Potentiometric Iodometry at Low Concentration m l . T I T R A N T (DERIVATIVE CURVE1
SIR: Iodometric end points are usually determined visually, using starch as a n indicator for the excess iodine. This method, however, is sensitive only to about 10-5N 1 2 , although the sensitivity can be increased somewhat if amylose is used instead of starch (4). The instrumental methods of end point detection which have been used are spectrophotometry, amperometry, and potentiometry. The spectrophotometric method has the greatest sensitivity-about 3 X 10-8N (2), while the amperometric method is sensitive to about lo-” (3). The most sensitive potentiometric work has been done by the “null-point” method, and sensitivities around 1.4 X lo-’ have been obtained (6). This method, however, is not especially convenient, as it calls for two identical electrodes and requires a “bottled end point” reference solution. A sensitivity of about 1.4 X 10-6N is the best that has hitherto been reported for the classical method of potentiometric titration against a fixed reference electrode (1). Since the classical method is one of the simplest to use, we have undertaken to determine its limiting sensitivity. Largely on the basis of convenience, we have chosen to work with the titration of arsenite with triiodide. This reaction has been extensively studied and is known to proceed quantitatively in neutral or slightly alkaline solution (5, 8 , 9). EXPERIMENTAL
The potentiometric titrations were followed with a Beckman Model G p H meter using a platinum wire indicating electrode. It proved helpful to heat the wire to yellow heat in a gas-air flame immediately before each use. A glass electrode immersed in the buffered reaction medium served as the reference electrode, avoiding the danger of introducing impurities through the liquid junction of a conventional ref-
Table I.
Method High concentration, visual end point
0.5
ANALYTICAL CHEMISTRY
8.90
0.95
9.00
I
I
I
I
0.4
m
0.3
5 >
0.2
0.I
ml. TITRANT (INTEGRAL CURVE)
Figure 1. Titration of 90 ml. of 9.99 X 10-5N arsenite in 0.01M borax with 1.007 X 10-3N triiodide
0-0-0
A--A--A
lntearal curve Derivative curve
erence electrode. Titrations were carried out using a special 10-ml. microburet made by the Kimble Glass Co. and equipped with a platinum tip that delivered 1O-pl. drops. Ordinary distilled water was distilled from alkaline permanganate, then from dilute sulfuric acid, and once more without additive before being used. An amylose suspension in 1-butanol prepared by the G. F. Smith Chemical Co. was used to make up a 1% amylose solution. All other chemicals were commercial products of reagent grade. The low concentration experiments were carried out by titrating potentiometrically 90 ml. of a solution 0.01M in borax and containing 9.91 X equivalent of arsenite with a solution 1.049 X lO-3N in iodine and 0.2M in KI. The high concentration ref-
Typical Experimental Results
Titer, ml. 9.45 9.44 9.45 9.45 9.45 9.45 9.448 f 0.004a 9.45 9.45 Low concentration, 9.46 9.46 potentiometric 9.46 9.45 end point 9.45 9.45 9.45 9.46 9.54 9.442 f 0.006a a Uncertainty expressed as standard deviation
298
8.85
Blank, ml.
Corrected av., ml.
0.002
9.446
0.02
9.432
erence experiments were carried out by titrating 20 ml. of a solution 0.2M in Na2HP04, containing 9.91 X equivalent of arsenite and a few drops of 1% amylose with a solution 0.1049N in iodine and 0.25;M in KI until the blue color persisted. Slightly different amounts of iodine and arsenite were used in the titration shown in Figure 1 . RESULTS AND DISCUSSION
Table I shows the results of a typical set of experiments. Titrations carried out a t appreciably lower concentration failed to give distinct and reproducible end points. Low concentration titrations buffered with borax seemed to give somewhat better end points than those buffered with phosphate. Figure 1 shows the integral and derivative curves of a typical low concentration titration. A ~ o - ~ arsenite N solution can be titrated with a precision of *0.06’%. If the high concentration titration is taken as a standard, the absolute error of the low concentration titration is 0.15 & 0 . 0 7 ~ 0 . Cncertainty in the blank of the low concentration experiments represents an unaccounted for source of error. The blank is due to the presence of impurities in the water which react with iodine, and it can be reduced to less than 1 pl. of 0.001N iodine if the water is pretreated with chlorine by the method of Megregian ( 7 ) . It appears, however, that in any event, we are close to the practical limit
of our method, and our realistic limiting sensitivity is probably not much better than 5 X lO-*A' iodine. The break in the titration curve of Figure 1 is considerably smaller than we would calculate from the redox POtentials of the As(III)-As(V) and I-IB- couples. Apparently the system prior to the end point does not establish an potentia' at the platinum electrode.
LITERATURE CITED
( 1 ) Bard, A. J., Lingane, J. J., Anal. Chzm. Acta 20, 463 (1959). ( 2 ) Grover, E. W., Reilley, C. N., ANAL. 26, 1750 (1954). (3) Knowles, G., Lowden, G. F., Analyst 78* 159 (1953). (4) Kolthoff, I. M., Belcher, R., "Volumetric ~ ~ ~ yol. l ~ 111, ~ pp, i 205-210, ~ , " Interscience, New York, 1957. (5) LiebhafskY, A., J . PhYs. C'hem. 35, 1648 (1931). ( 6 ) hlalmstadt, H. V., Pardue, H. L., ANAL.CHEM.32, 1034 (1960).
cHEM,
( 7 ) Megregian, S., U . S. Public. Health Reports 63, 137 (1948). ( 8 ) Roebuck, J. R., J . Phys. Chem. 6 , 365 (1902). ( 9 ) Washburn, E. W., J . Am. Chem. SOC. 30, 32 (1908).
FRANCI L. ANDERSON EVANH. APPELMAN Argonne National Laboratory Argonne, Ill. WORK erformed under the auspices of the 8 s .Atomic Energy Commission.
Carbon Determination in Hyper-Pure Elemental Boron Utilizing Gas Chromatography SIR: I n an earlier work ( I ) , we developed a method for oxidizing a boron sample and determining the carbon content by conversion to carbon dioxide. The procedure consisted of firing a powdered boron sample in an oxygen atmosphere with tin accelerator under carefully controlled conditions in a high frequency induction field. At that time the electroconductometric method was chosen for the carbon dioxide detection, and a sensitivity of approximately 20 p.p.m. carbon was suggested. I n subsequent papers, we were able to show a maximum detector sensitivity of approximately 5 p.p.m, carbon under ideal parameter conditions using gas chromatography (2) and combustion-gas chromatography ( 3 ) . I n the work presented here, the gas chromatographic method was applied to carbon analysis in elemental boron and the lower limit of sensitivity imposed by the electroconductometric detection system has been extended below the suggested 20 p.p.m. carbon. This method employs a programmed temperature gas chromatograph utilizing a molecular sieve column to trap the evolved carbon dioxide, a thermal conductivity detector, and a recorder equipped with a Disc chart integrator. EXPERIMENTAL
Preparation of Standard Curve. A standard curve was obtained by using different weights of NBS 170a, 55e, a n d 16d steel a n d observing t h e number of counts recorded for known weights of carbon. Linear results were obtained over the range of 700 p.p.m. down to less t h a n 20 p.p.m. T h e curve passed slightly above the origin when extrapolated t o zero carbon content. The number of counts above the origin corresponded to what was obtained by firing a Leco tin blank and was of the order of 5 p.p.m. This is the basis for the 5 p.p.m. sensitivity suggested above. At the 1% carbon level, the change in the slope of the
standard curve was less than 5%. The standard deviation in the range of 104 p.p.m. to 600 p.p.m. was +1.5oJ, or less. Procedure. T h e helium two-stage regulator was turned on a n d a helium flow rate was established. T h e column furnace a n d fan were turned on and the column temperature was set at 100" C. T h e bridge current was turned on and set at 130 amperes and t h e system was allowed t o a t t a i n equilibrium overnight. T h e helium flow rate was set at 150 ml./minute (12 p.s.i.), while the oxygen regulator was set a 11.5 p.s.i. which corresponded to a n oxygen flow rate of 100 m1Jminute. The prefire valve was opened and the crucible loaded with 0.600 + 0.0009 gram of 170a steel and 1.00 f 0.001 gram of Leco tin accelerator. The combustion tube was purged, the furnace was turned on, and the stop watch was started. The furnace was turned off after 15 minutes and the crucible was removed and loaded with sample after 20 minutes. While the combustion tube was purged for 1 minute, the attenuator was set on X512 and the O2 valve was opened and the helium valve was closed simultaneously. With 0 2 passing through the column, the sample was combusted for 7 minutes and swept for 1 minute. The helium valve was opened and the oxygen valve was closed simultaneously. After 2 minutes the attenuation was set properly (depending on C content and size of sample) , the temperature program started at 42" C./minute and the chart drive started. The C 0 2 was eluted a t 240-60' C. but the column temperature was allowed to reach 500" C., after CO, elution to maintain constant column parameters. The chart drive and oven were turned off and the oven door was removed. The oven was allowed to cool and stabilize a t 100" C. T o shorten analysis time per run, a crucible may be prefired while the C 0 2 from the previous samples is being eluted. The periods of prefiring, sample combustion, and column oven cooling afforded ample time for weighing of materials and recording of results. The time of analysis per run, including the prefiring of a crucible was less than 30 minutes.
The value of various parameters for each analysis were: Helium flow rate at the sample side: 150 ml./minute at 100' C. column temperature. Helium flow rate at reference side: 50 ml./minute a t room temperature. Oxygen flow rate at the sample side: 100 ml./minute at 100" C. column temperature. Program temperature rate: 42" C. per minute. Block temperature: 200' C. Bridge setting: 130 ma. Temperature limit setting: 500" C. Helium pressure: 12 p.s.i. Oxygen pressure: 11.5 p.s.i. Size of boron samples: 100 mesh. Weight of boron samples: 0.250 gram. Tin/boron ratio: 4/1. RESULTS AND DISCUSSION
The results of the boron metal analysis are shown in Table I. It was extremely important that parameters such as sample size, sample weight, tin/boron ratio, etc., be carefully controlled. Optimal conditions as found by Kuo, Bender, and Walker (1) gave maximum precision and accuracy in this work. I t was verified for the range of 9 to 16 p.s.i. that oxygen pressures have no effect on the combustion of boron samples. The oxygen pressure was carefully controlled, however, to maintain a constant flow rate. The length of the column did not effect the peak width so long as the flow rates remained constant. C 0 2 peaks obtained from 8-inch and 12-inch
Table I. Boron Metal Analysis Gas Chromatographic Carbon Sample No. rune Carbon, p.p.m, M 6312AN 4 420 f 8 M 6312 BG 4 1032 f 21 M 6401 A 0 4 448 i 27 M 6401 AR 4 760 f 34 M 6405 CP 5 644 f 24 M 6405 CJ 3 364 f 6 M 6405 CII 5 2416 i 87 .M 6312 BF 5 645 i 25 3 245 i 29 M 6404 A P M 6406 AQ 5 22,900 i. 400
VOL. 37, NO. 2, FEBRUARY 1965
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