LEONARD
P. PEPKOWITZ and JOHN
T. PORTER II
Knolls A t o m i c Power Laboratory, General Electric Co., Schenectady, N. Y.
The determination of low concentrations of carbon in sodium is required by the observation that grain boundary carburization sometimes occurs in stainless steel heat exchange systems utilizing sodium as the liquid metal coolant. The method involves the conversion of the sodium to the sulfate, wet oxidation of the carbon to carbon dioxide, and measurement of the low concentrations of carbon dioxide by a volumetric, differential freeze-out technique. The method is accurate with a standard deviation for precision of 1 0 . O O 1 ~ ~ c .
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NLESS steels are subject to grain boundary carburization by sodium contaminated with carbon, which results in embrittlement ( 2 ) . Therefore, the use of stainless steel in heat exchange systems which have sodium as the coolant requires accurate control of the carbon content of the sodium. A method capable of measuring carbon in the range of 0.01 weight 70or less is needed. A direct combustion method for the determination of carbon in sodium-potassium alloy has been published by Stoffer and Phillips ( 5 ) , but no recovery data were given in this low concentration range. The analytical problem can be divided into three phases: ( I ) the conversion of the sodium to a salt, (2) the oxidation of the carbon t o carbon dioxide, and (3) the measurement of the carbon dioxide.
This stopcock, as well as the ball joint and the stopcock in the line from the Van Slyke reservoir, is lubricated with sirupy phosphoric acid. The oxygen is purified by passing it over hot copper oxide t o decompose any hydrocarbons present and then through an riscarite trap prior t o admitting it to the system. Dry tank nitrogen is used as the inert cover during the solution of the sample and the subsequent evaporation step. The Van Slyke oxidizing fluid was made up of 13 grams of chromic acid, 85 ml. of orthophosphoric acid, and 165 ml. of fuming sulfuric acid (157, excess sulfur trioxide). The mixture was heated gradually to 140' C. with occasional swirling. After i t had cooled to approximately 80" C., suction from a water pump was applied during the remainder of the cooling time. The outlet from the combustion tube is connected via the threemay stopcock to the gasometer of the measuring system. This system ( 4 ) ,including the gasometer, is used nithout modification. Procedure. Wrap the glass tubing or cup containing the sodium sample in n thin sheet of copper and smash with a hammer blow. Cut the sodium into small pieces and dissolve in a small volume of water in the combustion tube under a stream of nitrogen. Keutralize the caustic solution with 5 M sulfuric acid and add 10% additional acid. Evaporate t o near dryness, maintaining the nitrogen flow during the evaporation to speed up the dehydration. Use of a "hot spot" electric heater, applied t o the side of the tube near the liquid surface, accomplishes dehydration more smoothly than the use of a burner.
Table I.
Sample Weight, Grams
EXPERIMENTAL
Conversion of Sodium to a Salt. tained using the metathesis reaction
(NHa)zCrOd
1.3 1.3 1.3 1.3 1.3
Partial success (3) was ob-
+ 2NaOH +NazCrOd + 2s"
Carbon Content of Sodium in Circulating Heat Transfer System
+ 2H20
followed by thermal decomposition of excess ammonium chromate. Two problems prevented application t o the final solution. It proved difficult to drive off the last of the ammonia, with the result that combustion led to nitrous oxide which is one of the few gases that interferes in the differential freeze-out technique for measuring carbon dioxide. Secondly, the strong chromate-chromic acid system attacked the carbon and led to low and erratic recovery. The method finally adopted involves solution of the sodium in water under nitrogen, addition of excess sulfuric acid, and evaporation to near dryness. As the boiling of the acid solution destroys the carbonate, there is no need for extensive precautions to avoid contamination with atmospheric carbon dioxide prior to this step in the procedure. Oxidation of Carbon to Carbon Dioxide. The Van Slyke sulfuric acid-chromic acid mixture (6) waa used for the oxidation of the carbon. Measurement of Carbon Dioxide. The measuring technique for the low concentrations of carbon was described previously (4).. Presumably other methods can be used if care is taken t o avoid interference from the acid spray from the Van Slyke solution. Equipment and Reagenfs. The combustion is carried out in a vessel shaped like a test tube, approximately 2 cm. in diameter by 10 cm. long, fabricated from borosilicate glass and fitted with a 38/25 ball joint. The socket into which it fits carried an inlet for the oxygen used to sweep the carbon dioxide into the measuring system, an outlet to the measuring system, and a delivery tube from the Van Slyke reservoir. The outlet tube is equip ed with a three-way stopcock to allow the combustion vessel t o ge swept with oxygen prior to connection with the measuring system.
Carbon Found, mg.
Conon., %
0.072 0.054 0.052 0.059
0.0055 0,0042 0.0040 0.0046 0.0074
0 096 I
41..
Std. dev.
0.0051 ?= 0,0014
Attach the combustion tube to the apparatus and pass oxygen through the vessel and out the bypass until the nitrogen and air are removed. Stop the oxygen flow and by means of the threeway stopcock connect the combustion tube with the gasometer. Add 5 ml. of the Van Slyke combustion fluid and heat a t incipient boiling for 5 minutes. Sweep the gaseous combustion products into the gasometer with oxygen until the gasometer is filled. Proceed with the freeze-out technique to measure the carbon dioxide as described by Pepkowitz and Moak ( 4 ) . RESULTS AND DISCUSSION
The major source of carbon contributing to the blank is the fuming sulfuric acid. This error was minimized by heating the combustion mixture t o 140" C. and cooling under suction t o remove the carbon dioxide formed. An over-all blank was determined on all the reagents used in the procedure. A typical average of four determinations amounted to 0.031 mg. of carbon with a standard deviation of f 0.006 mg. Because the usual sample is 1 gram, the blank correction was equivalent t o 0.0031%. The determination of the exact sample size taken is difficult. It is necessary t o smash the glass container in order to cut the
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V O L U M E 28, N O . 10, O C T O B E R 1 9 5 6 sodium into small pieces so that the vigorous reaction with water in the dissolving step can be controlled. I n practice the samples taken in small glass cups are obtained through a metering valve and the number of drops of sodium run into the sample cup is counted. I n this manner, the sample size is known to be & l o % as ascertained by the titration of a series of samples obtained in this way. The over-all error caused by this uncertainty is small and can be neglected a t the low carbon concentrations involved. Sample sizes for sodium contained in glass tubing are determined by measuring the length of the section taken for analysis. The sodium in one of the sections is dissolved and titrated to determine the amount of sodium per unit length. Care must be taken to be sure that no voids exist in the sample. Reproducibility in sample size is again about Itlo”,. The precision of the method applied to samples taken from a circulating system by the drop counting method is given in Table I. The standard deviation was found to be 10.00147& which is adequate for the control of the carburizing process and demonstrates the applicability of the method to the concentration range of 0.01 3 or less.
1607 The data in Table I11 are the recovery values for known amounts of carbon added as powdered graphite or uranium carbide. The latter was used because of its favorable weighing factor. In either case, the carbon was added to the water wrapped in a thin aluminum foil. The foil dissolved after the addition of a small portion of the sodium, thus exposing the added carbon to the elevated temperature and strongly basic conditions typical of sample solution. In all cases, 1 gram of the sodium for which analyses are shown in Table I1 was used. Therefore, the results are corrected both for the reagent blank and for the carbon content of the sodium.
Table 111. Recovery of Known Amounts of Carbon Carbon, Mg. Taken Found 0.47 0.27 0.06
0.29 0.08 0 21
Table 11. Carbon Content of Distilled Sodium Carbon Sodium, Gram
Concn., 70 0.0124 0.0107 0.0037 0.0097 0.0028
Found, mg. 0.124 0.107
0.037
0.097 0.028
0.16
0.35
0.27 0.07 0.23 0.05 0.22 0.17 0.06 0.16 0.16 0.40
Concentration, % Found
Powdered Graphite 0,047 0.040 0.027 0.027 0.006 0.007 0.029 0.023 0.008
0.021
0,005
0.022
Uranium Carbide 0,020 0.017 0.011
0.010 0.010
0,006 0.016 0.016
0.035 0,040 Algebraic s ~ i n iof differences = -0.00046 s ( d ) = ztO.0047 t = 0.32
Diff. -0,007 0 +0.001
-0.006 -0,003
+o. 001 -0.003 -0.005
+0.006 +0.006
+0.005
0.0046
0.046
Av. Std. dev. 0.5 0.5 0.5 0.5 0.5 0.5
0.20 0.11 0.10
0.40
Taken
0.035 0.046
0.011
0.029 0.045
0,044
Av. Std. dev.
0.0073 0.0041 0.0070 0.0092 0,8022 0.0058 0.0090 0,0088 0.0070 0.0030
To test the accuracy of the procedure by the recovery of known additions of carbon, the carbon content of the sodium used must be ascertained. For this purpose distilled sodium contained in glass tubing was used, and the sample size was determined by measuring the length of the section cut from the stick of sodium. The samples were analyzed as described and corrected for the reagent blank described above. Because of the larger deviation in the results, both 1- and 0.5-gram samples were run in order to determine whether there was any bias due to sample size which might reflect nonuniform dispersion of the carbon. The data in Table I1 show that there is no difference between the two sample sizes within the standard deviation of the two sets. However, the precision of these runs is much poorer than that for the set taken from the circulating system. This effect can be ascribed to the pickup of carbon from the glass of the distilling equipment, as well as the possibility that the saturation value of the sodium is exceeded and there is a degree of nonuniformity of dispersion of the precipitated carbon. The actual solubility of carbon in sodium as a function of temperature has not yet been determined.
The deviations between the amount of carbon taken and the amount found were evaluated by applying the t test, using the method of differences (1). The standard deviation, s(d), for the individual difference is 1 0.0047 and the calculated t value is 0.32. Because the significant ( 5 % ) t value for 10 degrees of freedom is 2.23, it is clear the deviation of the differences from zero is completely negligible, and, therefore, the method is accurate within the experimental errors after applying the blank correction. It should be borne in mind that the present method is directed toward the determination of elemental carbon and does not give a determination of the total carbon. Further work to develop a simple and satisfactory method for total carbon would be desirable. LITERATURE CITED (1) Davies, 0. L . , “Statistical M e t h o d s i n Research a n d Production,” Oliver a n d B o y d , London, 1949. (2) Jackson;,C. B., e d . , “Liquid M e t a l H a n d b o o k , Na-NaK SuppleGovernment P r i n t i n g Office, Washington, D. C., m e n t , U. 1955. (3) Pepkowite, L. P . , Downer, R . J . , ABAL. CHEM.25, 520 (1953) (ahstract). (4) Pepkowits, L . P., h l o a k , W. D . , Ibid.,26, 1022 (1954). (5) Stoffer, K. G., Phillips, J. H . , Ibid., 27, 773 (1955). (6) V a n Slyke, D. D., Folch, J . , J . BbZ. Chem. 136,509 (1940).
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RECEIVEDfor review December 5, 1955. Accepted July 7, 1956. The Knolls Atomic Power Laboratory is operated by the General Electric Co. for the Atomic Energy Commission. The work reported here W&B carried out under Contract h’o. W-31-109 Eng-52.
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