480
ANALYTICAL CHEMISTRY
This review and description of the Schoeller approach and all subsequent modifications of his approach to the analysis of simple and complex minerals containing the earth oxides are made in this brief form to lay a basis for the fundamental conclusions: 1. The analytical separative schemes of this approach do not give quantitative results without repeated precipitations and reworking of these precipitates. To achieve reasonable precision, therefore, requires long periods of time and many tedious and repetitive operations. 2. From the standpoint of the reactions themselves, it is clear that a t almost every stage of the analysis there is either a colloidal solution or a colloidal precipitate to contend with and almost never solutions or precipitates involving simple ions or compounds. I t is this condition which is responsible for the complexity of these analytical schemes, 3. Schoeller correctly diagnosed the main reason for the difficulties which had defeated the efforts of earlier norkers (except Marignac). Nevertheless, the reactions that he developed were subject to the same difficulties. Although he often started with complexes in true solution, hydrolysis was unavoidable in his scheme of operation, and the resulting precipitates were invariably colloidal in character, As such, adsorption was inevitable, as was "loss of chemical identity." In his massive and painstaking investigations, this master analyst extracted most of the potentialities of these complicated aqueous reactions, It therefore appears more profitable to seek an alternative approach, perhaps more difficult to develop in the preliminary stages, but basically simpler in the separation of the earth acids.
To this end the basic inorganic chemistry of the various elements was considered in a search for a class of compounds, simple in character and suitable for clean-cut separative reactions. The choice of compounds finally narrowed down to the anhydrous chlorides, where for reasons of ease of preparation, and a coinbination of suitable physical and chemical properties, a start n as made. In the next section soine of the reasons underlying this choice are developed. The essential difference of the approach described is that the main separations are made in nonaqueous systems \+here the difficulties of working with colloidal systems are evduded. While this initially poses many new difficulties it meets a requirement which all satisfactory analytical methods fulfill-i.e., handling compounds of simple and definite stoichiometry. This approach has already resulted in a method for the separation of titanium from the earth acids, so complete and satisfactory that in only one operation the concentration has been reduced beyond spectroscopic detection. This operation, involving an element intermediate in its chemical behavior to the two earth acids, is basic to the further development of their separative reactions. Its solution, therefore, confirms in a minor way the validity of the approach suggested above and a l l o s~ a direct attack on the sepal ation of niobium from tantalum.
( A n a l y t i c a l Chemistry o f N i o b i u m a n d Tantalum)
CHLORINATION OF OXIDES AND THOSE OF RELATED ELEMENTS a system. Hall (9) obtained similar results with sulfur inonoHISpaper reports a numberofqualitativeandafewquantitative investigations concerned with the atmospheric chlorination of chloride. The requirement of a sealed high pressure chlorinating appai aoaides likely to be found in tantalite and niobite minerals. The tus excludes the development of an easy distillation technique. decision to make this investigation developed from a review of To open the bomb tube and transfer the contents quantitatively current analytical methods for such minerals, from which it beto a distillation apparatus without hydrolyzing the reactive chlocame evident that a nonaqueous approach was necessary if cleanrides is difficult and time-consuming. I t was necessary therefore cut separations were to be achieved. The anhydrous chlorides to develop a chlorination technique operable at atniosphei ic preswere selected for the contemplated analytical scheme as a result sures and thus convenient for use in the boiler of a still. of several additional considerations. An examination of the boiling points of the elements of groups MATERIALS AND REAGENTS IV, I-,and VI blocked out in Table I1 (28) shows a real possibility Niobium(V) Oxide. This was the high purity grade niobium for separations involving a low boiling and a high boiling group. oxide sold by the Fansteel Metallurgical Corp., Sorth Chicago. This would mean the separation of titanium, tin, silicon, gerIll. The principal im urities are. 0.2% Ta20a,0.002% ,TiO., manium, vanadium, arsenic, antimony, and hexavalent uranium 0.001% Fe, and 0 . 0 3 4 SiO2. This reagent was used Tvithout further purification, except for ignition to remove water vapoi or from niobium, tantalum, zirconium, and tungsten. The 100" volatile constituents absorbed on it. difference in boiling points which exists between the titanium Tantalum(V) Oxide. This is the Type 400 grade obtained and the niobium and tantalum thus appears as an obvious chemifrom the Fansteel Corp. I t is made more reactive by being given cal difference to exploit. a pyrosulfate fusion followed by leaching into dilute ammonium hydroxide. The precipitate is filtered and then m-ashed with In the second place, a separation of titanium from niobium and successive portions of ammonium hydroxide, followed by ignition tantalum was considered a most desirable first step, as it results to constant weight. in a considerable simplification of complete analytical schemes for The titanium(1V) oxide and other oxides were C.P. analyzed chemicals used without further purification. these minerals (26, 3 7 ) . Thus it seemed best to develop first the chlorination reaction and then the distillation separation of titanium. Recent studies of reactions for the preparaTable 11. Boiling Points of Some inhydrous Chlorides tion of the anhydrous chlorides of niobium and Group V I Group IV Group V tantalum ( 4 , 1S, S1, 32, 38, 39) have used either SiCJr 57 carbon tetrachloride or carbon and chlorine as the reagents. The oxides were chlorinated generally in the 200' to 350" temperature range, using a sealed and evacuated bomb tube as the reaction vessel. Success with niobite ores was also reported, On the other hand, attempts a t chlorinating niobic oxide a t atmospheric pressure led to almost quantitative yields of the oxychloride, NbOCI8. Titanic and tantalic oxides could not be chlorinated in such
Tic14 136 ZrClr 331 HfClr 317 ThClr 922
GeClr 84 SnClr 113 PbClr decornp. t o PbCln
VClr 164 NbCla 243 TaCls 234
AsCla 122 8bCls (172) SbCla (219) BiCls 441
JloCla 268 WCla (337) WCla (276)
CCls (sublm. 50' C.)
UCUr (very high) FeCls 319
See14 Unstable TeClr 394 TeClr 322
481
V O L U M E 24, NO. 3, M A R C H 1 9 5 2 Table 111. Preliminary Chlorination Studies Reagent CCh
B.P.,
C.
78
121 205-215 210-212 236 320 CnCla
186
CIHCI~
'247-248
CaCls (tech. grade 85%)
210-270
1,4-C4HsCln
155-156
&HzCl @OCI n-CaHirC1 n-CaH.iCOC1 SlCll PClS
179 197 183-185 215 138 100
Observations and Conditions of Reactions After 2 hours a t 250' and 1 hour a t 300' C. After 2 hours at 250' C. Sealed a t a t m . pressure. Very slight reaction. Eyacuated before sealing. Extensive reaction. N o unreacted oxide remains. Oxide ignited a t ,l00O0 for a n hour prior t o 'treatment. Reaction started slowly b u t went rapidly to completion. Unsaturated Chlorocarbons No reaction Slight yellowing Decomposition of CLIa t o black Slight reaction oily liquid KO reaction N o reaction KO reaction S o reaction KO reaction N o reaction Saturated Chlorocarbons Small amount of oxide residue Slight reaction remains Unreacted oxide mixed with carbonaceous residue and black oily liquid Clean reaction. Small Complete reaction amounts of unreacted oxide Decomposition to black mass Miscellaneous Reagents Very slight reaction Some decomposition Slight reaction N o reaction Yellowing of liquid Slight reaction Incomplete reaction After cooling yellow-green viscous mass observed with some unreacted oxide
Niobite and Tantalite Ores. These ores were available in the department as samples analyzed for all their constituents. Chlorinating Agents. JIost of these were obtained from stock and used without further purification. They were dried in some cases with phosphorus pentoside followed by distillation. Octachloropropane. This reagent was obtained from Columbia Organic Chemicals, Inc., Charleston, S. C. I t was of 85% purity; the remaining 15%, was lesser halogenated chloropropanes, principally asyrn-heptachloropropane. This reagent is used as received or it is purified from the lesser halogenated constituents by recrystallization from hot isopropyl alcohol. Hot isopropyl alcohol is saturated with the reagent. Small y u a n t i t k of decolorizing charcoal are added to the hot, solution, which is then filtered through a heated Biichner funnel. The filtrate is allowed to cool, Is of octachloropropane separate. whereupon only white These are filtered free of the mother liquor, washed on the filter funnel several times with ice cold isopropyl alcohol, then dried over phosphorus pentoxide in a vacuum desiccator. PRELIMIKARY CHLORINATION S T U D I E S
The preliminary qualitative experiments had as their object the discovery of new high boiling chlorinating agents, without which atmospheric chlorinations would be impossible. To speed the research these studies were confined to the oxide of only one element, niobium, whose pentachloride has a characteristic yellowish green color and thus allows a visual determination of the course of the reaction. From Ruff and Thomas' work ( S I , 32) it was also known that this oxide was more reactive than that of tantalum. The reactions were performed in sealed glass tubes, usually with 0.2 gram of the oxide and 4 or 5 gram portions of the chlorine-containing reagent. Carbon tetrachloride was used as a control reagent, as the conditions of reaction were known in this instance. The degree of effectiveness of the various reagents was determined by watching for the disappearance of t'he white oxide and the appearance of the yellowish green vapor of the pentachloride. A Kichrome-wound tubular glass furnace permitted continuous inspection during the heating stage from behind a reinforced glass barrier, Some of the preliminary experiments made it apparent that an anhydrous oxide was necessary if chlorination was to be effected in the easiest, possible way.
Therefore all subsequent work was done a i t h freshly ignited oxide samples, eacept whei e specifically stated otherwise. The results obtained are summarized in Table
111. Several conclusions derived from this table were useful to the problem at hand. In the first place, only the saturated chlorocarbons were reactive with the oxide. Carbon tetrachloride was clearly the best reagent of those tested as regards chemical reactivity. Hexachloroethane and octachloropropane were also effective and possess melting and boiling points that appeared to be high enough for atmospheric work. The lesser halogenated hydrocarbons like heptachloropropane and 1,4-dichlorobutane showed extensive deconiposition, which may be due to a Friedel-Crafts reaction, with anhydrous chloride initially formed. I t ~vould be difficult to explain the black tars any other m y , as reduction of the oxide would not be expected at these temperatures. The fully chlorinated compounds possessing double bonds and the other miscellaneous coinpounds tried proved to be inert. The promising character of the hexachloroethane and the octachloropropane reaction prompted a somewhat more quantitative measurement of the degree of reaction under the experimental conditions of Table 11. ,
The reaction vessel used was a U-tube with the reactants loaded in one arm only. After reaction, which was conducted with theentire tube in the fuinace, it was possible to distill the volatile products to the chilled empty limb. In this way only unieacted oxide would remain in the first arm along xith any carbonaceous residue. Even the partially chlorinated niobium oxvchloride is volatile and would distill. Follon . ing distillation, thetube was opened and the contents of thereactor arm were analyzed for niobium by the Schoeller tannin procedure (56).
-
The charge taken in the first two expcrinlents was 0,100 grain of the oxide and 1.0 gram of octachloropropane; 0.019 and 0.020 gram of unreacted oxide remained. In three additional determinations 2 grams of octachloropropane were used; 0.001, 0.000, and 0.000 gram of unreacted oxide were found. The use of this reagent purified by recrystallization from isopropyl alcohol gave identical results. Similar results were obtained n-ith the hesachloroethane, except that it was necessary to work near 300" C. to get apparently complete reaction. \Then this was done all the niobium could be volatilized. Available samples of niobite and tantalite ores were tested with these reagents in exactly the same Tvay. If the heating treatment involved at least 3 hours at 300", reaction was complete for these ores. With the quantitative character of these reactions i n sealed tubes established, the next' step was to investigate their potentiality under atmospheric conditions. ATMOSPHERIC CHLORINATIONS
A 50-ml. round-bottomed flask fitted with the female end of a ball joint was attached to the bottom of a 50-cm. reflux column 9 mm. in diameter. This column could be uniformly heated by means of a jacketed Sichrome ribbon heater which surrounded it for its full length. Provision was made a t the top of the column for evacuation of the assembly. A still head and receiver of conventional design were part of this end of the column. The receiver v a s also fit,ted with a drying tube to prevent moisture from entering the apparatus during use. A tall-form beaker, also wound with Nichrome ribbon and insulated on the out'side, was used to heat the still pot and that portion of the column not covered by its own heater. By means of glass wool insulation between the inner surface of the beaker and the outer surface of the column heater in the overlap region, an effective seal could be made, allowing for temperature control at all positions.
ANALYTICAL CHEMISTRY Most of the reactions were conducted as described below. Deviations are specifically noted. In general, a portion of ignited oxide (0.100 gram) was placed in the still pot with some quantity of chlorinating agent, and the apparatus was assembled. The entire apparatus was then evacuated to exhaust all moisture from it. Thoroughly dried air was admitted next, after which contact with the atmosphere was provided by means of the drying tube attached to the receiver. The ground-glass joint near the reaction zone, previously lapped to ensure a tight fit, was lubricated with a fluorinated grease. The heater for the beaker was next turned on, while the column heater was left off. After the reaction had progressed for the time interval desired, the column heater mas turned on and heated to about 250" to ensure distillation into the receiver of the volatile metal chlorides.
The flask and its contents are slowly brought to 275' C. and kept under total reflux a t that temperature for 5 to 6 hours. The flask is cooled, disconnected, stoppered, and quickly transferred to a dry box. After the air in the box has been dehydrated, the flask may be opened, the supernatant liquid drained off, and the crystals of the pentachloride washed repeatedly with chilled carbon tetrachloride. Finally the crystals are dissolved by heating them with carbon tetrachloride almost to its boilicg point and then filtered through a heated, sintered-glass filter funnel to remove ucreacted oxide and silica, The filtrate is now cooled and the anhydrous chloride is recovered by filtration or by evaporation of the solvent. Yield is about 90%.
In this way the effectiveness of hexachloroethane, asym-heptachloropropane, and octachloropropane as a reagent for the atmospheric chlorination of niobic(V) oxide was determined. With pure hexachloroethane a serious difficulty was immediately encountered, due to its tendency to sublime into the cold column before the chlorination had progressed appreciably. Heating the column sublimed it into the receiver without furthering the reaction. With the heptachloropropane, reaction could be initiated, but on continued refluxing of the reagent, decomposition set in with the formation of black tarry masses. This reaction seemed so unpromising that it, too, was abandoned, With octachloropropane the reaction appeared most promising almost from the start, and a large number of experiments were performed with it. With large excesses of this reagent, the reaction proceeded in the following way.
A number of studies were next made to see the effect of octachloropropane as a chlorinating agent for tantalum oxide.
As the still pot was warmed, the reagent melted to a clear liquid a t about 160' C. At this temperature the white niobic oxide could be seen unreacted on the bottom of the flask. From about 225' until the onset of reaction was revealed by an initial frothing, the rate of temperature rise was slow. Then the temperature was raised to 275", where it was observed that in 3 hours as much as 0.50 gram of the oxide would be comdetelv chlorinated. The test for-completeness of reaction was made b$ warming the reflux column to about 250" C. to distill all the reaction products out of the still pot. Then niobium was determined in the residue in the still pot by dissolving with aqueous hydrofluoric acid, decomposing the fluoniobate, oxidizing the organic material by boiling with a hot concentrated sulfuric-nitric acid mixture, and finally making a tannin precipitation a t a p H of 5 to 6. By this means, and as a consequence of experiments in which the octachloropropane-metallic oxide ratio as well as the time of refluxing was varied, it was found that the minimum conditions for the chlorination of samples 0.5 gram or less included a reagent-oxide ratio of not less than 10 and a reaction time of at least 3 hours at 2 7 5 O . The amount of niobium found in the still pot was reduced to 0.1% or less by operating in this way. In spite of the fact that the reflux column was kept a t room temperature during the chlorination, no deposits of octachloropropane were observed. Instead, a clear yell on^ liquid kept refluxing a t the bottom of the column. When the octachloropropane was heated in the absence of metallic oxide, the same thing happened. It was subsequently established that the octachloropropane was thermally decomposing at an appreciable rate into carbon tetrachloride and tetrachloroethylene. This finding is in accord with previous observations ( 2 , 80). These lower boiling liquids apparently dissolve any octachloropropane that condenses below them in the reflux column and return it to the still pot. This accounts for the absence of solid reagent in the column and receivers and ensures that the reagent can disappear only as a result of reaction with a metallic oxide or as a consequence of thermal or catalytic decomposition a t the reaction temperature. By a few minor changes this analytical reaction allows the opportunity of preparing sizable quantities of anhydrous niobium(V1 chloride. Five grams of ure ignited niobium(V) oxide are mixed with 50 grams of octachforopropane (the commercial product recrystallized from isopropyl alcohol) in a 100-ml. flask connected to a reflux condenser. There must be provision for evacuating the apparatus to remove moist air and dry air during the synthesis.
CHLORINATlON OF TANT4LUMW) OXlDE
The oxide sample used was the untreated Fansteel product (Type 400 grade) of high bulk density and extremely refractory to chemical attack. In a preliminary study 0.100 gram was heated with 3 grams of octachloropropane. The starting temperature was 275'. After an hour the temperature was raised to 300' and then t o 325" and 350' for an additional hour each. The unreacted oxide was determined as in the case of the niobium-Le.,' by subliming the excess reagent and chloride into the cold column, followed by an analysis of the still-pot residue using the tannin method. Conversion was only 2%. A second experiment in which the reaction temperature, was maintained at 290" to 310" for 20 hours gave a 3% conversion. To bring the oxide into a more reactive state, a portion was fused with potassium bisulfate until a clear melt resulted; this was leached into water and precipitated with ammonia, filtered, and ignited to a fluffy powder. This preparation when subjected to the chlorinating conditions of the second experiment gave conversions of 36 and 50% on two successive tries.
Table IV.
.
31ole TazOs Ratio Wt. of Taken, G. Nb/Ta Residue, G. 0.0114 1.7 0.1005 0.0184 0.85 0.1000 0.43 0.0243 0.1006 0.0538 0.1001 0.17 0,0178 0.1000 0.17 0 . OlOOa Reaction time 3.5 hours a t 300' to 310' C.
NbzOa Taken, G. 0.1015 0 0499 0.0248 0.0101 a
Effect of Niobium on Tantalum Oxide Chlorination Tar05 Chlorinated,
%
88.6 81.6 76.7 46.2 82 2
Ruff and Thomas (SI) had reported that the tantalum oxide chlorinates much more readily in the presence of the niobium oxide. Some preliminary experiments tended to confirm their observation and thus led to the study of the effects of various niobium-tantalum mole ratios. The conditions of reaction were in this case a temperature of 275' for 3.5 hours. Five grams of octachloropropane were used in all cases to ensure an excess of reagent. The data obtained have been summarized in Table IV. From Table IV it is evident that the niobium has a profound catalytic effect on the chlorination and when present in equal weights with the tantalum induces nearly complete chlorination. By operating at the higher temperature, the per cent of reaction was nearly doubled from what it was at 275". Raising the temperature or the niobium content leads to complete conversion. In several additional experiments where the niobium-tantalum mole ratios were 3.3, 6.6, and 16.6 chlorination was complete, zm shown by visual inspection of the reaction vessel. CHLORlNATlON OF TITANIUMCIV) OXIDE,
A major effort was made in studying the conditions for the atmospheric chlorination of titanium dioxide, as this is a key element in the program. The problem of its chlorination a t the concentration levels found in niobites and tantalites has been solved. It has not been solved for ilmenites or rutiles, however, or for the pure oxide.
V O L U M E 24, NO. 3, M A R C H 1 9 5 2 Table V. Oxide AIzOr Ti02 ZrOz ThOz 8n0z VPOS
As201
SbrOa CrOr CrzOs MoOz WOI MnOz Re08 Fez01
483
Chlorination of Miscellaneous Oxides
Chloride Observed None None None None SnClr vc1, AsCla SbCla CrOzClz CrZOa None MoCla WOClr MnClz ReCla and some ReCla FeCh
Gas Evolved h'one None None None coc12 c12 COClP COClr c12
Wt. of Sample,
G.
Wt. of Residue,
G.
0.1024 0.1024 0.025 0.026 0.1028 0.1034 0.1017 0.1093 0.1028 0.0129 0.1027 0.0026 0.1068 0.0592 0.0994 0.0079 See discussion
7%
Chlorinated 0 0 0 16 88
97 41 92
None c12 COClz c12 ClP
0.1008 0.1000 0.1048 0.1020 0.0500
0.1009 0.0230
....
0 77 96 10 90
coc12
0 1002
0 0032
97
0.0038
0.0901
CHLORlNATION OF OTHER OXIDES
To see the extent to which the octachloropropane reaction would apply to oxides commonly found in the ores of the earth oxides, a large number of additional cases were studied in a preliminary fashion. The method of study was that previously described, except that it was occasionally necessary to sublime or distill the chlorinated products with the aid of a vacuum pump. The metallic chloride was usually identified by its physical appearance as it condensed in the receiver or in the reflux column of the apparatus. No further analysis was attempted. After decomposition of the organic matter in the still pot, the residual oxide was determined and from this the per cent chlorination was calculated. The oxide samples were taken directly from the laboratory shelves and used without purification. The more refractory ones such as titanium( IV), zirconium(IV), thorium( IV), aluminum(111) oxide, etc., were dissolved, reprecipitated, and ignited to ensure a reactive state of subdivision. They were reacted a t 275" for 3 hours with 5 grams of octachloropropane. The data obtained are summarized in Table V. Thorium(IV) chloride is nonvolatile a t the temperature used.
It seems that the increase in weight observed, which is outside the probable error, must represent some chlorine uptake to give basic chlorides. The actual weight increase represents a 16% conversion if assumed to be the tetrachloride. With the chromium( VI) oxide it was observed that soon after the melting point of the octachloropropane had been reached the red chromyl chloride began to distill. This substance, like the oxide, is a powerful oxidizing agent. Soon after the appearance of the red liquid the chlorinating agent began to decompose, with copious evolution of chlorine. The residue appeared to be the green chromium(II1) oxide. On the other hand, when chlorination with octachloropropane was performed in a sealed bomb tube, the reduction and chlorination proceeded violently to the pink trichloride. The obvious conclusion to be derived from Table V is that the more acidic the oxide in the Berzelius or G. N. Lewis sense, the greater the ease of chlorination. This is demonstrated in several ways. As the periodic table is crossed horizontally-Le., titanium, vanadium, and chromium each in its highest valency-the ease of chlorination increases steadily. Within a given group the ease of chlorination falls with increasing atomic weight-vie., vanadium, niobium, and tantalum. For a given transition elementLe., chromium-the higher and more acidic oxide is highly reactive nhile the trioxide is inert. The basicity of the latter is comparable to that of alumina, which is also not chlorinated under the conditions of the experiment. The higher valences of such elements as rhenium, arsenic, antimony, and germanium will be completely chlorinated under these conditions, perhaps n-ith reduction to lower valence states.
When chlorination occurs with reduction, there is copious evolution of chlorine gas. The instances in which this was observed are listed in the table. Phosgene was also present. It was expected, in view of the difficulties with the tantalum chlorination, that the state of subdivision of tin(1V) oxide would show a pronounced effect on the degree of chlorination, and because cassiterite is often a constituent of these earth oxide minerals, a few supplemental chlorination reactions on this form of stannic oxide were made. It was observed that in 2 hours a t 30O0, cassiterite was chlorinated only to the extent of about 5 or 10%. Prolonging the reaction time to 16 hours and raising the temperature to about 325' assisted the chlorination and brought it to about 25%. It was not found possible, however, to achieve complete chlorination of this oxide. Some special observations should be recorded concerning the chlorination of ferric oxide. This chlorination reaction proceeds most easily and at even lower temperatures than those normally used. With the same conditions that would be used in the chlorination of niobium, the ferric oxide was speedily chlorinated and the still pot soon became dry and devoid of any undecomposed octachloropropane. It may be anticipated, therefore, that this oxide is functioning .as a catalyst for the thermal decomposition of this reagent ( 1 5 ) and that difficulty may be encountered in the chlorination of ores containing appreciable quantities of iron oxide. NATURE OF CHLORlNATION REACTION
B few semi-quantitative studies were made of the chlorination reaction itself. It has previously been shown that octachloropropane a t these temperatures undergoes thermal decomposition according to the folloning reaction: C3Cls +CCl,
+ CzCI4
Under atmospheric conditions of reaction, the carbon tetrechloride is capable of chlorinating the niobium pentoxide to the oxychloride SbOCla. This has been observed on numerous occasions and may be demonstrated by merely heating the pentoxide in a flask at 350' or thereabouts, meanwhile passing carbon tetrachloride vapor over the pentoxide. The oxychloride, which is volatile at this temperature, will sublime out of the reaction flask and condense in the cooler parts. One of the possible steps of the reaction will then involve chlorination of the niobium to this extent by the carbon tetrachloride formed as a result of the decomposition of the octachloropropane. The tetrachloroethylene would, of course, be inert to this oxide under the conditions of reaction normally prevailing. There is also the likelihood that the octachloropropane itself is the principal chlorinating agent, yielding as products of the chlorination, phosgene and tetrachloroethylene as well as the niobium(V) chloride. The over-all reaction may be postulated as: 5CaC18
+ SbzOa --+
5COCIz
+ 5CzC14 + ZNbCls
and this, very likely, is one of the principal reactions. Analysis of the reaction products reveals that trichloroacetylchloride is present and sometimes in considerable amounts. This reaction product has been identified by its boiling point and by its esterification ;eaction with ethyl alcohol. An over-all reaction which may be written for the formation of this substance is: 1OC3CIs f 2Xb205 + lOCC13COCI
+ 5CzCla + 5NbCIs
In a preliminary study, it is not possible to hazard an estimate regarding the principal or dominating reaction. KO doubt, these vary with the oxide that is being chlorinated and are affected to a pronounced degree by the presence of acid or base catalysts in the reaction mixture. With a base present, such as sodium oualate, chlorination of niobium pentoxide at 275' led to the generation of
ANALYTICAL CHEMISTRY
484 relatively large amounts of phosgene with little trichloroacetylchloride, the predominant inorganic product being the white niobium oxychloride. In this reaction carbonization was extensive. Carbon in small amounts is a normal by-product of the chlorination reaction. It seems, therefore, that before any complete mechanism can be written for this reaction a step by step investigation of the kinetics in the case of several different oxides must be effected.
CONCLUSIONS
It now seems evident that with such a reagent as octachloropropane the requisite conditions for a method of attack on the problem of quantitatively analyzing ores of the earth oxide minerals may be anticipated. With the chlorination methods described above a direct attack on the problem of the chlorination and distillation of titanium from niobium and tantalum becomes the neut step in developing this system of analysis.
( A n a l y t i c a l Chemistry o f N i o b i u m a n d Tantalum)
CHLORINATION OF TITANIA AND DISTILLATION SEPARATION FROM HAS been demonstrated that the pentoxides of niobium and I Ttantalum can be chlorinated at atmospheric pressure using octa-
chloropropane. This reagent and technique have noK been applied to synthetic mistures of these oxides, to which titania and other geochemically associated oxides have also been added. The principal effort has been directed toward finding the conditions under which the titania could be quantitatively chlorinated. Once converted to the low boiling tetrachloride, it iyas planned to distill the titanium a m y from the other higher boiling chlorides as had been done by Ruff and Thomas (31,3&) and by Schiifer and Pietruck (33). A practical quantitative separation of the titanium n-odd greatly simplify the subsequent separation of the niobium and tantalum as well as increase the precision of determination of these elements, regardless of the choice made among the tannin technique ( 5 5 ) ,t'he pyrogallol reaction of Platonov ( 2 6 , 2 7 ) and others ( I d ) , and the indirect methods ( 3 3 ) . Therefore, these studies were concerned as much nith the distillation technique as with the chlorination itself. By a systemat,icvariation of the composition of the oside mixtures, the effect of each oside on the total process was evaluated. APPARATUS, REAGENTS, AKD IKCIDEKTAL ANALYTICAL METHODS
The atmospheric chlorination apparatus was much the same as that employed in the previous investigation except for modifications at the top which permitted different kinds of take-off. The apparatus consisted of a reactor still pot, a reflux column, and one of the three column heads shown in Figure 1. A was used for straight take-off, B tyas an air-cooled head which permitted total refluu, total take-off, or any desired reflux ratio, and L' was a water-cooled still head which functioned like B except that the total internal surface was very drastically reduced. Conditions of operation of the reactor, column, and take-off in the final procedure were anhydrous to avoid hydrolysis or the formation of hydrates of titanium tetrachloride with attending loss of volatility. The apparatus was dried by heating under moderate vacuum. Thoee parts not heated by the Kchrome resiatance? were lightly flamed. After this treatment, dry air was slov ly admitted to the apparatus through the drying tube attached to the still head. The oxides were always freshly ignited and were placed along with the chlorinating agent in the cooled reactor still pot after the system had been treated as above. The reactor still pot was disconnected momentarily while the reagents v-ere inserted, and then quickly replaced. The system was once more evacuated; meanwhile only the column and the still head were kept hot. The reactor had to be left cold to avoid volatilization of the oetachloropropane. The apparatus was again filled with dry air, after which the reaction was initiated by heating the reactor still pot. This drying technique did not always yield a completely dry system. It is probable that there were always small amounts of water present which did not interfere because they were destroyed during the course of the reaction. When there was more than this minimal amount of water, a white precipitate of titanium tetrachloride monohydrate formed in the moist parts of the
apliai atus. The presence and amount of thia pi ecipitate sewed as a rough CI iterion of the dryness of the apparatus. A large number of titanium determinations were made on the distillates, on the column holdups, and on the reactor-still-pot residues. In general, these determinations were made by transferring the titania arid any other oxides involved to small beakers by Iyashing with water. The last traces of the oxides were remove by rinsing several times with small quantities of concentrated sulfuric acid. The total volume of concentrated sulfuric acid was adjusted to about 10 to 15 ml. The composite solution was then evaporated until fumes of sulfur trioxide were observed. rlny organic matter not distilled or destroyed at this point was eliminated by treating with concentrated nitric acid, followed by boiling to remove any excess. After dilution with water to a volume of about 125 nil., 25 ml. of 3y0hydrogen peroxide were added. The final volume was now adjusted to 250, 500, or 1000 ml., depending upon which of these volumes would give an absarbance value (log loll)of about 0.5. The absorbance was measured at 410 mp in a Beckman DC' spectrophotometer and the amount of titania was computed using the relation: grams of Ti02 = o*0261 where V = final 250 volume, A = absorbance, and the constant term is the reciprocal of the absorptivity coefficient derived per gram of titania. The uncertainty or limit of detection v a s usually about 1 0 . 2 mg. In the case of the residues where the niobium concentration was high, it was usually necessary to a-ork at the larger volumes in
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Figure 1. Still Reads
13.
