ANALYTICAL CHEMISTRY
1422
The linearity of the log R us. 1/T relationship over short temperature ranges is indicated in Figure 3. Where resistance readings are made to =t1ohm, and temperature readings accurate to 0.05 O C. are desired, a linear interpolation over a 15’ C. range is permissible. Figure 4 gives time-temperature curves obtained with water and mercury, xhich indicate the precision of the data obtained to be of the order of 0.01 ’C. or better. The abscissa for each curve in Figure 4 is started from zero to avoid confusion on the time base. The curve illustrated in each case is only that portion of the run immediately adjoining the crystallization points and has no reference to the time a t which the experiments were started. The accuracy of measurement obviously depends upon the care observed in calibration. The three determinations with water were made over a 2-month period, and the freezing points agree within 1 ohm. The temperature coefficient of resistance a t 0 ” C. is 250 ohms per C., indicating a difference of less than 0.01 O C. and good stability of the Thermistor element. FURTHER APPLICATION OF THERMISTORS TO CRYOSCOPY
An obvious application of Thermistors is to the analysis of mixtures where very small temperature differentials must be measured. This technique has been described by Glasgow, Streiff, and Rossini ( 3 )and elsewhere
An indirect application of Thermistors to freezing point measurement in this laboratory was the use of a Type V611 disk Thermistor as a temperature regulator in the water bath used to calibrate the 14B above 0 O C. ACKNOWLEDGMENT
The authors wish to express indebtedness to H. C. Biggs and W. F. Still for construction of the cryostat used, and to D. A. Littleton for freezing point determinations. Thanks are extended to J. E. Tweeddale of the Western Electric Company for technical information received. LITERATURE CITED
(1) Becker, J. A, Green, C. B., and Pearson, G. L., Elec. Eng., 65,
711 (November 1946).
(2) Drummeter, L. F., Jr., and Fastie, W. G., Science, 105, 73
(1947). (3) Glasgow, -4.R., J r . , Streiff, A . J., and Rossini, F. D., Research s a t l . Bur. Standards, 35, 355 (1945); Research Paper 1676. (4) Goodyear, R. S., Product Eng., 16, 93 (1945). (5) Muller, R. H., ANAL.CHEM.,19, 29A (April 1947). (6) Richards, L. A., and Campbell, R. B., Soil Sci., 65, 429 (1948). (7) StUll, D. R., IND. ENG.CHEM., ANAL.ED., 18,234 (1946). (8) Tweeddale, J. E., “Western Electric Oscillator,”December 1945.
RECEIVEDSeptember 11, 1948.
Spectrochemical Determination of lanthanum in Praseodymium Metal WILLIAM M. SPICER AND WALDE-MAR T. ZIEGLER Georgia Institute of Technology, Atlanta, Ga. T H E course of an investigation of the properties of the Iof Karare earth elements a t low temperatures, a study Bras made sample of praseodymium reported by the supplier to contain
interfere, because they are far enough from the lanthanum line to be resolved by the spectrograph used.
1.3% iron and to be “essentially free from other associated metals.” Qualitative spectrographic studies, however, indicated the presence of more than a trace of iron, calcium, silicon, aluminum, neodymium, and, especially, lanthanum. The quantitative determination of certain of the rare earths, including neodymium, can be conveniently made by spectrophotometric means. However, this method is not applicable in the case of lanthanum, because its ion exhibits no suitable absorption band, The present paper describes a spectrographic method of analysis for lanthanum. An extensive review of the literature has appeared in a recent paper ( 3 ) . It was decided to use the copper spark method ( 4 ) of spectrographic analysis, because it requires only a very small sample and avoids the formation of cyanogen bands which mask many important lanthanum lines. An obvious method of attack would have been to prepare a series of mixtures containing known amounts of lanthanum and praseodymium and to compare a solution of the praseodymium metal with this series. Although both pure lanthanum and praseodymium oxides were on hand, only a small amount of the latter was available, and it was thus necessary to use it sparingly. Furthermore, preliminary work here had indicated that the lanthanum content in the sample was so high that the praseodymium might not serve as a satisfactory internal standard. Therefore, it was decided to add an internal standard. The choice of manganese for this purpose was based on the usual considerations (1). The possibility of interference was carefully considered in the light of the qualitative spectrographic investigations already it was found made. Using the M.I.T. Kave Length Tables (j), that the 3438.974 A. Mn(I1) line is free of interference from the elements knovp to be present in the praseodymium sample. The 4086.714 A. ka(I1) line lies betyeen the two praseodymium lines at 4086.24 A. and 4087.206 A. However, these do not
Equipment. The equipment used included a grating spectrograph, multisource unit, and comparator-densitometer, equipped with a voltage regulator, supplied by the Applied Research Laboratories. Photometric Procedure. Eastman spectrum analysis No. 2 film was processed in the ARL-Dietert film-developing machine maintained a t 70” F. Films were developed with mechanical agitation for 4 minutes in Eastman D-19 developer, immersed in a 3y0 acetic acid solution short stop for 30 seconds, and fixed in Eastman x-ray fixing bath for 4 minutes. The film was calibrated by use of the two-step filter, preliminary curve method ( 2 ) . 4 gamma of 1.4 was obtained. Background corrections were made by subtracting the background intensity from the total intensity (6). Preparation of Solutions. Stock solutions of lanthanum chloride and manganese chloride were prepared containing 0.00414 gram of lanthanum and 0.1316 gram of manganese per ml. The lanthanum chloride solution was prepared by dissolving in 0.5 A- hydrochloric acid a weighed quantity of pure lanthanum oxide which had been previously ignited to constant weight to remove moisture and carbon dioxide. [The lanthanum and praseodymium oxides used in these studies were obtained from Adam Hilger and Sons, Ltd. They were reported to be of high purity (>99.75%), Qualitative spectrographic analysis confirmed the general correctness of these analyses.] The manganese chloride solution was made by dissolving B weighed quantity of Baker’s C.P. manganese chloride tetrahydrate in water. Preliminary studies indicated that the praseodymium sample might contain 30 to 40y0 lanthanum. Accordingly, four standards were prepared to cover the range from 28.2 to 42.3% lanthanum in the praseodymium sample under study. These standard lanthanum-manganese solutions were prepared by adding to 1 ml. of internal standard 0.40, 0.50, 0.55, and 0.60 ml., respectively, of the lanthanum chloride solution. The resulting mixtures were first diluted to 15 m]. with water and then mixed with an equal volume of 0.5 hydrochloric acid solution. These dilutions were used because it was found that with more concentrated solutions much of the sample was often lost during excitation, owing to sputtering. The excess hydrochloric acid seemed to improve the reproducibility. The only reason deemed assignable to this phenomenon is that the hydrochloric acid may remove whatever oxide exists on the copper electrode and thus may decrease any tendency of the spark to become localized.
EXPERIMENTAL METHOD
V O L U M E 2 1 , NO. 1 1 , N O V E M B E R 1 9 4 9 The stock solution of the praseodymium sample was made by dissolving a sufficient quantity of impure praseodymium metal in 0.5 N hydrochloric acid to yield a solution containing 0.0147 gram of the sample per ml. The solution used for analysis was prepared by adding 0.40 ml. of the stock solution to 1 ml. of the internal standard solution, diluting to 15 ml., and mixing with an equal volume of 0.5 AT hydrochloric acid. The electrodes 4ere cut from 0.25-inch copper rods, as described by Fred, Xachtrieb, and Tomkins ( 4 ) . To decrease oxidation before use, the electrodes were stored in a jar Kith their tips immersed in benzene and %ere always used uithin 12 hours after they were cut. -2 hot plate was used to evaporate the solutions on the electrodes. I t was found that ahout 0 0286 nil. ma5 requiied to cover the t i p of an electiode. EXPERI\IEYThL RESULTS
K i t h each of the four standard lanthanum-manganese solutions, 21 spectra (three films containing seven each) ere made, and the ratio of intensities of the 4086.714 b. La(I1) line to the 3438.974 Iln(I1) line was determined in each case. The average value of this ratio as found for each standard solution and a log-log plot of the results made. The working curve so obtained was linear. From a determination of the intensity ratio for a solution of the pi aseodymium metal containing a known amount of added manganese chloride, the lanthanum content of the praseodymium can be calculated with the help of the working curve. Treating an average of five repeat determinations as a single result, the lanthanum contents found Rere: 38.5, 37 4, 39.7, 37.5, 33.7, and 35.3%, or a mean value of 37.0’%. The average deviation from the mean corresponds to a variation of 1.7% in the lanthanum content of the praseodymium metal, or 4 6% in the analysis of the sample.
a.
As a check of the accuracy of the analysis, a synthetic known to contain 38% lanthanum-i.e., very nearly the same amount as that found in the sample-was made up from pure lanthanum and praseodymium chlorides. This solution differed from that containing the praseodymium metal sample only in being free of minor impurities such as iron, neodymium, etc. Portions of the solutions of the synthetic sample and the praseodymium metal sample, each containing the same percentage of manganese as that used i n the analysis of the praseodymium metal, were
1423
compared by photographing spectra of both on the same film. Seven exposures were made tor each sample. The ratio of the intensities of the 4086.714 A. La(I1) line to the 3438.974 b. Mn(I1) line was 0.56 =t0.04 for the synthetic sample and 0.55 =t 0.05 for the metal sample. This agreement was taken as proof of the essential correctness of the analygo%) of the unaccounted for balance Tvas found to precipitate as an oxalate in acid solution. Because spectrophotometric analysis failed to show absorption bands for any other rare earth in the range 380 to 1000 mp, and qualitative spectrographic studies had shown gadolinium to be absent, it was presumed t h a t this unidentified balance was lanthanum. This is in general agreement with the spectrographic resultACKNOWLEDG\IEhT
The authors wish to express appreciation to J. B. Downs, Jr., for carrying out the chemical and spectrophotometric analyses. LITERATURE CITED
dverbach, B. L., IXD. ESG.CHEM..-4s.~~. ED.,17, 341 (1945). Churchill, J. R., I b i d . , 16, 653 ( 1 9 4 4 , . Fassel, V. A., and Wilhelm, H. A , , J . Optical SOC.A m . , 38, 518 (1948).
Fred, &I Nachtrieb, ., N. H., and Tomkins, F . S., I b i d . , 37, 279 (1947).
Harrison, G. R., “Massachusetts Institute of Technology Wave Length Tables,” New York, John Wiley & Sons, Inc., 1939. Pierce, W. C., and Kachtrieb, X. H., ISD. ENG.CHEM.,AXAL. ED.,13, 774 (1941). Rodden, C . J., Bur. S t a n d a r d s J . Research, 26, 557 (1941). RECEIVED October 8, 1948. Presented a t the Meeting-in-Miniature, Georgia Section, AYERICAKCHEMICILSOCIETY,Kovember 19, 1948. Joint contribution from t h e School of Chemistry a n d t h e State Engineering Experiment Station. Work carried o u t with t h e assistance of t h e Office of Xaval Research under Contract S6-ori-192.
Determination of Carbon by Wet Combustion PAUL S. FARRINGTON, C i R L NIE-MANY, A h D ERNEST H. SVIFT California ZnstitzLte of Technology, Pasadena, Culif.
OR situations where a Van Slyke manometric apparatus Fcannot be provided, a scheme has been devised to use the Van Slyke-Folch combustion mixture (3) in the analysis of organic solids and relatively nonvolatile liquids. -4sample of 10 to 12 mg. is heated n i t h combustion solution in a stream of carbon dioxide-free air, and the carbon dioxide evolved is absorbed in sodium hydroxide. The carbonate thus formed is precipitated as barium carbonate and determined acidimetrically. A determination requires about 45 minutes. The efficiency of the appaiatus has been tested by analyzing pure samples of several compounds. The results were accurate to +0.05 mg. of carbon. Halogens, nitrogen, and sulfur do not interieie; however, compounds that decompose to release hydrocyanic acid will give low carbon values. In comparison with other methods for the determination of caibon, this procedure has the advantage of an inexpensive apparatus which can be assembled quickly and does not require much space. I n addition, only one weighing is required for each determination. The Van Slyke-Folch combustion solution has been used in other procedures (1, 2 ) , but in these the carbon dioxide or barium carbonate has been determined gravimetrically.
REAGEhTS
Combustion Solution. Pour 60 ml. of 30% fuming sulfuric acid into a flask containing 40 ml. of 8570 phosphoric acid, and add 10 grams of chromium trioxide and 1 gram of potassium iodate. Heat to 140 O to 150’ C. and swirl or stir for 1 to 2 minutes. Cool and store in small glass-stoppered bottles. If fuming sulfuric acid is not available, use 20 grams of phosphorus pentoxide, 85 ml. of 95% sulfuric acid, and 15 ml. of 85% phosphoric acid with the stated quantities of chromium trioxide and potassium iodate. Sodium Hydroxide. Dissolve 5 grams of sodium hydroxide in 20 ml. of water, add 1 ml. of 1 F barium chloride, and centrifuge. Dilute to 250 ml. Barium Chloride Reagent, 1 F in barium chloride and 0.001 F in hydrochloric acid. APPAR4TUS
The Iiraissl tube, A , shown in Figure 1 is packed with 20 to 30-mesh Ascarite or soda lime, and 95% sulfuric acid is placed in the bubble counter, B. The 15-ml. centrifuge tube, M , is connected to head D by means of a 14/20 T joint. The stopcock, E, is lubricated with phosphoric acid, but grease may be used in the other stopcocks. A plug of glass wool moistened with distilled water is placed in tube F to remove any hydrofluoric acid. The disperser, K , is made from a short length of Zircofrax tubing (12-7P.89, Carborundum Company) sealed to Pyrex tubing and
