Point in Tests of Latex Mechanical Stability ALFRED C. MEYER’, U. S. Rubber Co., North Bergen,
N. J.
In tests of the mechanical stability of natural rubber latex by the standard ASTM procedure, considerable variation occurred among the results obtained at four laboratories. Accordingly, possible causes of observer error in the test were investigated. A new technique, involving only a minor modification in the endpoint determination by the ASTM method of testing the mechanical stability of natural rubber latex, has been developed. Particle formation during a run is studied throughout the periods of incipient and complete coagulation. This is made possible by successive dips with narrow 80-mesh Monel screens, which are then washed free of adhering latex. Variance analysis shows lower error than with the currently used rod-dipping method on unknown samples.
I
N THE ASTM procedure for measuring mechanical stability of latex (I), an alternative method recommends that the end point be taken as the start of formation of visible clots in the latex, recorded as time in seconds from the start of the test until clots begin to form, and verified by dipping a glass rod into the latex and drawing it once over the palm of the hand. Small pieces of coagulated rubber may readily be seen, if they are present in the film deposited. This presupposes that latex may be stirred on a high-speed stirrer for a given length of time before any coagulation occurs, and that the method is one by which the point of incipient coagulation is observed. McColm (4) reported the results of a round-robin test of mechanical stability carried out on eight latexes obtained from all the large American importers, by four different laboratories. He gave the error standard deviation obtained a t the different laboratories in tests on the same latex as follows: Laboratory
A B C
C (repeat t a t ) D
Error Standard Deviation, Seconds
48.5 44.4 30.1 13.3 11.4
These large differences in error, characteristic of laboratories which regularly run thousands of tests annually, indicate the possibility that some factor in the latex contributes t o the in1
Present addrem. P.O. Box 31, West Englewood, N. J.
elusion of a Iarge observer error, reached to a greater or less extent by an unintentional forcing of replicated results. Gradual coagulation under agitation, rather than sudden coagulation, would fit such a supposition. With coagulation slow and the rate of the increase of coagulated particles low, the odds in favor of the operator’s observing an end point in the samples withdrawn with his sampling rod nrould change relatively slowly during the early stages of the test. Accordingly, if the operator were confronted with a latex of previously unknown stability, the reported end point might be anywhere within a range of several hundred seconds. Assuming a likelihood of some unconscious forcing of results in the routine of even an honest experienced operator, the error on an unknown sample between separate coded replications in which the operator had no means of knowing the value of previously obtained results should therefore be much greater than the corresponding error between replicated observations made on a known latex. This situation waa actually found to exist after a more objective means of determining end points had been developed, in which the observer error could be reduced to a minimum. PROCEDURE DEVELOPED
The glass sampling rod is replaced b a set of narrow %mesh Monel screens, 8/16 X 1.5 inches, whicx, held a t one end by forceps, are successively quickly dipped perpendicularly to the direction of flow into the latex being tested, a t desired intervals of time, and withdrawn. This may be seen in Figure 1, though the screen shown there has not yet come in contact with the latex. The small section cut out of the upper sample cup holder for this
