colloidal lithium metal, oxide, and hydride may occasionally be present (3). Of these possible impurities, lithium butoxide interferes seriously in the double titration method; Kamienski and Esmay (6) found relative errors as high as -17% when a considerable amount of alkoxide was present. It does not interfere in the butanol-thermometric titration method, the V2O5method, o r (presumably) in the iodination method. Liz0 would not be expected to interfere in any of these methods; metallic Li and LiH might be expected to interfere in all of them. Table I shows comparative values for samples of sec-butyllithium which were analyzed by the double titration method ( 4 ) , the V205method ( 2 ) , and the butanol-thermometric titration method; values for the butoxide content (by dichromate oxidation) are also given. The iodination method of Clifford and Olsen ( I ) was not tested because of the general inconvenience involved in handling standard solutions in diethyl ether; Collins et al. ( 2 ) considered the method unattractive for similar reasons. The results indicate reasonable agreement between the butanol-thermometric titration method and the V,05 method; only a few tests were made with the latter, and the low values for Sample A may reflect lack of experience. (It was noted that in some cases a slight green color developed in the titrated
solutions on standing, probably indicating incomplete extraction of reduced vanadium from the excess v&.) The double titration method gave high and erratic values for the impurity correction and correspondingly low values for butyllithium. Dichromate oxidation showed that the actual butoxide content was considerably lower than the values obtained for the benzyl chloride correction. If the lowest of these values (0.19%) be taken as most nearly correct, it can be concluded that the metallic Li and LiH content (expressed as equivalent butyllithium) is probably not over 0.1%. The difference between the butanol titration value and total lithium minus butoxide is of this magnitude (0.09%) and would also include Li20 if present. Experience indicates that the butanol-thermometric titration method gives precise results, duplicate determinations seldom differing by more than 1% of their mean. (Repeatability is largely a function of the operator’s success in avoiding extraneous losses from oxidation and hydrolysis during sample handling prior to titration; this is a problem common to all methods for analysis of lithium alkyls.) The chief merit of the method, however, is its convenience. Only one reagent is needed; the procedure (“add sample; titrate”) approaches the ultimate in simplicity and speed.
The butanol-thermometric titration method has been applied to both nand sec-butyllithium solutions. n-Butanol has been used as titrant in most cases, but sec- and tert-butanol work equally well. The method has been in routine use for over a year, and no difficulties in its application have arisen during that time. No reason is apparent why the method should not be generally applicable to compounds containing Li-C bonds. ACKNOWLEDGMENT
The author thanks EveIyn Ramirez for numerous analyses made in the course of developing the method. LITERATURE CITED
(1) Clifford, A. F., Olsen, R. R., ANAL. CHEM.32, 544 (1960). ( 2 ) Collins, P. F., Kamieneki, C. W.,
Esmay, 11. L., Ellestad, R. R., Ibid., 33, 468 11961). (3) Foote Mineral Co., 18 W. Cheiten Ave., Phila. 44, Pa., Technical Data Sheet TD-109(Dec. 1961). (4) Gilman, H., Haubein, A. H., J. Am. Chem. Sac. 66. 1515 11944’1.
ANAL. ( 6 ) Kamienski, C.
CHEM:28, 1294 (1956).
D. L., ANAL.
RECEIVED for review Kovember 6, 1963. Accepted January 13. 1964.
Conductometric Dete rminuti on of Car bon Using Carbonate-Saturated Barium Hydroxide Absorbent EDWARD J. VIOLANTE Research Services Department, Engineering and Research Staff, Ford Motor Co., Dearborn, Mich. Barium hydroxide is preferred to sodium hydroxide as an adsorbent in the conductometric determination of carbon because, with it, the change in conductance is more sensitively related to changes in concentration. Use of Ba(OH)z, however, has been limited to the determination of carbon contents of less than O . O l ~ o because some investigators have shown it to produce erratic results at higher carbon levels. It has been determined that a barium hydroxide absorbent which is previously saturated with barium carbonate eliminates erratic results and shows a standard deviation of *0.0001~0 carbon a t the 0.01 %carbon level.
I
INTEREST in producing ultrahigh purity metals makes i t imperative to determine accurately carbon contents of less than 0.01%. NCREASING
856
ANALYTICAL CHEMISTRY
.I number of reviews (5, 6, 8) describe the various methods that are available; of these, low pressure and conductometric methods are the most commonly used. The low pressure method offers the highest accuracy because the COc f rom the combustion is measured directly. Because of the complexity of the vacuum equipment, however, efforts are being made to improve the precision of the condurtometric method to make it more competitive. Much work has been done to improve conductometric equipment ( I , 4, 7 , 9) since its introduction by Cain and Maxwell ( 2 ) . However, little has been done to increase sensitivity of the absorbent, although use of more dilute solutions has been suggested. Cain and Maxwell ( 2 ) suggested that Ba(OH), would be the most suitable absorbent “since CO, precipitates barium without
leaving reaction products in the solution to increme the conductivity. . . .” Still, Dauncey, and Chirnside (9) observed that this absorbent produced erratic results. Similar behavior was observed by Dailly and Elliott (4). Because of this, the more sensitive Ba(OH), has been largely replaced by KaOH. This investigation was made to determine the conditions under which Ba(0H)z could be used as the absorbent to take advantage of its greater sensitivity and thereby increase the precision of the method. EXPERIMENTAL
Conductometric Carbon Determinator. Leco X o . 515 conductometric carbon deterniinator was used in this investigation. Preliminary work with this unit showed t h a t t h e reference cell and measuring cell were highly temperature dependent when
previously boiled to remove any disused with dilute alkali absorbents. Also, i t was difficult to balance the cell solved COz, which is the recommended circuit because thew was a slight difpractice. The Ba(OH),.8H20 crystals were dissolved in a small amount of ference in the temperatures of the two cells when heat was supplied to the conwater and filtered into the boiled water. Approsimately 0.25 ml. of n-octyl alcostant-temperature water jacket. As hol was added for each liter of solution the heater is just below the electrodes prepared to reduce the surface tension in the absorption cell, these are heated to a slightly higher temperature than of the solution and allow more efficient absorption of the GOz. the reference cell. As the water circuProcedure. Analytical working lates, the cells are heated alternately curves were prepared for each of t h e and the cell circuit continuously three concentrations of Ba(OH), using wanders. t h e National Bureau of Standards This situation wa!, remedied by insample, 55e, which contains 0.011270 stalling a 75-ohm, E h v a t t resistor in C . Use of different sample weights the heater circuit so that heat was varied t h e amount of CO, produced supplied almost continuously, rather during combustion and produced workthan in large surges. Also, by reversing the mater flow, the heated water was ing curves over the range of 0 t o 0.17 mg. of carbon. mixed with the tot il volume in the water jacket before it passed the cells. The absorbing cell was filled with the Ba(OH)2 solution and allowed to come I n thiq \ray, the t u o cells were kept at a more constant temperature and i t to thermal equilibrium. The crucible Ras posrible to maintain almost perfect containing the sample was put into the furnace and, without heating, the comelectrical balance of the cell circuit for approsimately 20 minute. when a bustion tube was flushed with oxygen flowing at the rate of 500 ml. per minute. dilute (0.002,11) l$a(OH)z solution was This was vented through the three-way used. stopcock to the atmosphere. ExpeOXPGFNPURIFICA'I ION. The oxygen rience showed that 2 minutes nere resupply for combustion of the sample was connected to a &liter bottle which quired to remove any COz that entered served a4 a reservoir to supply the imthe system while the sample was being placed in the furnace. The osygen mediate demand for osygen. From this reservoir, the o y g e n n a s passed flow was reduced to 250 ml. per minute through a heated copper oxide catalyst and diverted through the three-way and then through a volumn of ascarite stopcock into the absorbing cell. The cell circuit was balanced and the furnace and magnesium perrhlorate before i t entered the combustion tube. power was turned on to ignite the sample. Combustion was allowed to All components of the apparatus were proceed until the cell circuit maintained connected with glass tubing and short polyethylene connections. a balance for a t least 2 minutes. With F U R N A C F A S D h 3 5 O R P T l O N TRAIX larger sample weights, u p to 20 minutes A Leco S o . 1H-15 induction furnace were required before the cell circuit wa5 used. h t the exit end of the could be balanced. Resistance readings quartz combustion tube a short length were then plotted as resistance us. milliof quartz tubing was fused so t h a t grams of carbon. glasq connections and polyethylene The procedure was repeated with the connectors would k separated from solution which contained 0.6 gram of heat produced during combustion. Ba(OH)z per liter; however, the solution Small bore glasq tuling was used t o was first saturated with BaCOJ by connect the cornbustion tube to a small the blowing of a small amount of air sulfur trap n hich contained MnOp and into the solution until a slight prea h i c h wai connecatec to a three-way cipitate formed. Fourteen different stopcock, then to the> conductometric sample lveights of the NBS sample, carbon determinator. With the three55e, were used to obtain a carbon range way stopcock, the ehit gasei could be of 0.05 to 0.16 mg. After the sample dil erted to the atmoqihere during the was put into the furnace, the atmosflushing of the combiistion tuhe, prior pheric COZ which entered the combusto combuqtion of the !>ample. tion tube was flushed through the BaC O M B C ~ T I CRUCIBLES. OK Leco por(0H)z solution as would be done in a n ous ceramic combust on crucibles and unmodified unit. ,ifter the cell circuit covers nere used. These were heated maintained a balance for 2 minutes, the in a muffle furnace a t 1000" C. for a sample was combusted; the change in minimum of 12 hours, with free access resistance of the Ba(OH), solution was to air; removed individually just prior recorded after the cell circuit again to uqe; and cooled for five minutes in maintained a balance for 2 minutes. a desiccator. These values are plotted as resistance SAhIPLE: AND FLUX PRCPARATIOX. us. mg. of carbon. The samples, standards, and tin fluu were aashed in reagent grade acetone RESULTS AND DISCUSSION and dried in warm air. They were stored in glass vials uiitil ready for use. When COS is absorbed in a Ba(0H)Z The vial caps were lined a i t h alusolution, resistance of the solution inminum foil 90 that the material would creases because of the removal of the not touch the paper lining inside the highly mobile hydroxyl ion. Rate of cap. B A ( O H ) ~ABSORBL~ T PREPARATION.change of the resistance increases as the concentration of hydroxyl ion decreases. Ba(OH), solutions t h a t contained 1.0, However, since, in this study, the change 0.8, and 0.6 gram of Ba(OH2).8Hz0 in concentration was incremental, a per liter were prepared. The water was
linear relationship between resistance and hydroxyl ion concentration can be assumed with negligible error, Figure 1 shows the typical analytical working curves that are obtained with Ba(OH)z solutions prepared with C0,free water. Resistance changes linearly in two distinct steps. During the first stage, the COY t h s t reacts with the Ba(0H)z does not produce enough BaC03 to saturate the solution. This introduces two opposing electrical effects: Removal of the hydroxyl ions tends to increase the resistance, while formation of carbonate ions tends to decrease it. Since the hydroxyl ion has a higher mobility than the carbonate ion, the net effect is shown as an increase in the resistance of the solution. When the solution becomes saturated with BaC03, the slope of the curve increases. This change in slope is solely due to the removal of hydroxyl ions from solution, since the solution i q now saturated with carbonate ions and BaCos is precipitating. This change occurs after approximately 0.08 mg. of carbon has been absorbed. The slope and location of the lower part of the curve are constant and depend only on the original concentration of the Ba(OH)S solution. The upper part of the curve also has a constant slope, but its location depends on the degree of initial carbonate saturation. When starting with an essentia!ly carbonatefree Ba(0H)z absorbent, the two parts of the curve intersect a t a point approximately equivalent to 0.08 mg. of carbon. If the Ra(OH)2 absorbent is initially partially saturated with BaCos, the curves will intersect a t some point between zero and 0.08 nig. of carbon, depending on the degree of saturation. Thus, when a sample is put into the combustion tube of a standard conductometric setup, C 0 2 from the atmospherr is flushed through the Ba(OH)2 absorbent prior to combustion of the sample and ('auses a partial but unknown amount of carbonate saturation. Thiq causrs the working curve to shift an indefinite amount to the left. I t is obvious, then, that carbon contents greater than approximately 0.08 mp. cannot be determined with a Ba(OH)z absorbent which has been prepared with Cos-free water, since they will be taken from the part of the curve that is not completely defined. Figure 1 shows that the sensitivity of the Ba(OH)2 solution increases rapidly with slight decrease? in solution concentration. However, there is a limiting concentration below which the resistance change of the solution is extremely sensitivr to temperature and concentration differences, and it becomes virtually impossible to stabilize the cell circuit. A solution concentration of 0.6 gram of Ba(OH)?.8H20 per VOL. 36, NO. 4, APRIL 1964
0
857
3U
I
d
30
45
-
/'
40 -
35 -
w 200 z
a
z!p
15-
MILLIGRAMS OF CARBON
Figure 1. Typical analytical working curves for Ba(0H)z absorbents prepared with COz-free water
liter offered high sensitivity with good cell circuit stability. When a Ba(0H)z solution which is originally saturated with BaCOa is used, the lower part of the analytical working curve is effectively eliminated and the upper portion of the curve is displaced to the left and passes through the origin, as shown in Figure 2. The carbonate-saturated solution has more than twice the sensitivity of the carbonatefree solution over the range of 0 to 0.08 mg. of carbon and the working curve is now usable over the range of 0 to 0.17 mg. of carbon. The change in resistance of the Ba(0H)z absorbent is not a linear function over a wide range of carbon contents; thus, the use of different solution concentrations for various ranges of carbon contents is necessitated. This laboratory uses a solution that contains 0.6 gram of Ba(0H)z 8Hz0 per liter for carbon contents up to 0.20 mg. and a solution of 0.8 gram per liter in the range of 0.20 to 1.00 mg. of carbon. COz Evolution. The COz which is formed during the combustion of the sample is not liberated in a steady stream as might be expected. Instead, the bulk of the COz is liberated rapidly as the sample burns violently, b u t as the reaction subsides the COz is liberated in small bursts. At one point in the cycle the cell circuit could be balanced for about one minute before more COZ was absorbed. With large samples which attain very high combustion temperatures, the time for complete evolution of the COZ is considerably more than when smaller samples are used. Since this work was done with an induction furnace, the last traces of e
858
ANALYTICAL CHEMISTRY
MILLIGRAMS OF CARBON
Figure 2. Effect of carbonate saturation on a 0.6-gramper-liter Ba(0H)z absorbent
COZappear to be evolved as the crucible is cooling; this may be caused by a phase change or by inversion of the crucible material. Cook and Speight (3) have indicated, inconclusively, that carbon analyses with an induction furnace tend to be higher than when a constant temperature resistance furnace is used. Precision. The most important factor in obtaining high precision, and one which cannot be overemphasized, is cleanliness. Extreme care must be exercised to avoid contact of the sample with any carbonaceous material. A single solid sample is preferred to chips since i t offers considerably less surface area for carbon contamination. This laboratory uses cold-drawn wire samples, approximately 0.1-inch diameter. These are cleaned by alternately sanding the surface with an aluminum oxide-type paper and then deep etching in dilute HC1, because the drawing compound which is used during the forming operation becomes imbedded in the metal surface. Silicon carbide-type papers should be avoided because carbide particles could remain on the sample surface. After the sample has been cleaned it must be handled only with clean tongs or tweezers. These tools are best cleaned by heating them to dull red in a Bunsen burner and placing them, when not in use, on a wire gauze which has also been heated to dull red. Care
should also be exercised in handling the combustion crucibles and covers after they have been fired a t 1000" C. Clean tongs should be used to transfer the crucibles. Carbon contamination can be avoided by placing the crucibles on wire gauze when they are being loaded. From the 14 data points used to plot the working curve for a carbonatesaturated Ba(OH)2solution in Figure 2, the standard deviation was found to be =t0.0001$70 carbon. This is a fivefold increase in the precision from the =t0.0005~0 reported by Gardner, Rowland, and Thomas (7). LITERATURE CITED
( 1 ) Bennet, E. L., Harley, J. H., Fowler, R. hl., ANAL.CHEM.22. 445 (1950). (2) Cain. (
(3: (4:
,
(5: , , (1960). ( 6 ) Fowler, R. M., Guldner, W. G., Bryson, T. C . , Hague, J. L., Schmitt, H. J., ANAL.CHEM.22, 486 (1950). (7) Gardner, K., Rowland, W. J.. Thomas, H., Analyst 75, 173 (1950). (8) Iron and Steel Inst. J . London 183, 287 (1956). (9) Still, J. E., Dauncey, L. A., Chirnside, R. C., Analyst 79, 4 (1954).
RECEIVED for review August 12, 1963. Accepted January 6, 1964. Presented at the Detroit Anachem Conference, Detroit, Mich.. 1962.
