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
V O L U M E 25, NO. 9, S E P T E M B E R 1953 purpose can he closed a t other times by a piece of ruhher stopper cut to proper shape. The flabmeshed surfsce of the screens permits a much more complete and representative sampling under conditions of rapid flow than does a rod, without slowing down thc flow auvreciablv.
1349 hering latex but leaves m y particles that were caught on the mesh. The cleaning is then completed by just barely touching the lower side of the sereen to the surface of ammonia water or blotter saturated with it, hut not deeply enough to loosen any uarticles a t the u m e r Side which were obtained in the diD. Fi-
ANALYTICAL CHEMISTRY
1350 Table 1. Coded Unknown Saniples of Soreen-Rod Comparisons of Feur Lathes Original Total Solids and Stability
67.37%. 958 set. 67.11%. 1175 see. D2.08%, 1235 see. til.SS%, 1688 8ec.
Cream A Cream B Centrifuged A Centrifuged B
1st day
780 810 780
825
840 930
...
828
L"..
1185 1203
1142 824 1048 llijY 1094 fuged 4 1187 1660
2""
uay
"'al
'"70 I0
io
1445 1082 987 818 1202
1320
1192
1140 1320
1220
1185
1367
1170 1140 1213
1281 1231
1139 1238
Centirfuged B 1680 1849 1920 1131 1680 1495 1755 1392 1800 1140 1680 1045 1753 1342
In c.d, case a sufficient batch of the latices to be examined was made on each of the two days rtnd run into separate jars for the coded samples or into quart bottles for the known sample, previously being adjusted to 51.5% total solids. Thus, six runs were made by each method on the coded Samples, vhich consisted of two creamed and two centrifuged latices (Table I); and 20 runs were made by each method on a sinale known creamed latex (Table 11). DISCUSSION OF RESULTS
Figure 2 gives s. complete set of sere en^ for a stability test of creamed latex taken a t the intervals (in seconds) indicated by the numbers shown above eachscreen. The early- screens in this set afford a good illustration of random particle formation, apparently- indicating a slow-preparatory process that must precede incipient coagulation; the formation in the first eight screens gets nowhere and it is not until those taken a t 1110 or 1140 seconds
Comparisons (One later used.
EXPERIiMENTAL DETAILS
Original total solids 66.90%. stability 1237, oream)
1st (1:
The work WBS divided into two parts. The first consisted of three runs each on four different samples of latex, in nhich t,he operator was not permidied to known thr identity of the material he was testing; all bhe runs u-e11?carried out by both the new screening method and the usual rod-dipping method. The entire sets m r e repeated on the following d q . The second part consisted of 10 runs on B single latex known to the operator, by both methods, and again the entire serics r z s repeated the next day.
Stand:
.. Figure 3.
Complete Stability Test of Centrifuged Later X1.375 ~ n l a r e ~ m e n t Sorceninp at inferv& indicated
..-
V O L U M E 2.5, NO. 9, S E P T E M B E R 1 9 5 3 Table 111. Yariance . h a l y s i s of Table I Source of Variation
Variance Significance Of Entire Data Between rods and screens 100,650 Significant a t odds of 95 to 2 Between latices 709,556 Between days within laticrs 119,983 Experimental error 23,615 Standard deviation = 1.34 seconds B. Of Screen Data Between latices and days 395,602 Experimental error 12,628 Standard deviation = 112 seconds C. Of Rod Data Between latices and day? 97,038 Experimental error 34,602 Standard deviation = 186 seconds Ratio, rod error variance = 2.74, signifirantly greater than screen error screen error variance a t odds between 95 to 5 and YR to 1.
A.
are reached that coagulation actually gets under way, increasing until complete coagulation is attained a t 1230 seconds. Therefore, 1140 seconds u dc chosen as the end point of this run and the interval between incipient and complete coagulation in this case is 1230 - 1140 or 90 seconds. Figure 3 gives a complcte iet of screens for a stability test of centrifuged latex. h:tlogous conditions appear. The particles in the early screens, though somewhat larger, again show random formation and no definite trend of sustained coagulation begins to set in until the screen at 1620 seconds is reached. I t was therefore selected as the vnct point (incipient coagulation) for this run; but here the interval to complete coagulation is longer1’740 - 1620 or 120 second5 This typr always showed a longer interval than creamed Iatey. Even if such intervals wv(w canceled out, by going directly to the screen first showing complrte coagulation, the results would not become more reproducible. Somewhere within that range the material becomes less homogeneous, as indicated by the condition in which it is found a t tlir end of the run‘i. which could be a considerable source of error were end point\ to be chosen on a basis of “completeness” rather than incipienry. The final condition, even among replicates, m u prove to be liquid. pasty,
1351
lumpy, or often, though not invariably (3), a single uniform mass of coagulum. This was noticed after a means had been found of preventing all shaft coagulation during a run; the shaft (not the propeller disk) n-as coated with collodion, as was done in all the work reported here. Turning now t o the numerical data of Tables I and 11, the results show that the rod operator attained reasonably good checks on the known latex, obtaining a standard deviation of error of 36.8 seconds. Corresponding results were obtained with the screens. However, with the unknown coded samples the rod operator definitely had a much larger error and did not equal the Pcreens’ precision. The screens also gave larger error on the unknown than on the knonm sample, because when a screen operator has an idea of the approximate end point he can dip more frequently when approaching the region in which it lies (up to about once every 15 seconds for a single operator) and thus increase his chances for having les- error between replicates; with an unknown the end point ma>-come while he is still dipping a t longer intervals. VARIANCE ANALYSIS
The results of variance analysis on the data obtained are given in Table 111. From these results it was shown that: The stability mean values obtained with screens are significantly higher than with the rod dipping method. The screen dipping method has significantly lower error than the rod dipping method in the standard ASTM test on u n k n o m samplv. LITERATURE CITED
(1) Am. Soc. Testing Materials, Philadelphia, Pa., “Tentative Specifications and Methods of Test for Concentrated, Ammonia
Preserved, Creamed and Centrifuged Natural Rubber Latex,” Designation D 10764-9T, 1949. (2) Dalfsen. J. W. van, Rubber Chem. & Technol., 14,316 (1941). (3) Dawson, H. G., A N ~ LCREM., . 21, 1071 (1949). (4) XIcColm, E. M., memorandum entitled ‘‘Latex Mechanical Stability,” submitted to Latex Subcommittee, Committee DII, Am. SOC.Testing Materials, July 1952. R E C E IED ~ for re\
ICW
February 2, 1953.
Accepted June 24, 1953.
Polarographic Studies with a Stationary Mercury-Plated Platinum Electrode THOiM.4S L. M4RPLE AND I,. R . ROGERS Department of Chemistry and Luboraiory for Nuclear Science, .Massachusetts Institute of Technology, Cambridge 39, Mass.
T
H E adaptability of solid electrodes, either stationary or rotated, for continuous analyses in closed or flowing systems has increased the desirability of finding a better substitute than a platinum electrode for the dropping mercury electrode (DME). The work of Miller (8) suggested that deposition of a mercury film onto a solid electrode of another element should provide an electrode with nearly as high overvoltage toward hydrogen as the dropping electrode. From the work of Zlotowski (IS), one would anticipate difficulties from contamination in using a stationary mercury surface for polarographic studies. However, it appeared that the use of such an electrode would a t times be advantageous in spite of these difficulties. Two years ago H. M. Hershenson in this laboratory (6) compared various methods of plating mercury onto platinum, including a suggestion by Uhlig (12)involving depositions ont,oplatinum which had previously been heated and quenched in mercury. In
no case was the mercury retained sufficiently well to be certain that reliable results would be obtained when the electrode was rotated rapidly. Independently, Cooke ( 4 ) and Perley (IO) suggested the use of silver as the supporting metal because of its ability to amalgamate. Although Cooke ( 3 ) has recently reported the successful use of such an electrode, Hershenson’s attempts to use a silver support for the mercury film, while more successful than with platinum, were not entirely satisfactory. Furthermore, coulometric studies (6) in this laboratory have indicated that an amalgamated silver electrode will change its calibration with time. Consequently, attention was again directed toward the mercury-plated platinum electrode for use only as a stationary electrode. When the present study was nearly complete, Arthur et al. ( 1 ) reported a similar study on a mercury eIectrode formed by forcing mercury up a small-diameter glass tube to present a mercury sur-
