Determination of Iodide in Lead Te Iluride Semiconductor LOUIS SILVERMAN Atomics International, Division of North American Aviation, Inc., Canoga Park, Calif.
F A method for the determination of 0.05 to 0.07 mole Yo lead iodide in Lead lead telluride is described. telluride (99.5-99.970) is dissolved in dilute nitric acid under reflux. The lead iodide is partially converted to iodine by the oxides of nitrogen, and completely converted with the aid of hydrogen peroxide. The iodine is extracted into o-xylene, and the absorbance is measured at 495 mu.
(1) may be considered an introduction to the analytical chemistry of semiconductors. The particular intermetallic compound, PbTe, a face-centered cubic crystal, is synthesized from its pure constituents and is made either "positive" or ''negative" by the addition (doping) of impurities. The addition of lead iodide forms the n-type semiconductor, and the effects of varying amounts of the lead iodide have been described ( 2 ) . A compromise of desired properties of the semiconductor prescribes a range of 0.0.5 to 0.07 mole % PbI2. These limits call for a range of 0.38 to 0.53 mg. of I2per gram. Of the methods considered, the colorimetric iodine was favored since solution of the sample in dilute nitric acid has already converted most of the iodide to iodine. There is no filtration required for the removal of graphite or other insoluble materials which might be found, and a previous paper (4) had shown suitable absorbance values for 0.15 t o 0.30 mg. of iodine (0.5-gram sample) per 10 ml. of solvent. RECENT PAPER
paratus on the warm plate and heat until a few tiny bubbles appear (about 100" C.). Cool in air, then insert the flask in an ice bath and cool to 10" to 15" C. Wash down the reflux condenser several times with cold water and detach. Add l ml. of HzOz (30% grade) to the flask, stopper, shake, and let stand for about 1 minute. Prepare a 125-ml. separatory funnel and pipet in just 10 ml. of o-xylene. Transfer the contents of the flask to the prepared separatory funnel, stopper, and shake under running water for a t least 1 minute. Allow the liquids to separate for several minutes in the stoppered funnel, then drain the water and top-pour the xylene through filter paper into a cuvette. Obtain the absorbance vs. an o-xylene blank a t 495 mp. 3. Preparation of the standard curves. Standard solution for iodine: dissolve 0.059 gram of NaI in 500 ml. of water. 1 ml. = 0.1 mg. of 1 2 . a. Prepare a series of flasks containing 75 ml. of cold H2O and 5 ml. of HN03. Pipet in amounts of iodide as follows: 0, 0.1, 0.15, 0.20, 0.25, 0.3, 0.4, and 0.5 mg. of 12. Add 1 ml. of H202 to each. Complete as in the procedure. Plot the curve. b. Prepare five flasks, each containing 0.5 gram of iodine-free PbTe. Add 0, 0.15, 0.2, 0.3, and 0.5 mg. of Iz to the respective flasks. Dissolve and make the runs, as in the above procedure. c. Prepare three flasks, each containing 0.5 gram of iodine-free PbTe. Dissolve the samples as usual, cool, wash down the condensers, and separate. To the respective flasks. add 0.0, 0.3, and 0.4 mg. of Is. Complete the procedure as described.
the quality of NaI is sufficient to be used without checking each new batch. DISCUSSION
Solution of Sample. The PbTe dissolves rapidly in concentrated HNOs with the evolution of brown fumes, and slowly in diluted "01. I n dilute HK03 plus sulfamic acid (S), the reaction does not take place until sufficient nitrite is added to destroy the sulfamic acid. This point indicates that the lower valence nitrogen compounds are the dissolving agents. Another detail is that the bulk of the Pb12 has been converted to I2 during the solution of the sample. The source of this reaction may be : PbTe
+ 8HN03
=
+
Pb(NO3)? Te( NOa),
+
+ 2N0
4Hz0
and/or PbTe
+ SHY01 =
+
+
Pb(N03)~ T e ( l Y 0 ~ ) ~
Table 1.
Sample Batch Number 1
2
3
4 5 6 7 8
3HS02
+ 3HzO
Determinations of Pblz in PbTe
Mole Per CentSominal Found 0 05 to 0 07 0 050 0 05 to 0 07 0 055 0 05 to 0 07 0 055 0 05 to 0 07 0 058 0 05 0 058 0 05 0 058 0 05 0 061 0 05 0 055
PROCEDURE
1. Preparation of sample. Powder the sample to 60 mesh or less. 2 . Transfer 0.500 + 0.002 gram of the powder to a 250-ml. Erlenmeyer flask with ground glass neck. iidd 50 nil. of water and 10 ml. of HN03 (sp. gr. 1.4) in this order. Insert a thermometer and attach to a water reflux condenser with ground glass connection. Warm to about 60" C. on a hot plate, and, when the black powder seems to turn completely gray (gas formation), move the flask and condenser to the desk top where the reaction usually goes to completion without further heating. Again place the ap-
RESULTS
The duplicability of the results and recoveries were shown in the use of the three sets of standard runs. They are discussed below. A spread of 0.005 absorbance unit (0.002 mole yo) between duplicate samples is allowable. Table I shows typical results on eight sample shipments by one analyst. Duplicate determinations on five samples by another analyst are given in Table 11. The ultimate standard for the procedure is potassium dichromate (4), but
Table II.
Pb
62.04 6'2.17 62.21 62.27 61 79 61 70 62 20 62 13 62 11 62 04
PbTe Semiconductor, n-Type
Weight Per Cent Te 37.80 37 80 37.81
__
I 0.031 0 031
37.81
0.033 0.035
37 38 37 37 37 37
0 0 0 0 0 0
96 11 89 89 84
81
VOL 34, NO. 6, MAY 1962
021
019 038 037 040
043
701
+ 2HXOj = Pb(SO,), + 2HI H I + HSOt = H?O + NO1 2x01 = 2x0 +
PbI2
I2
The sample is dissolved in dilute HS03 to prevent the formation of any m a l l amounts of H I 0 3 n hich n ould not be recoverable as 1 2 after the disappearance of thc HI. The sample is dissolretl under refluv a t about 60” to 65’ C.to prevcnt loss of thc iodide or iodine. If the sample is d r a c t e d without further heating, or if the sample is heatcd in a boiling ~ a t e r bath (about 90-95” C.), the results are consistently low by about 0.005 absorbance units. On the other hand, if the sample is dissolved as dcwribed, placed on the hot plate, and heated to the formation of small. colorlw bubbles (95-100” (2.). then removed from the heat, the recovery of 1 2 from a sample of PbTe S a 1 will equal the recovery from standard S a 1 solution that has not been put through the qolution and hcating cyclc. The e\planation of low results when the boiling water bath is usrd, compared nith stoic~hionietricre-
+
sults when a very short “to boiling” time is used, together with the formation of colorless bubbles may lie in the decomposition of YO1 to colorless bubbles of s o 12. Use of H202. H202 is added for t n o reasons. First, the oxidation of HI by the H S O z or the oxides of nitrogen is not quite complete; and second, when the standard solutions of S a 1 are used to make the norking curve, H202is as satisfactory as any other oxidizing agent. The addition of H z 0 2 makes sample and standard more comparable. Extractant, o-Xylene. Previous work (4)had shown the superiority of o-xylene for the absorbance measurements of the iodine. The present working curve and that previously reported (4) are close matches. It is not knoivn if the same Beckman DU was used in each case. Statistical Data. A statistical evaluation of the procedure with 10-mm. cuvettes s h o w d : (1) Siu determinations (two per day) on the same sample: s = 0.0028 in absorbance units, iThich number, di-
+
vided by the average value, 2 = 0.053, equals 5%, the relative standard deviation. (2) The correlation coefficient of the standard graph (iodide taken plotted us. absorbance) is 99% (recovery value). The slope of the graph is 0.27, and the bias is 0.002 mole 7cPb12. ACKNOWLEDGMENT
The author thanks Mary Shideler Hanson for furnishing the data in Table 11. LITERATURE CITED
(1) Burkhalter, T. S., A h a ~CHEV. . 33, 21A (1961). (2) Cadoff, I. B., Miller, E., “Thermonuclear Materials and Devices,” Chap. 10, Reinhold, Nen York, 1960. ( 3 ) Silverman, L., IXD. EXG. CHEM., ANAL.ED. 17,270 (1945). (4) Silverman, L., Bradshaw, W.,Anal. Chirn. Acta 12, 526 (1955). RECEIVED for revieir. September 15, 1961. Accepted Derernber 11, 1961. Pittsburgh Conference on Analytical Chemistrj- and Applied Spectroscopy, 1962. Based on studies conducted for the U. S. Atomic Energy Commission under Contract AT11-1-GEN-8.
A Simple Atmospheric Carbon Dioxide Analyzer JAMES P. LODGE, Jr.,l EVELYN R. FRANK,’ and JAMES FERGUSON2 Robert A. Tuft Sanitary Engineering Center, Cincinnati 26, Ohio
b A device for the measurement of carbon dioxide in gases has been adapted for measurement of carbon dioxide at atmospheric concentrations. Fine marble chips are suspended in continually aerated distilled water, and the equilibrium pH is measured b y an expanded-scale pH meter. Performance of the device has been compared with that of a nondispersive infrared analyzer and found to be substantially equivalent. The cost of the device is markedly less, however, than that of the infrared instrument,
I
there has been a marked increase in interest in the measurement of atmospheric concentrations of carbon dioxide. In addition to its probable impact on the long-range climatology of the world, carbon dioxide constitutes an excellent traccr for the effects of human activities. Haez ( I ) concluded that carbon dioxide concen-
tration correlated very well with other manifestations of pollution in Mexico City. Keeling (4) has shown that meaning can be attached to local fluctuations as small as 0.5 p.p.m. of carbon dioxide. Heivson ( 2 ) recommended that routine measurenient of carbon dioxide be undertaken on a national basis. SUCTION
SIDE
The primary obstacle to this undertaking appeared to be the lack of an inexpensive instrument for accurate routine measurement of carbon dioxide. Manual methods such as the excellent titration technique of Holm-Jensen (3) are seldom satisfactory for evtended field studies. Sondispersire infrared analyzers are generally evpensive. A P R E S S U R E
,q
SIDE
NEEDLE ,VALVE
ri RECENT TEARS
DIAPHRAGM PUMP
DIAPHRAGM PUMP
I~~ROTAMETER HUMIDIFIER+
IL,,,
1
Present address, Sational Center for
.1tmospheric Research, Boulder, Colo.
* Present address, Chemistry Department, RlcXicholas High School, Cincinnati, Ohio. 702
e
ANALYTICAL CHEMISTRY
SAMPLE Figure 1 ,
Schematic of carbon dioxide analyzer
+COCO,
