material in an atmosphere of carbon dioxide as proposed by Richards and Hoover (8). LITERATURE CITED
.,
(1) Balis. E. \I7.. Bronk. L. B.. Lieb-
hafsky, H.‘ A., PfeBer, H. -G., ANAL.CHEM.27, 1173 (1955). (2) Duval, C., Anal. Chim. Acta 13, 32 (i955j.
(3) Easterbrook, W. C., Analyst 82, 383
(1957). (4) Howarth, J. T., Maskill, W., Turner, W. E. S., J . SOC.Glass Technol. 17,25T (1933). Howarth, J. T., Turner, W. E. S., Ibid., 14,402T (1930). Kolthoff, I. XI., Stenger, V. A., “Volumetric Analysis,” Vol. 11, p. 80, Interscience, New York, 1947. (7) hlotzfeldt, K., J. Phys. Chem. 59, 139 (1955).
Richards, T. W., Hoover, C. R., J . Am. Chem. SOC.37, 95 (1915). Simons, E. W., Kewkirk, A. E., Aliferis, Ifigenia, ANAL. CHEM. 29,48 (1957). Smith, G. F., Croad, G. F., IND. ENG.CHEM..ANAL. ED. 0. (1937~,_ ,_141 __ ~~
~
Waldbauer, L., hlcCann, D. C., Tuleen, L. F., Ibid., 6 , 336 (1934).
RECEIVED for review August 9, 1957. Accepted December 28, 1957.
Titrimetric Determination of Fluorine Particularly in Aluminum Fluoride LAWRENCE V. HAFF General Chemical Division, Allied Chemical & Dye Corp., Morristown, N. 1. C. P. BUTLER and J.
D. BISSO
General Chemical Division, Allied Chemical & Dye Corp., Porf Chicago, Calif. ,Discordant results for total fluorine were reported from different laboratories analyzing identical samples of aluminum fluoride. Serious losses of fluorine occur in distilling and titrating large amounts of fluorine by conventional methods. Recoveries are especially low and erratic when borosilicate glassware i s employed. Details of a rapid and accurate procedure for pyrohydrolytic assay of aluminum fluoride were worked out. Routine analyses of aluminum fluoride are greatly expedited by the procedure, which, however, requires a standard sample of aluminum fluoride. In assaying the standard sample, appropriate correction must be made for the losses incurred in the distillation and subsequent concentration of the fluorine. The number of replicate samples required to establish the correction and the magnitude of the correction can be greatly reduced by avoiding use of borosilicate apparatus.
A
of conventional volumetric methods for determining fluorine in aluminum fluorine indicates that recoveries of large amounts of fluorine from the distillation procedure are only about 98% complete and further losses are incurred when glassware containing boron is used. Under standardized conditions recoveries are reproducible. Analysis by a pyrohydrolytic method of samples standardized by conventional methods established that pyrohydrolysis offers a rapid, accurate, and convenient procedure for assaying aluSTUDY
984
*
ANALYTICAL CHEMISTRY
in reported assays. It quickly became apparent that recovery of fluorine was never complete by either method. While the relative percentage of fluorine LOSSES I N CONVENTIONAL PROCEDURES lost would not be important in microFor a number of years two procedures analysis, it assumed serious proportions for determining relatively large amounts in assay of a material containing some of fluorides have been used throughout 50% of fluorine. Because some of this company. As applied to aluminum these errors will affect almost all fluoride, both involve fusion of the macrodeterminations of fluorine, some sample with sodium carbonate, followed details of this work are presented. by acidification and steam distillation Standard Samples. No standard fluoride sample was available to the as described by Willard and Winter authors a t the time this investigation (22). The analysis may be concluded as was initiated. Cryolite has been sugby Armstrong’s procedure (1, i?), modified by Ron-ley and Churchill gested as a standard ( T ) , but its purity (20). Practically, this titration is is problematical. Sodium fluoride, used limited to determination of small by Hoskins and Ferris (8), Kimball amounts of fluorine, and the small aliand Tufts (9), Reynolds and Hill quot which must be titrated involves (l7), and hIatuszak and Brown ( l a ) , is of uncertain assay. Kimball and serious magnification of small errors. Alternatively the fluorine in the disTufts (9) reported fluorspar unsatistillate may be converted quantitatively factory as a standard and suggested to fluosilicate, precipitated as potassium analyzed lead chlorofluoride. The presfluosilicate, filtered off, and titrated. ence of large amounts of either calcium or lead in samples intended to simulate This procedure is essentially a macrosodium carbonate fusions of aluminum method and all of the fluoride distilled from a 0.2-gram sample may be titrated fluoride was considered undesirable. conveniently. Unlike the preceding I n this work, the exact weight of titration, this procedure is insensitive to fluorine involved in each analjs’is was the interferences of small amounts of determined by titrating about 0.2 gram phosphates and sulfates and the end of 48% hydrofluoric acid to the phenolpoint is familiar to most analysts. Acphthalein end point with standard 0.1N cordingly it was designated the standard sodium hydroxide solution. The titrations were performed in platinum dishes procedure for the analysis of aluminum and concluded a t the boiling point to fluoride. As rather lengthy evaporaeliminate interference by fluosilicic acid tion and filtration procedures are inin the hydrofluoric acid or dissolved volved, manufacturing locations were silica in the standard base. allowed the option of using the thorium The titrated solution, after cooling, is nitrate titration on all but critical transferred to the still, acidified, and samples. distilled. The distillate is then analyzed An investigation of the procedures was by one or both of the above methods to initiated when discrepancies appeared determine the weight of fluorine reminum fluoride. Recoveries were reproducible and about 99% complete.
7
covered. As the weight of fluorine taken is determined only by the alkalimetric titration, the weight and concentration of the hydrofluoric acid and possible loss of some of the acid by volatilization are immaterial and a pure solution of sodium fluoride containing an accurately known weight of fluorine may be quickly and conveniently prepared. Special Reagents. Hydrofluoric acid, Baker and Adamson reagent grade, is manufactured by dilution of silica-free 99.95% anhydrous hydrogen fluoride, followed by redistillation in graphite equipment. It was not further purified in the laboratory. It contains less than 0.1% of foreign acids, almost exclusively fluosilicic acid. The sulfate, chloride, and phosphate contents of the acid are consistently well below 0.001% of each. The absence of significant amounts of metals is assured by the lorn nonvolatile matter content of 0.0017,. Sulfuric acid, 72%. The reagent grade acid was diluted to about 72% and boiled for 2 hours to remove traces of fluorine. The evaporated water was replenished continually from a dropping funnel to keep the boiling point a t about 165°C. Alcoholic potassium chloride solution. Sixty grams of potassium chloride were dissolved in 400 ml. of water, 400 ml. of ethyl alcohol were added, and the solution was neutralized to the phenolphthalein end point. Monochloroacetate buffer. Monochloroacetic acid (9.4 grams) was dissolved in water and diluted to 60 ml.; 38 ml. of this solution mere neutralized with 20% sodium hydroxide solution using one drop of Alizarin Red S solution as the indicator. The remainder of the acid solution was then added and the mixture diluted to 100 ml. A fresh solution should be prepared every 2 weeks. Standard thorium nitrate solution, containing 3.0 grams of thorium nitrate tetrahydrate per liter, should be standardized daily by titrating an aliquot of a sodium fluoride solution, prepared as outlined above, containing an accurately known weight of fluorine. Apparatus and Distillation. Distillations were carried out automatically in stills of the type shown in Figure 1. The dropping funnel side arm was connected to a source of distilled water, the connecting tubing passing through the jaws of a Fisher Electrohosecock. A thermostat in the flask thermowell, acting through a relay, energizes the hose cock to shut off the flow of water when the temperature in the flask is below 165" C. and opens the hose cock to add water when it exceeds 165" C. The temperature is easily held to 165" =t0.5" throughout the distillation without requiring- the attention of the analyst. Standard samdes of sodium fluoride containing a b 0 4 0.1 gram of fluorine were transferred t o the distillation flask which contained six to eight glass beads. Aluminum potassium sulfate was added in most cases to approximate the effect of the aluminum in aluminum fluoride. The tip of the condenser was immersed in water or dilute sodium hydroxide solu-
buffer solution was then added and the solution titrated with standardized thorium nitrate solution to the initial appearance of a pale pink color. The end point is a difficult one for inexperienced analysts and individual differences in color perception will often cause operators to disagree as to when the end point appears. For this reason it is well for each operator to determine the titer of the thorium nitrate solution for himself, Blanks are determined and deducted from all titrations.
Figure 1,
Fluorine still
tion in a 600-ml. beaker. After 60 ml. of 72% sulfuric acid had been slowly introduced into the flask by way of the dropping funnel, the contents were heated and distilled until about 470 ml. of distillate had been collected. Potassium Fluosilicate Procedure. The distillate was made alkaline t o litmus with 1N sodium hydroxide solution and evaporated to about 75 ml. After transfer t o a 250-ml. beaker, the evaporation was continued to a volume of 25 ml. The solution was then cooled, 0.3 gram of silicic acid was added, and the mixture was acidified to methyl orange plus an excess of 2 ml. of 12N hydrochloric acid. The beaker was then covered and the contents were boiled for 60 seconds. Four grams of potassium chloride were added to the cooled solution and when it had dissolved, 25 ml. of ethyl alcohol were added, and the mixture was allowed to stand for 1 hour with frequent stirring. Following filtration through a Shimer filter tube, the precipitate was washed with alcoholic potassium chloride solution until 10 ml. of the washings did not noticeably decolorize a solution containing phenolphthalein and 2 drops of 0.10N sodium hydroxide solution. The contents of the Shimer tube were transferred to a casserole and 1 ml. of phenolphthalein solution and 100 ml. of recently boiled water added. After titration with standardized 0.1ON SOdium hydroxide solution nearly to the phenolphthalein end point, the solution was heated to boiling and the titration completed to a faint pink end point which is permanent in the boiling solution. Thorium Nitrate Procedure. After the distillate had been diluted to 500 ml. in a volumetric flask, a 20.00-ml. aliquot was pipetted into a 100-ml. beaker containing 25 ml. of water and 3 drops of 0.2% Alizarin Red S solution. The solution was stirred continuously with a magnetic stirrer and neutralized with 0.10N sodium hydroxide and 1 to 100 hydrochloric acid solution to the pale yellow end point of the indicator. One milliliter of monochloroacetate
Results and Discussion. From time t o time the literature has reported that recoveries of steam-distilled fluorine are not complete (8, 9, 18, 19). Other investigators either report complete or high recovery (6, 10, 15) or do not discuss the point, apparently assuming or implying complete recovery. Nearly all the reports are concerned with trace analysis and it would seem that the small errors reported mould be insignificant in macroanalysis. Distillation of eleven samples of neutralized hydrofluoric acid containing known weights (about 0.1 gram), of fluorine in the presence of 1.2 grams of potassium alum gave an average recovery of 95.1% with a standard deviation of 1.5%. Similar analyses without alum gave 95.570 recovery and a standard deviation of 1.2%. Losses of this magnitude cannot be tolerated in assaying aluminum fluoride. Similar losses have not been encountered in distilling trace or small amounts of fluorine, For such determinations the Willard and Jt'inter distillation is practically ideal and has been used in a number of this company's laboratories for many years without detecting any loss of milligram quantities of fluorine. On the contrary, a small, positive blank is invariably found. It is possible that the fluorine is lost, not in the distillation, but in the subsequent evaporation and precipitation. These points were investigated by preparing a known quantity of sodium fluoride in a platinum dish, precipitating the potassium fluorosilicate in the same dish without intervening distillation, and then filtering, and titrating. As shown in Table I,A, the recoveries were very nearly complete, averaging 99.7%. The loss of 0.3% is probably due to the solubility of potassium fluosilicate. When known amounts of fluorine in slightly alkaline solution were evaporated from 50 to 25 ml. and then precipitated in borosilicate glass beakers, recoveries were much lower, averaging 98,6y0 (Table 1,B). Evaporation of larger volumes of these alkaline solutions in borosilicate glassrare resulted in a further decrease in recoveries and an increase in the standard deviation (Table 1,C). Similar results have been reported (11, 17). On the other hand, VOL. 30, NO. 5, MAY 1958
985
-
12" before coiling
Table 1. Recoveries of Fluorine by Fluosilicate without Distillation
Treatment Recovery, % A. Direct precipitation 99.9, 99.6, 100.0, in platinum 99.6, 100.0, 99.4,99.7 Av. 99.7 Std. dev. 0.23 B. D4ect precipitation 96.7, 98.7, 100.1, in borosilicate 99.9, 100.4, 97.9,96.9, glass 98.4, 97.6, 98.2. 98.9. 98.9; 97.9; 98.3,gg.O Av. 98.6 Std. dev. 1.08 C. Standard diluted to 95.9, 97.5, 99.4 500 ml. Evapo- Av. 97.6 rated t o 25 ml. Std. dev. 1.75 and precipitated in borosilicate ware D. Standard in plati- 99.6. 99.0. 99.6. num dish diluted 99.9, 100.4, ' 98.3, 100.0 to 100 ml. and e v a p o r a t e d . Av. 99.5 Water replen- Std. dev. 0.70 isheduntiltotal of 500 ml. added and evaporated E. Standard made al- 99.6, 99.4, 99.8 kaline, diluted t o Av. 99.6 Std. dev. 0.20 600 ml., and evaporated t o 3040 ml. in copper beaker. Transferred t o latinum dish iefore acidifying Table
II.
Per Cent Recovery Checks
(Distilled in boron-free apparatus and titrated by fluosilicate method) Boron-Free Fused Glass Flask Quartz Flask Analyst Analyst hnalyst Analyst -4 B A B 98.25 98.48 98.40 98.73 98.59 99.15 98.19 98.45 97.23 98.01 98.44 98.60 98.71 98.65 98.47 98.41 97.88 ..
Av.98.22 98.49 Std. dev. 0.04 0.06
98.28 98.22 98.19 98.19
98.64
0.44
.47
concentration of the alkaline solution in glassware has been recommended
($1.
Data in Table I, D and E, indicate clearly that the loss is due to reaction with the glass and not to simple volatilization, precipitation, or spattering. In all subsequent work platinum or copper ware was used in evaporating and precipitating th'e distillates. This practice not only increased recoveries but also greatly improved the precision of the method. The over-all losses can now be assigned as follows: 986
ANALYTICAL CHEMISTRY
--Alundum tube , Platinum
Silica suprhsater
E- @] Figure 2.
Pyrohydrolysis apparatus
Over-all loss (100.0% - 95.1) Precipitation, filtration, titration Evaporation in glassware Disti1I:ition (by difference)
4.9% 0.3% 2.1%
2.5%
It was established by collecting large volumes of distillate and redistilling and titrating the still residues that no significant amount of fluorine remains in the still or that, if it does, it is in an unavailable form. Distillation temperatures of 200" C., attained by substituting phosphoric for sulfuric acid, did not improve recoveries. The situation was not improved when the thorium nitrate procedure was used. Distillation loss mechanisms which have been suggested include formation of a nonvolatile silicon oxfluoride, SiOF2, suggested by Daniel (5), leakage of gaseous fluorine compounds from the still, and interference by boron derived from the glassware (4, 17, 22). Silica gel, which has been reported to retain fluorine (5, 12, 13, 16, 18, 22), was not present during the distillation. Loss by escape of gaseous fluorine compounds from the distillation apparatus is unlikely. The system is open a t only one point, the end of the condenser, and this is kept submerged during the distillation. Except at the beginning of the distillation, there is always a slightly reduced pressure (2 to 10 inches of water) in the still, eliminating the possibility of leakage through the joints. If absorption of fluorid- in the sealing liquid in the early part of the distillation were incomplete, results when sodium carbonate is added to the still should be considerably lower, because, on acidification, the evolved carbon dioxide would sweep the fluorine out of the flask very rapidly. Investigation revealed no such effect. To determine the possible effect of boron, another distillation apparatus was assembled from Corning glass No. 7280, which contains only 0.06'% boric oxide. Either of two flasks was used: one of fused quartz, and one of glass No. 7280. Beakers of boron-free glass, stainless steel, or copper were used in the subsequent evaporation with equal success. The data (Table 11) show recoveries
L
-1'
Figure 3. Reactor and condenser
tube
Condenser racket copper; all other parts platinum Substitution of nearest standard tubing diameters permissible
in excess of 98%, about 3% higher than from borosilicate ware. Student's t test indicates that the recoveries of the two analysts on a single apparatus are significant of operator bias. Confidence levels are between 80 and 90% for the quartz flask and better than 99% for the boron-free glass flask. There is no evidence of a significant difference between the two sets of apparatus with a single operator, confidence levels for existence of a real difference being about 50% for Analyst B and less for Analyst A. Conclusion. The recovery of fluorides in the steam distillation of macro amounts is incomplete. Part of the loss results from the presence of boron in the glassware and is probably due to formation of fluoborates. A smaller loss incurred in the absence of boroncontaining glassware is unexplained. Aside from the greater losses attributed to boron, the precision of the determination is adversely affected by use of borosilicate glassware. For this reason quartz or boron-free glassware is to be preferred in analyses requiring high accuracy. The recovery from a given still by a single analyst is sufficiently constant to permit establishing a reliable factor to be used in correcting the error.
yW7 ball j o i n t '
-
"I,.
,of
- , 8 1 14
8 8
121'
Figure 4.
Steam superheater
PYROHYDROLYSIS METHOD FOR TOTAL FLUORINE IN ALUMINUM FLUORIDE
Preliminary Work. Neither of the methods discussed above has been completely satisfactory. The thorium nitrate method gives difficulty with end points, plus possible errors in handling semimicro amounts of fluorine. The potassium fluosilicate method requires too much time for a control method and it does not offer possibilities for better reproduction of results. I n the past several years, various modifications of distillation methods have been tried with little improvement. A search through the literature produced nothing until the work of Warf, Cline, and Tevebaugh on pyrohydrolysis (21) showed that fluorides and other halides could be hydrolyzed with steam at 1000" C. to produce hydrofluoric acid or other halogen acids which could be absorbed in water and titrated with standard base. Of the several hydrolysis curves presented, one showed a good recovery for aluminum fluoride. Some w6rk on this means of separating fluorine was immediately begun. First results obtained with a Monel tube reactor were discouraging, in that low recoveries and high variable blanks were obtained. Following this, a specially fabricated platinum reactor tube and condenser were obtained. Fused quartz was used for a steam superheater. The tubes joined in the center of the furnace with a tapered fit. Heat was supplied with an electric tube heater of the hinged type. Temperatures over 920' C. could not be obtained. Recoveries of fluorine were 98% or under. To obtain higher temperatures, a ribbon-type gas heater was built, operating on a mixture of natural gas, air, and oxygen. A temperature of 1075" C. was measured by a platinum-platinum-rhodium thermocouple located inside the quartz tube. With this equipment a good deal of work was done to show that recoveries of slightly over 99% could be obtained consistently from samples of standard aluminum fluoride. During 5 months this method was used for plant control of aluminum fluoride assay with frequent comparison of results by the thorium nitrate method. The gas-fired burner was somewhat difficult to control and the heating apparatus could not be easily duplicated.
Figure 5. Pyrohy. . drolysis rates
.4
35f!\
V '0
2
4
8
b
10
1'2
TIME
Pyrohydrolysis Equipment. Figure 2 shows the general arrangement of apparatus used. A Burrell tube furnace, Model H-1-9, was selected for a source of controlled heat. Other equipment was mostly of special fabrication to fit. The platinum reactor tube is 14 inches long, 0.75 inch in diameter, and 0.015 inch thick. Attached is a condenser of coiled 6-mm. platinum tubing enclosed in a copper jacket for water cooling (Figure 3). Fused quartz is used for the steam superheater (Figure 4). This is 12.75 inches in over-all length and 7/8 inch in outside diameter with a tapered end to slide into the open end of the platinum tube. Opposite the taper is a ball joint for attachment of the 500-ml. steam generator. A thermocouple pocket was built into the quartz tube to explore temperatures a t this point. For routine work, the furnace temperature alone is sufficient. I n place of a platinum dish, a polyethylene tall-form beaker is preferred by some analysts. The platinum delivery tube is extended by use of a close-fitting polyethylene tube. Using this equipment the chances of loss by spraying are reduced. Standard Aluminum Fluoride. As aluminum fluoride alone is nearly completely hydrolyzed, whereas other fluorides commonly used for fluorine standards, as sodium fluoride, do not hydrolyze completely, it was necessary to have a source of fluorine from aluminum fluoride. Several samples of technical grade aluminum fluoride have been extensively tested a t two laboratories, by two different methods in General Chemical Division, and also by Kaiser Chemical Corp. a t Mead Works. Of these, sample C (Table 111) was selected because of the quantity available.
-
'h
lb
Ib
20
22
24
20
$0
MINUTES
Table 111. Fluorine Determination Sam le SRP-1, % ' C, Alcoa General F 56.7 59.17 59 .20, 59 .19, 59 .16, 59 .16 F as AIFs 8 3 . 6 87.16 4.24 1.82 NanO 0.55 SiOZ 0.46 0.22 0.06 Fer03 1.1 0.19 so1 0.13 PZOS A1 total 64.4 62:26 '
6
(RzOs)
ai3
A1208
Another sample was being used in the industry, originating at Aluminum Co. of America's (Alcoa) Research Laboratories and described as SRP-1. They advised (14) that the total fluorine value was based on 36 separate determinations by the thorium nitrate method. Values obtained on this sample are in very close agreement, as shown in Table I11 for SRP-1. Temperature Variations. Recoveries were checked a t temperatures up to 1700' C. Most work was done between 1000' and 1200' C. At 1000° and 1200' C. recoveries were 99.2%, but a t 1700" dropped off to 98.9%. It is possible that the evolution of hydrogen fluoride a t this temperature was rapid enough to cause some loss while the quartz tube was being joined to the platinum tube. At any rate, the results obtained at 1100" to 1130" C. were the highest a t 99.35% recovery of fluorine. Therefore 1125" C. was set as the optimum temperature for future work. VOL. 30, NO. 5, MAY 1958
987
Table IV.
Recoveries
Sample Wt., Gram (Nominal) 0.25 SRP-1 C SRP-1
0.15 . -_
F as % AlF,
C
Recovered 82.88 86.42 83.06 86.60 Residuea 1.02 0.59 99.06 99: i 5 99.35 99: 36 Recovery, % Determined by fusion with sodium carbonate, distillation, and titration by thorium nitrate.
Effect of Sulfate Impurities. At temperatures of 1075' C. or lower, sulfur in aluminum fluoride, pi-esent as sulfates from gas impurities, is partially recovered as sulfate in the distillate. About 40% of the sulfur could be accounted for as sulfates when pyrohydrolysis was carried out a t 1050" C. This would require a correction to be applied to the titration with sodium hydroxide. With temperatures a t 1100" C. or higher, the distillate from sample C, containing 1.1% sulfur as sulfur trioxide, did not show more than 0.1% sulfur in three separate hydrolyses. Many other control samples containing about 1.0% sulfur trioxide were checked without showing more than a trace of sulfate. In an effort to determine if other sulfur compounds were distilled over, the filtrate was oxidized with bromine, without increasing the sulfate recovered. Evidence of sulfur dioxide formation on hydrolysis was shown by adding an excess of iodine to the water in the absorption dish and back-titrating with sodium thiosulfate. In this may, 0.5% sulfur as sulfur dioxide was obtained, while an iodine titration of a finished distillate shows only a trace of sulfur dioxide. It was concluded that some sulfur goes to sulfur dioxide but is not absorbed in the hydrofluoric acid solution and therefore no correction is necessary. Hydrolysis Rate. Using sample C or SRP-I, the ultimate recovery of fluorine was obtained in 8 minutes. This was determined by titrating the acid continuously as distilled. Distillation was continued for 30 minutes t o be certain that all fluorine was recovered. Samples of sodium fluoride and sodium fluosilicate were also checked with calcined alumina added, as would be present in aluminum fluoride. Hydrolysis of sodium fluosilicate is incomplete a t 90% recovery in 30 minutes, Sodium fluoride is only hydrolyzed in 30 minutes. Cryolite, with alumina added, was 53% recovered in 30 minutes. Figure 5 shows these results on a time recovery basis. A blank of 0.1 ml. is found with calcined alumina. Silica added to calcined alumina or to a standard sample had no effect on results. Commercial aluminum fluoride contains an average of 2.5% sodium calcu988
ANALYTICAL CHEMISTRY
lated as the oxide, The silicon content varies with the raw material fluorspar. Analysis of the product shows from 0.4 to 0.8% as silica. Calculations will show that there is not sufficient silicon present to account for all of the sodium as sodium fluosilicate. It has been suggested that chiolite (5NaF.3AlFa) could be present in the product. Recoveries are too high to allow for the existence of much cryolite or sodium fluoride. Sample Weight and Recoveries. The rapid rate of hydrolysis permits an increase in sample weights from a nominal 0.15 gram first used to 0.25 gram. The choice of weight mas influenced by the average titration desired. It was convenient to standardize on a 40-ml. titration using 0.2N sodium hydroxide solution. Recoveries of 99.3% were obtained (Table IV). Procedure. A 0.25-gram sample of aluminum fluoride ground t o about 80-mesh is accurately weighed in a platinum boat, The boat is placed with a narrow forceps 4 inches in from the open end of the platinum reactor tube. The furnace temperature should be about 1125' C. The steam generator is adjusted to give 3 to 3.5 ml. of condensate per minute. The temperature of the silica steam heater has been determined to be 993" C. with above conditions. After the steam vent is closed to atmosphere, sufficient time is allowed to discharge air from the silica preheater. A shallow platinum dish or tall-form beaker of 250-ml. capacity is filled with enough deionized water to submerge the delivery tube about inch. When ready to start, the platinum reactor tube containing the sample is slid into the Alundum furnace tube to engage the silica tube. During this procedure the receiver dish or bottle is kept in position to submerge the condenser tube under water. This manipulation is the most critical in the method. A little practice is necessary to make this connection quickly and firmly. It is not necessary to have a gas-tight joint, as there is a slightly reduced pressure inside the tube, due to condensation of steam or to a Bernoulli effect arising from the tapered silica tube, The joint should be tightened by twisting and pushing slightly on the cold ends of both tubes after the connection is made. The air that is first expelled should be clear and not cloudy
with uncondensed gases. The latter condition mill cause a lower recovery factor but can be corrected by more practice to reduce the time required for joining the tubes. The longer time allows for too high a temperature in the platinum tube and probably an excessively rapid initial reaction. After 15 minutes of reaction, some 50 ml. of distillate have been collected. The receiver is lowered to allow for flushing and draining the condenser without submergence. The distillate is titrated cold with 0.2147 sodium hydroxide solution, using 0.5 ml. of 1% phenolphthalein for an indicator. The first light end point that holds for 15 seconds is taken. A 50-ml. buret equipped with Teflon cock and sidefilling T is convenient for this purpose. A titration of about 40 ml. is usually made. On heating to boiling, about 0.1 ml. of additional base can be added; as a blank of 0.1 ml. is also found, the two corrections cancel out. Reagents. Sodium hydroxide solution, 0 . 2 N , is made from 50% liquid stored in a gallon polyethylene jug to precipitate carbonates. The water used is distilled and deionized. Phenolphthalein is a 1% solution in methanol. Standardization. Sodium hydroxide is standardized with potassium acid phthalate, National Bureau of Standards sample 84D. As the recovery is not loo%, it is necessary to run standardizations on a sample of aluminum fluoride of known strength. Results obtained on a series as compared to theoretical will give the percentage recovery. The theoretical equivalent is divided by the recovery factor to obtain a working aluminum fluoride vaIue. The recovery on equipment described here has been about 99.3%. Samples of aluminum fluoride suitable for check purposes are Alcoa SRP-1 or General's standard sample C, which has been very thoroughly checked by several laboratories. Both contain impurities found in a technical grade of aluminum fluoride. Summary of Results. With the furnace left a t operating temperatures during the day, samples can be run at intervals of a quarter of an hour with duplication of results to =4=0.1%, These results have been obtained on a series of six determinations with both standard samples. In contrast to this, some 2 hours are required to complete an analysis by the distillation method. It is also difficult to duplicate results closer than 0.2870 even for the most experienced analyst. The pyrohgdrolysis method has been used on several hundreds of aluminum fluoride samples with a large saving in time and improved accuracy of plant control work. Because the method was developed exclusively for aluminum fluoride, it has a limited application and cannot be applied t o other fluorides shown in the hydrolysis curves.
ACKNOWLEDGMENT
The authors gratefullv acknowledge the help given by R. S. Bederka, L. G. Jordan, and E. J. Weber in performing the many analyses and thank A. J. LaVine for drawing their attention to the work of Warf, Cline, and Tevebaugh. LITERATURE CITED
(1) Armstrong, W. D., IND. ENG.CHEM., ANAL.ED. 8, 384 (1936). (2) Armstrong, W. D., J. Am. Chem. Soc. 55, 1741 (1933). (3) Boruff, C. S., Abbott, G. B., IND. ENG. CHEM..ANAL. ED. 5. 236 (1933). (4) Clark, H. S., ANAL. CHEX.23, 659 (1951).
(5) Daniel, 2. anorg. Chem. 38, 290 Aluminum Co. of America, private communication. (1904). Reynolds, D. S., J. Assoc. Ofic.Agr. (6) Ebere, W.F., Lamb, F. C., Lachele, Chemists 17, 323 (1934). C. E., IND.ENG. CHEM.,ANAL. Ibad., 18, 108 (1935). ED. 10, 259 (1938). Reynolds, D. S., Hill, T (7) Fedoruk, J. C., General Chemical Division, Morrietown, N. J., private communication. (8) Hoskins, W. M., Ferris, C. A., IND. Ibid., 3, 366, 371 ENG. CHEM., ANAL. ED. 8, 6 (1936). (9) Kimball, R. H., Tufts, L. E., ANAL. ists 11; 225 (1928).” CHEM.19, 150 (1947). (20) Rowley, R. J., Churchill, H. V., (10) McClendon, J. F., Foster, W. C., IND.ENG. CHEM.,ANAL. ED. 9, IND.ENG.CHEM.,ANAL.ED. 13, 551 (1937). (21) Warf, J. C., Cline, W. D., Tevebaugh, 280 (1941). R. D., ANAL.CHEW26,342 (1954). (11) McClure, F. J., Ibid., 11, 171 (1939). (22) Willard, H. H., Winter, 0. R., JND. (12) Matuseak, M. P., Brown, D. R., ENG. CHEM., ANAL. ED. 5, 7 Ibid., 17, 100 (1945). (1933). (13) Milton, R. F., Liddell, H. F., Chivers, J. E., Analyst 72, 43 (1947). RECEIVEDfor review January 3, 1957. (14) Moss, hi. L., Research Laboratory Accepted January 16, 1958.
Extraction of Chromium with Trioctylphosphine Oxide from Acidic Solutions of Alkali Metal Salts Determination in Situ as Chrom um-D phenylcarbazide Complex C. K. MANN and J. C. WHITE Oak Ridge National laboratory, Oak Ridge, Tenn. For determination of microgram amounts of chromium in concentrated solutions of alkali metals, chromium is extracted in the sexivalent state with a 0.2M solution of tri-n-octylphosphine oxide in benzene from either a chloride or sulfate solution of the alkali metals. The chromium-diphenylcarbazide complex is then formed directly in the benzene solution b y addition of an alcoholic solution of diphenylcarbazide. As chromium is extracted b y tri-noctylphosphine oxide only in the sexivalent state, an oxidant is added to ensure complete oxidation of chromium. Argentic oxide was a satisfactory oxidizing agent in sulfuric acid solutions. When hydrochloric acid is present, bromine water is the preferred oxidant. The coefficient of variation of the method is 9% for 2 to 10 y of chromium extracted from approximately 10 grams of sodium and potassium chlorides in 80 ml. of solution. O n a weight basis this range is equivalent to 0.2 to 1 pap.m.
A
for the spectrophotometric determination of chromium is based on the formation of the characteristic red-violet color with diphenylcarbazide, after extraction of chromium from aqueous solution into an immiscible nonaqueous solution of tri-nMETHOD
octylphosphine oxide (TOPO). White and Ross (4) found that dichromate can be extracted from acidified aqueous solutions into hydrocarbon solutions It of tri-n-octylphosphine oxide. seemed possible, therefore, that chromium might be separated from solutions having such a high ionic strength as to vitiate the usual diphenylcarbazide method and concentrated into a smaller volume so as to increase the sensitivity of the method. REAGENTS
Standard solution of sexivalent chromium, 1.000 mg. per ml. in approximately 0.05M sulfuric acid. Dissolve 2.828 grams of dried potassium dichromate from the National Bureau of Standards in about 800 ml. of water to which 3 ml. of concentrated sulfuric acid has been added. Dilute to 1 liter with water. Standard solution of trivalent chromium, 1.000 mg. per ml. Transfer an aliquot of the standard solution of sexivalent chromium to an Erlenmeyer flask. Pass a stream of sulfur dioxide through the solution for 5 minutes; then boil the solution gently for 30 minutes to remove the excess sulfur dioxide. Use this solution directly or dilute it with water as required. Tri-n-octylphosphine oxide (TOPO), 0.2M. Dissolve 7.72 grams of tri-noctylphosphine oxide (Eastman Organic
Chemicals No. 7440) in 100 ml. of benzene. Protect from light by storing it in an opaque container. Diphenylcarbazide, 0.25 (w./v.)%. Dissolve 125 mg. of diphenylcarbazide in 50 ml. of absolute ethanol immediately before use. Argentic oxide, Ago, reagent grade, hIer ck . PROCEDURE
Transfer an aliquot, no larger than 80 ml., which contains preferably 3 to 10 y of chromium and not less than 1 y in approximately 5 volume sulfuric acid, to a 150-ml. beaker. Add 3 ml. of concentrated sulfuric acid and approximately 100 mg. of argentic oxide. Stir the solution well; then heat to boiling and boil vigorously for 5 minutes. Cool the solution to room temperature. Dilute the cooled solution to approsimately 50 ml., if necessary, with water; then transfer the solution to a 125-ml. separatory funnel and add sufficient 5M sulfuric acid to make the solution about 1M. Complete the following procedure for color development without interruption. Add 5 ml. of 0.2M tri-n-octylphosphine oxide in benzene; then shake the sample for 2 minutes. Allow the phases t o separate and collect the organic phase in a test tube that contains silica gel. (Do not use indicator silica gel, as the color is extracted by the organic solution.) Transfer 2 ml. of diphenylcarbazide solution to a 10VOL. 30, NO. 5, M A Y 1958
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