208
Vol. 18, No. 3
INDUSTRIAL AND ENGINEERING CHEMISTRY
by the method described. The results of these determinations (Table IV) are in very good agreement with the certified value, when the variation in the values reported to the Bureau of Standards by the collaborating analysts is taken into consideration.
and analyses of standard samples indicate that nickel may be determined with satisfactory precision when present in amounts of 1microgram or greater.
SUMMARY
(1) Denighs, G., Bull. SOC. phann. Bordeaux, 20, 101 (1932). (2) Holland, E. B., and Ritchie, W. S., J. Assoc. Oficial Agr. Chem.,
A colorimetric micromethod for nickel is described which is applicable to the determination of traces of nickel in foods and biological materials as well as in carbon steels. It is based upon the isolation of nickel by solvent extraction of the dimethylglyoxime complex and the subsequent determination by IneanS of sodium diethyldithiocarbamate. Recovery determinations
LITERATURE CITED
22, 333 (1939).
(3) Lindt, T.V., 2. anal. Chem., 53, 165 (1914). (4) Murray, W.R.I., Jr., and Ashley, S. E. Q., IND.ENG.CHEM., ANAL.
ED.,10, 1 (1938). ( 5 ) Sandell, E. B., and Perlich, R. W., Ibid., 11, 309 (1939). (6) Yoe, H. J., and Wirsing, F. H., J. Am. Chem. Soc., 54, 1866 (1932).
Amperometric Microtitration of Very Dilute Chromate Solutions Using the Rotating Platinum Electrode I. M. KOLTHOFF AND D.
R. MAY’, School of Chemistry, University
Chromate (chromic acid) can be titrated rapidly b y amperometric titration with ferrous iron, using the rotating platinum wire microelectrode as the indicator electrode. The potential of the indicator electrode is maintained at 1.0 volt vs. the saturated calomel electrode which i s used as the reference electrode. The method it accurate and precise to within 0.5% at concentrations as small as 1 to P x 10-4 M chromate. The method can also b e used in the reverse titration of traces of ferrous iron with dichromate.
C
HROMATE in very dilute acid solutions can be accurately determined by amperometric titration with ferrous sulfate, using a rotating platinum electrode as indicator electrode. This method does not involve a n indicator correction; this latter is appreciable when very dilute solutions are titrated. The principles and technique of amperometric titrations with the rotating platinum wire microelectrode have been discussed and described by Laitinen and Kolthoff (1,8). The method described here for chromate involves the titration of an acidified chromate solution with a ferrous iron solution. The end point is found graphically as the point of intersection of the “residual current” line before the end point and of the anodic diffusion current line of the ferrous iron after the end point, Under the experimental conditions used in this work the residual current was zero. The anodic oxidation of ferrous iron t o ferric gives a well-defined diffusion current a t a rotating platinum microelectrode. Current-voltage measurements using 5 x 10-6 M ferrous ammonium sulfate in 0.1 M perchloric acid are shown in curve 1, Figure 1. I n curve 2, the residual current of 0.1 M perchloric acid solution is shown. At a potential of +1.0 volt of the rotating electrode, as measured against a saturated calomel reference electrode, proportionality was found between the concentration of the ferrous ammonium sulfate and the diffusion current, as may be seen in Table I. At potentials of f1.0 volt no current is obtained by electrode reactions with Cr++++++, Cr+++, or Fe+++. Thus when an acid solution of chromate is titrated with a ferrous ammonium sulfate solution no current is observed at a rotating platinum electrode potential of 1.0 volt until all the chromate has been reduced and the solution contains an excess of ferrous iron. By measuring the current after adding successive amounts of ferrous solution and obtaining two or more values in the presence of an excess of ferrous solution the end point is found by graphical extrapolation to 1 Present
address, American Cyanamid Co., Stamford, Conn.
of Minnesota, Minneapolis, Minn.
zero current. The volume of ferrous solution a t that point is equal to that a t the equivalence point. Typical titrations are &own in Figure 2. PROCEDURE
The chromate solution to be titrated is placed in a beaker of suitable size and a rotating platinum microelectrode and salt bridge are placed in position. A saturated calomel electrode is used as an outside reference electrode. If the solution is not already acid, sufficient acid is added to give a. concentration of 0.1 M (perchloric, hydrochloric, sulfuric, or nitric acid may be used). A 1-volt difference of potential is applied across the platinum (anode) and saturated calomel electrodes, using a salt bridge of low resistance. A 0.01 N standard ferrous ammonium sulfate, 0.05 M in sulfuric acid, is added to the chromate solution from a microburet until a current is observed on the microammeter or other suitable current-indicating instrument. The sides of the beaker are rinsed with distilled water and the diffusion current of -
Table 1. Diffusion Currents of Ferrous Ammonium Sulfate Solutions in 0.1 M Perchloric A c i d Solution (Rotating platinum electrode. Molar Concentration
of Ferrous Iron X 10s 10 5 2
4.5 1.8 0.9
1
+ 1.2 POTFNTLAL
+
Potential 1.0 volt electrode) Diffusion Current, Microamperes 8.9
YS.
+1.0 S41WATtD
+0.8 ChLOMfL
8s.
saturated calomel id/C
x
los
0.89 0.90 0.90 0.90
+O.b 0.4 €L€CTkODt (VOLT)
Figure 1. Current-Voltage Curve of 5 X M Ferrous Ammonium Sulfate in 0.1 M Perchloric A c i d 1 5 X 1 0 - b M fenour ammonium sulfate in 0.1 M perchloric acid 9: 0.1 M perchloric acid
ANALYTICAL EDITION
March, 1946
-4
t
Table 11.
209
Amperometric Titration of 50 Solutions
Molar Concentration Volume of 0 . 0 1 N of Chromate X 10‘ Ferrous Solution, MI. 1.560 1.00 2.086 1.333 2.358 1.510 2.603 1.667 2.590 1,667 2,000 3.122 3,120 2,000
2.0 2.1 VOLUM€ Of
2.2
2.3
2.4
2.5
2.6
STANDARD fERROU3 SOLUTION (ML.) Figure 9. Amperometric Titration OF Potassium Chromate in 0.1 M Perchloric A c i d with 0.00969 N Ferrous A m monium Sulfate 1. 2.
50 ml. of I .333 X I O - ’ M chromate 50 ml. of 1,510 X IO-‘ M chromate
the ferrous iron is measured. The current is measured again after the addition of 0.2 ml. more of ferrous solution. One or two more readings are made after successive addition of more ferrous iron. On graph paper, points representing the microamperes (ordinate) and volume of ferrous solution (abscissa) are plotted. These points lie on a straight line. The volume of ferrous solution at zero current is obtained by drawing a line throu h these points. For the most accurate work the current should Ee corrected to compensate for dilution (9). Several dilute chromate solutions were analyzed in this manner. The ferrous solution used was prepared by diluting a 0.098 N ferrous ammonium sulfate solution tenfold. The diluted solution was standardized against potassium chromate, using sodium
M1. of Dilute Chromate
Calculated Molar Concentration of Chromate Solutions x 10‘ 0.999 1.336 1.511 1.668 1.660 2.000 1.999
Error,
%
-0.1 $0.2
+o.
1 +0.1 -0.4
0.0 -0.1
diphenylamine sulfonate as indicator and correcting for the indicator blank. The concentration found was 0.00961 N . In Table I1 are the results obtained in the amperometric titration of dilute chromate solutions with 0.0096 N ferrous ammonium sulfate solution. The solutions were 0.1 N in perchloric acid. The rapid method, which is accurate and precise to 0.5% even a t concentrations as small as 1 to 2 x 10-4 M chromate, can also be used for the reverse titration of traces of ferrous iron with dichromate. The current decreases continuously during the titration and becomes zero a t the end point. Again the exact location of the end point is found graphically. LITERATURE CITED (1) Kolthoff, I. >I., and Lingane, J. J., “Polarography”, New York, Interscience Publishers, 1941. (2) Laitinen, H. A., and Kolthoff, I. M.,J . Phys. Chem., 45, 1079 (1941). FROM part of a thesis submitted by D. R. May to the Graduate School of the University of Minnesota in partial fulfillment of the requirements for the doctor’s degree (1944).
Rapid Estimation of Rubber in Guayule Latex Dispersions R. T. WHITTENBERGER AND B. A. BRICE, Eastern Regional Research Laboratory, Philadelphia 18, Pa.
S‘
I CE (4) has shown that the rubber of guayule (Partlienium argentaturn Gray) is recoverable as a latex which yields a product of high purity and superior quality. His observations on the quality of the latex rubber have been confirmed recently by Clark and Place ( 1 ) . The occurrence of the latex in small separate cells rather than in a continuous duct system makes necessary a fine comminution of the shrub, and as a result the dilute dispersions initially obtained are highly contaminated with nonrubber plant and soil fragments. A microscopic method for the rapid estimation of rubber in these dispersions has been developed. The microscopic method, which consists essentially in the identification and counting of rubber latex particles in a small volume of dispersion, is advantageous in several respects. It requires a minimum amount of equipment and is rapid, taking from 10 to 20 minutes for a determination. A rapid method is important because of the instability of some dispersions and the need of knowing the approximate concentration of dispersed rubber during ‘a recovery experiment. Furthermore, only a small quantity of dispersion, as little as one drop, is required for the microscopic method, and it is possible to distinguish between dispersed and partly agglomerated latex in the samples. I n contrast, the chemical methods of analysis (6, 7) are time-consuming, do not distinguish between disperse$ and partly agglomerated latex, and require la7 ger samples and more extensive equipment.
STANDARDIZATION AND PROCEDURE
The method is standardized by making counts on a number of guayule latex dispersions of known rubber content, as deter-
mined by duplicate analyses by the tetrabromide method (7). I n this method, the dispersions are extracted in a Waring Blendor Kith benzene, and rubber hydrocarbon is then determined gravimetrically by precipitation as the tetrabromide. Only dispersions free from agglomerated or partly agglomerated rubber particles should be used for the standardization. Since some dispersions are unstable, the microscopic counts should be made a t the time the chemical analyses are begun.
A small sample (usually 1 ml.) of a dispersion of known rubber content is mixed with neutral distilled water to give a known dilution in which the rubber latex particles can be accurately counted in a Petroff-Hausser bacteria counter. Probably a hemacytometer .Mould serve as well, although such a chamber Tas not tested in the present case. Owing to the tendency of the latex particles to accumulate a t the surface on standing, the dispersion must be thoroughly agitated, not only just before sampling for dilution but also after dilution, immediately before mounting in the counter. A dilution which gives 2 to 5 recognizable latex particles per square (1/20,000 cu. mm.) has been found most satisfactory. To ensure a uniform film of dispersion for the mount, the usual precautions must be taken, such as carefully adjusting the cover glass and blotting the excess liquid.. For one determination, all identifiable rubber latex particles, irrespective of size (usually 0.4 to 3.5~in diameter), in 36 squares in each of two mounts are counted. If agreement between the number of particles counted in the two mounts is not within 5%,, pounting should be continued on a third mount, or op an additional number of mounts until such agreement is obtained. The count is readily made with a magnification of approximately 800 to 900 diameters, such as is obtained with a 4 m m . objective and 20 X ocular. Each particle must be brought into clear focus for identification; those too small for identification should not. be counted. Since all flow of particles must be eliminated during counting, i t is sometimes necessary t o readjust the cover glass
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