Chapter VII, is now almost the sole source of the thorium nitrate of

commerce. Small quantities are obtained from thorianite, the separation of the pure material presenting, in this case, very little difficulty by reason of the solubility of the mineral in acids and the very high percentage of thoria.

[497] _Vide_ Böhm, ‘Die Thorium Industrie,’ _Chem. Ind._ 1906, ~29~, 450 and 488.

The extraction of pure thorium compounds from monazite is a process of very great technical difficulty. The percentage of thoria is small, whereas that of the ceria oxides is high. The mineral is almost always decomposed by heating with concentrated sulphuric acid, and when the resulting pasty mass is taken up with water, a large amount of free sulphuric acid must be present in order to hold the rare earth phosphates in solution. For the separation of thoria from ceria and yttria compounds in acid solution no processes were known until quite recently. When it is remembered that the thorium nitrate used for the manufacture of mantles must be of a degree of purity which very few commercial products ever approach, some idea of the difficulties of the extraction may be obtained.

~Decomposition of the Monazite.~--Two processes have been used for the working up of monazite. The first of these consists in fusing the mineral with soda, and extracting the sodium phosphate with water; the earths may then be taken into solution with acid, and the separation effected as outlined below. This method is very rarely used. A process has been proposed, in which the monazite is fused with carbon in an electric furnace; the cooled mass is treated with mineral acids, which take the earths into solution free from phosphoric acid. No technical application has so far been made of this proposal.

The method commonly used is that in which the sand is decomposed by means of sulphuric acid. The charge usually employed, about two to three hundred kilograms, requires from four to six hours’ heating, about twice the weight of concentrated acid being needed. The operation is carried out in cast-iron vessels, and an efficient draught must be maintained to remove the acid fumes; the factories are usually isolated. The treatment with sulphuric acid converts the phosphates chiefly into sulphates; when the reaction is finished, the liquor fumes strongly and begins to thicken, heating being stopped when a thick broth is obtained. The cooled mass is extracted with water, care being taken to maintain a degree of acidity sufficient to prevent any precipitation of the phosphates.

It has been already stated in Part I (_vide_ p. 73) that a strongly radioactive product. Radiothorium, has been obtained from the mineral thorianite. This body is produced by the atomic degradation of thorium, and an intermediate body, mesothorium, has been found to be formed during the change. Mesothorium is a substance which, though it appears to be chemically identical with radium, has an activity equal to three hundred times that of radium, and when in equilibrium with its degradation products the ‘rays’ it emits are very similar to those of the latter element. Since mesothorium is a degradation-product of thorium, it occurs in minute quantities in all thorium minerals, and by reason of the possibility of using it as a substitute for radium, its extraction becomes a matter of importance. Soddy[498] has shown that if a barium compound be added to monazite before the treatment with sulphuric acid, the mesothorium remains with the barium sulphate; this is readily separated from the heavy unchanged grains of sand, and is purified, and finally obtained as chloride by treating the solution with hydrogen chloride. On recrystallisation of the barium chloride, the active products are concentrated in the less soluble part, and it is possible to prepare on the commercial scale a mixture which, though it contains only 0·25 per cent. of mesothorium, has an activity equal to that of pure radium bromide. This mixture contains 25 per cent. of radium compounds, radium being present as an original constituent of monazite; owing to the chemical identity of radium and mesothorium,[498] the latter cannot be separated, but Soddy, by removal of much of the barium compound in the laboratory, has obtained a product four times as active as the pure radium salt.

[498] _Proc. Chem. Soc._ 1910, ~26~, 336, and _E._ 25504, November, 1910. See also Hahn, _Chem. Zeitg._ 1911, ~35~, 845.

It is probable that the treatment of monazite will in the future be modified by the addition of barium sulphate before the sulphuric acid decomposition, to allow of the commercial extraction of its mesothorium.

~Separation of Thorium.~--The separation of a crude thorium product from the acid solution obtained after decomposition of the mineral can be effected in two ways, both of which are based on the fact that thoria is less basic than the oxides of the cerium and yttrium metals. In the first, the rare earth elements, including thorium, are precipitated as oxalates by the addition of oxalic acid to the acid solution. These are again taken into solution by the action of hydrochloric acid on the hydroxides, obtained by prolonged digestion of the oxalates with sodium hydroxide; the acid solution is then treated carefully with sodium hydroxide, or pure powdered magnesia, until about one-sixth of the bases has been precipitated, the liquid being constantly stirred. Thorium hydroxide being very weakly basic is precipitated before the other hydroxides, and the precipitate obtained, after one or two repetitions, contains most of the thorium originally present in the monazite. In the second process, thorium is partially separated from the other metals by adding gradually to the solution obtained after the treatment of the mineral with sulphuric acid, the quantity of magnesia calculated to precipitate a suitable fraction of the earths, with constant stirring; this throws down a mixture of phosphates containing almost all the thorium and some of the other elements. The slimy phosphate precipitate is dissolved in hydrochloric acid, and the earths precipitated as oxalates; the precipitate must be washed thoroughly in order to remove phosphoric acid. It will be seen that these two methods differ only in that in the first the phosphoric acid is removed before the precipitation of thorium, whereas in the second the thorium is precipitated as phosphate, and this transformed into oxalate.

Quite recently, methods have been proposed by which the thorium can be separated in a fairly pure condition from the acid solution obtained from the sulphuric acid treatment. Rosenheim, Meyer and Koppel[499] protect the use of hydrofluosilicic acid (H₂SiF₆), and its salts, for this purpose. The sodium salt, added to the hot acid liquid, produces a quantitative separation of thorium silicofluoride; the precipitate is washed by decantation, and treated with sulphuric acid, the thorium sulphate being then purified directly by the sulphate method described below. A second method proposes to make use of the insolubility of thorium hypophosphate, ThP₂O₆,11H₂O, which was found by Kaufmann in 1899 to be insoluble in water, and in acids and alkalies. This method has already been in use for some years for analytical work;[500] it appears to be readily susceptible of adaptation for the technical extraction,[501] the sodium hypophosphate, Na₂H₂P₂O₆,6H₂O required as the precipitating agent being obtainable in large quantities by the electrolytic oxidation of copper phosphide, employed as the anode in an electrolytic cell.[502] This method also gives a thorium compound sufficiently free from other earths to be subjected at once to the refining process; the hypophosphate has in fact been suggested as a very suitable compound for the impregnation of artificial silk mantles directly. The thorium nitrate of commerce, however, is still prepared almost entirely from the crude product obtained by one or other of the two methods of fractional precipitation first described, so that it becomes necessary to outline the method generally employed for separating from this a compound pure enough to be suitable for the final refining process.

[499] _D. R. P._ 214886, October, 1909.

[500] Rosenheim, _Chem. Zeitg._ 1912, ~36~, 821; also Koss, _ibid._ 686

[501] Wirth, _Zeitsch. angew. Chem._ 1912, ~25~, 1678.

[502] Rosenheim and Pinsker, _Ber._ 1910, ~43~, 2003.

The crude oxalate or hydroxide is thoroughly digested with a concentrated solution of sodium carbonate. The carbonates of the cerium elements are much less soluble in sodium carbonate solution than is thorium carbonate. After thorough digestion the liquid is filtered from the undissolved carbonates. The thorium is reprecipitated from the filtrate, either as oxalate, by the addition of hydrochloric acid (if the crude material was in the form of oxalate), or as hydroxide, by the addition of sodium hydroxide. The process is again repeated, and a final digestion is then made with ammonium carbonate; addition of an alkali to the clear filtrate now gives thorium hydroxide sufficiently pure to be used for the last refining.

~Purification of the Thorium Compounds.~--The object of this last stage is to remove from the thorium compound small quantities of cerium and yttrium salts which cannot be separated by the carbonate method. The chief process is the sulphate crystallisation, the principles underlying which have been thoroughly examined in the patient researches of Koppel and Holtkamp.[503] Since the process is based on the solubilities of the various thorium sulphate hydrates, it is necessary to consider these in some detail.

[503] _Zeitsch. anorg. Chem._ 1910, ~67~, 266.

The solubility-curve of thorium sulphate was examined by Demarçay and by Roozeboom. Three important hydrates are known, viz. Th(SO₄)₂,9H₂O, Th(SO₄)₂,8H₂O, and Th(SO₄)₂,4H₂O, other unstable intermediate compounds being said to exist. From a study of the diagram it will be seen that the hydrate with 8 molecules of water is labile, whilst the 9-hydrate and the 4-hydrate have a transition temperature at 43°C., the transition temperature of the 8-hydrate and the 4-hydrate being just below this.

Since the 8-hydrate is labile with regard to the 9-hydrate, and the transition temperatures are so near, the former will be formed first as a solution cools, and by reason of the great similarity of the solubility-curves for the 9- and 8-hydrates the rate of change of this to the 9-hydrate will be very slow. In practice, therefore, it is always the 8-hydrate which is formed, and it is on the separation of this compound that the success of the process depends. The anhydro-compound, Th(SO₄)₂, which can be obtained by heating any of the hydrates to 300°-400°C., is very soluble at 0°, but slowly hydrates itself and separates from the solution as the 8-hydrate, which has a very low solubility. The sulphates of the cerium metals, compounds of which form the chief impurities to be removed, are considerably more soluble, and can be separated by repeated crystallisations.

The thorium hydroxide to be purified is dissolved in sulphuric acid, and in the first form in which the method was employed, the thorium sulphate obtained by evaporation of the solvent was heated until it became anhydrous. This was dissolved to saturation at 0°, and the solution raised to the boiling-point, the 4-hydrate being precipitated; this treatment was repeated several times. It was pointed out by Bunsen, from theoretical grounds, that this method could never yield a pure thorium salt, and Krüss and Nilson accordingly introduced a modification. The impure sulphate, after dehydration, as before, is dissolved at 0°, and allowed to come to ordinary room temperature, 20°; the hydrate which separates (the 8-hydrate) is collected and dried at high temperature and the crystallisation repeated. This method gives a fairly pure salt after three recrystallisations, but the process is very tedious, owing to the time required for drying and heating the hydrate. For this reason the method was further modified by Cleve and Witt. The crude sulphate is boiled with ammonia, and the hydroxide obtained dissolved in hydrochloric acid; addition of sulphuric acid to the concentrated solution in the cold transforms the chloride into the sulphate, which separates as the 8-hydrate at ordinary temperatures. Three repetitions give a satisfactory product, and in this form the method is now much used.

The work of Koppel and Holtkamp referred to above has placed the process on a sound basis. These authors have examined the solubilities of the various hydrates in presence of hydrochloric, nitric, and sulphuric acids, and mixtures of these, at different temperatures. They find that hydrochloric acid is to be preferred to nitric acid, in the process of Cleve and Witt, as besides its lower price, its use involves less loss than that of the latter acid; excess of hydrochloric acid is not harmful within wide limits, whilst a slight excess of sulphuric acid over the quantity required to form the sulphate is desirable, to secure the greatest yield. Finally, the temperature at the addition of the sulphuric acid must not be allowed to rise above 25°, for in the presence of so much acid the transition temperature to the 4-hydrate, normally 42°, is considerably lowered; it is necessary to avoid separation of the 4-hydrate, which is a flocculent unworkable precipitate.

Recently it has been proposed to carry out the purification by use of alkyl hydrogen sulphates,[504] as it is stated that the differences of solubilities of the alkyl sulphates of thorium and the cerium metals are greater than in the case of the sulphates themselves. It is also claimed that the presence of a small quantity of the alkyl sulphate in the thorium nitrate which forms the final product has a good effect on the quality of the mantles made from it.

[504] Kreidl u. Heller, _D. R. P._ 233023, March, 1911; _F._ 414463, June, 1910.

Another process of purification which has found considerable commercial application is the acetate crystallisation, thorium acetate being considerably less soluble than the acetates of the cerium elements. The impure hydroxide is dissolved in acetic acid and the solution evaporated to dryness; repeated washing with small quantities of water removes the cerium acetates, and a fairly pure salt is obtained. This is repeatedly damped with nitric acid and heated to dryness, but even after this treatment a certain amount of unchanged thorium acetate is usually present.

In a second form of this method, due to Haber, the impure hydroxide is dissolved in hydrochloric acid, and the acetate precipitated by addition of sodium acetate. The precipitate is filtered off and re-dissolved in acid, and the acetate again thrown down by means of sodium acetate. The precipitate is then dissolved in nitric acid, and the solution evaporated to dryness. In this form the method gives very good results, even from a comparatively crude product; but the process is, of course, considerably more expensive than the sulphate purification.

The high price of the necessary reagents, again, is a bar to the technical application of the very simple and efficient process of Wyrouboff and Verneuil. These authors suggest the precipitation of thorium peroxide from a warm dilute neutral solution by means of hydrogen peroxide, a process which is quantitative and yields a very pure product. The last traces of the cerium metals can be completely removed by a second precipitation. The cost of hydrogen peroxide is too high, however, to allow its employment on such a large scale, and the method has not, in consequence, come into general use.

The thorium nitrate obtained after purification by the sulphate method, or by the less generally employed acetate method, is usually considered sufficiently pure for technical purposes. Even now, however, it may contain traces of sulphate, of iron, of alkalies, and of cerium metals. If absolute purity is desired, the salt may be dissolved, and freed from all impurities, except the cerium compounds, by precipitation with ammonium oxalate and thorough washing; the oxalate may then be dissolved in chromic acid, and potassium chromate solution added drop by drop; the precipitated thorium chromate is nearly free from other rare earth compounds, and repetition of the process will give a pure salt. The separation from cerium metals may also be effected by the hydrogen peroxide process. If the technical processes are carefully carried out, however, a thorium nitrate of a very high degree of purity may be obtained, and the laboratory purification need only be undertaken if material is needed for very accurate quantitative work.

~Preparation of Thorium Nitrate from Mantle-ash.~--Since the ordinary incandescent mantle, in use, consists only of the pure thoria and ceria, with small quantities of alumina, lime, and magnesia, which have been employed to strengthen the ‘head,’ the working-up of mantle-ash gives an easy means of obtaining the nitrates, and high prices are accordingly paid for the ash in quantity. At one period of great competition between rival manufacturers, canvassers went from house to house in many large towns buying up mantle residues, to be used for the extraction of the thorium for ‘lighting-fluid.’

For this purpose, the oxides are treated with hot concentrated sulphuric acid, the cooled residue dissolved in water, and the thorium and cerium precipitated free from compounds of aluminium, magnesium, and calcium by oxalic acid. If pure thorium nitrate, free from cerium, is required, the oxalates are added to the last precipitate from the double carbonate purification in the treatment of monazite (_vide supra_), and the ordinary processes of refinement continued; more often, however, the mixed nitrate for impregnation of the mantle-fabric is required, and this is obtained by ignition of the oxalates and solution of the oxides so obtained in nitric acid, more cerium nitrate being added if necessary.

~Extraction of Cerium Nitrate.~--Since monazite is primarily a phosphate of the cerium metals, the percentage of thoria being usually quite low (_vide_ Monazite, Chapter VI), very large quantities of compounds of the cerium group of elements are annually produced in the process of extraction of thorium. There is at present a very limited demand for these compounds (_vide_ Chapter XXI), no important uses having yet been found for them. In the ordinary process of extraction of the thorium, these elements remain as the sparingly soluble double carbonates, whilst the thorium double carbonate is removed in solution. From the mixed salts which contain 50-60 per cent. of the cerium compound, the cerium nitrate required for the manufacture of mantles is prepared, but the amount so used is a small fraction of the whole, and large quantities of compounds of cerium and the allied elements are available as soon as profitable uses can be found.

Three processes are in general use for the preparation of cerium nitrate from the mixed carbonates; all of these are based on the fact that cerium can become tetravalent, forming in this condition compounds which can readily be separated from those of the allied elements, which can be obtained only in the trivalent condition. When ceria is dissolved in hot nitric acid, ceric nitrate, Ce(NO₃)₄, is formed, though the action of nitric acid on cerous carbonate or oxalate gives rise to cerous nitrate. Two of the three processes are based on this reaction, and for these the mixed carbonates are dissolved in hydrochloric acid, freed from foreign elements by precipitation with oxalic acid, and the oxalates ignited to the oxides, which are then dissolved in the required quantity of nitric acid. In the first process the cerium is precipitated from this solution by merely pouring it into a large excess of very dilute nitric acid, when a yellow basic ceric nitrate is precipitated; this is washed with dilute nitric acid by decantation, dissolved in concentrated acid, and purified by a second precipitation in the same way. In the second process, separation is effected by addition to the nitric acid solution of the calculated quantity of ammonium nitrate; the solution is concentrated to incipient crystallisation, and on cooling the double ceric ammonium nitrate, Ce(NO₃)₄,2NH₄NO₃, separates. This is collected, washed with dilute nitric acid, and recrystallised until a pure salt is obtained. The double nitrate can be readily decomposed by ignition, leaving ceria, which is dissolved in nitric acid; the nitrate is obtained by evaporation.

The third method, due to Drossbach, is based on the oxidation of cerium salts in neutral solution by potassium permanganate. The mixed carbonates are dissolved in hydrochloric acid, a further quantity of the carbonates stirred in, to neutralise excess of acid, and a solution of the required quantity of potassium permanganate added. The reaction is said to proceed according to the equation:

3Ce₂O₃ + 2KMnO₄ + H₂O = 6CeO₂ + 2KOH + 2MnO₂

The precipitated solid is separated, and dissolved in acid; the cerium is then precipitated as the oxalate, which is transformed into nitrate in the usual way. The solution contains the other elements of the cerium group, which are precipitated by means of sodium hydroxide. The yield obtained by this method is very good, practically the whole of the cerium being separated without loss; whilst it has the further advantage that the remaining elements of the group can be precipitated at once after the separation.

~Analysis of a Monazite or Monazite Sand for Thorium.~--Since the commercial value of a monazite sand or concentrate, or of the pure mineral, depends, at present, entirely on the percentage of thoria, it is important to have a rapid and reliable method of estimating this constituent. The only reliable methods of quantitatively decomposing the mineral, however, all involve acid treatment, and excess of acid must always be present to prevent precipitation of phosphates. Until recently, no way was known for estimating thorium in an acid solution, and all the earlier methods therefore involved tedious processes for complete removal of phosphoric acid, so that the salts could be obtained in neutral solution. This was usually effected by precipitation of the whole rare earth content with oxalic acid, and thorough washing of the oxalates; these can then be dissolved directly in fuming nitric acid on the water-bath, or ignited to the oxides, which may then be dissolved in the same reagent. The solution of nitrates is evaporated to dryness, to effect removal of the excess of acid, the nitrates dissolved in water, and the thorium estimated in the neutral solution.

Among the earliest methods employed for the estimation in neutral solution was the thiosulphate precipitation.[505] Thorium thiosulphate is not known; when sodium thiosulphate is added to a neutral solution of a thorium salt, a precipitate of thoria mixed with sulphur is obtained, by hydrolysis of the potential thiosulphate, and decomposition of the unstable thiosulphuric acid. The method, however, leaves much to be desired; other earths are partly precipitated, and the separation of thoria is not complete. For analytical purposes the precipitate obtained is redissolved in hydrochloric acid, and a second precipitation with thiosulphate effected. The filtrates from the two precipitations are collected, and the whole earth-content precipitated from these with ammonia; the hydroxides are dissolved in hydrochloric acid, and again treated with thiosulphate to throw down any thoria which has escaped the previous precipitations. The three precipitates of thoria are then collected, dried, and ignited for weighing as pure thorium dioxide, ThO₂.

[505] Full accounts of this and the two following methods will be found in an important paper by Benz, _Zeitsch. angew. Chem._ 1902, ~15~, 297

Even more tedious and unsatisfactory is the method based on the solubility of thorium oxalate in excess of ammonium oxalate in neutral solution. The solution is boiled, ammonium oxalate added, and after some moments a small quantity of ammonium acetate solution. On cooling, the oxalates of the cerium metals separate, and can be collected; thoria is precipitated from the filtrate by addition of ammonia. The process must be repeated two or three times, the solution being allowed to stand for one or two days each time, and finally the thoria must be precipitated by thiosulphate to remove traces of the other bases before it can be weighed. Benz (_loc. cit._) gives a complete account of this method, and quotes numerous analyses carried out to test its accuracy.

Far more satisfactory than either of the above is the peroxide method used by de Boisbaudran and Cleve, and later by Wyrouboff and Verneuil.[506] Thorium is completely precipitated as a ‘peroxide salt’ (Th₂O₇,SO₃ or Th₂O₇,N₂O₅ respectively) from warm neutral solutions of the sulphate or nitrate on addition of dilute hydrogen peroxide, a second precipitation being necessary to free it from cerium compounds. Wyrouboff and Verneuil state that the process is rendered difficult by the fact that the peroxide cannot be converted into the dioxide by heating, either alone or with acids, as decrepitation takes place and may cause loss; they accordingly reduce the compound in presence of hydrochloric acid by ammonium iodide, and precipitate thorium hydroxide by ammonia. Benz (_loc. cit._) does not find this difficulty; he states that small quantities of the peroxide dissolve easily in acids without loss, and further finds that if an ammonium salt be added to the neutral solution of the thorium compound before addition of hydrogen peroxide, the precipitate forms much more readily and is very easily handled. Borelli[507] states that the precipitated peroxide can be ignited without loss to the dioxide, and weighed as this.

[506] _Compt. rend._ 1898, ~126~, 340.

[507] Abstract in _J. Soc. Chem. Ind._ 1909, ~28~, 625.

* * * * *

The azoimide method of Dennis[508] is of interest rather than of use. He finds that addition of potassium azoimide, N₃K, precipitates thoria quantitatively from a neutral solution, the reaction being expressed by the equation:

Th(NO₃)₄ + 4N₃K + 2H₂O = 4KNO₃ + ThO₂ + 4N₃H

Cerium, however, if present, is always precipitated with the thorium, and cannot be removed by re-precipitation; this fact, together with the cost of the reagent and the difficulty of obtaining it pure, renders the method quite useless for mineral analysis.

[508] _Zeitsch. anorg. Chem._ 1897, ~13~, 412.

Numberless experiments have been made with organic acids in the hope that an easy method of separation might be found, but though some useful results have been obtained, precipitation has always to be effected in neutral solution, so that all such processes involve the tedious preliminary work of which an outline has been given above. Metzger[509] finds that a quantitative separation of thorium can be effected from a solution in 40 per cent. alcohol by use of fumaric acid; a second precipitation is needed to secure the complete removal of the cerium elements. Neish[510] uses meta-nitrobenzoic acid, which precipitates the thorium salt from a boiling solution; cerium earths, if present, are carried down in small quantities, and are removed by dissolving the precipitate in dilute nitric acid, adding a further quantity of the organic acid, and treating carefully with ammonia to almost complete neutralisation. The compound obtained by this second precipitation is the pure thorium salt. More recently, Smith and James[511] have shown that sebacic acid gives a quantitative precipitation of the thorium salt, from boiling neutral solution, as a voluminous granular precipitate, readily filtered and washed; sebacic acid is very sparingly soluble in cold water, but dissolves readily at 100°, and since, in virtue of this property, it can be readily recovered after use, the authors suggest it as a suitable reagent for the technical separation of thorium from monazite. In all cases where thorium is precipitated as an organic salt in quantitative analysis, the precipitate is dried and ignited, and the residue weighed as the pure dioxide.

[509] _J. Amer. Chem. Soc._ 1902, ~24~, 275 and 901.

[510] _Ibid._ 1904, ~26~, 780.

[511] _Ibid._ 1912, ~34~, 281.

An interesting method has been worked out by Giles.[512] If pure moist lead carbonate be stirred into a neutral solution of rare earth compounds, thoria is completely precipitated. Only the tetravalent elements are separated by this method, so that if ceric compounds are present, they must first be reduced by means of sulphuretted hydrogen or sulphur dioxide; zirconium, if present, must afterwards be separated from the thorium. One precipitation is said to ensure almost complete separation from the trivalent elements. The precipitate is collected, washed, and dissolved in hydrochloric acid; after filtering, if necessary, the solution is saturated with sulphuretted hydrogen, to ensure complete removal of the lead, and thorium hydroxide is then precipitated by ammonia. The drawback to this method lies probably in the fact that it is necessary to use absolutely pure lead carbonate, a substance which, as the author’s elaborate process of purification seems to show, could not be obtained very cheaply on a large scale.

[512] _Chem. News_, 1905, ~92~, 1 and 30.

An account has recently been published[513] of a volumetric method for the estimation of thorium. The mixed oxides are dissolved in concentrated acetic acid, and the solution titrated with a standard solution of ammonium molybdate. This reagent effects complete precipitation of thorium, but does not react with compounds of the cerium elements; excess of the molybdate is shown by a solution of diphenyl carbazide, CO(NH·NH·C₆H₅)₂, used as an external indicator. The carbazide, which is obtained by the action of phenyl hydrazine on urea, has the property of producing definite, though evanescent, colourations with compounds of many of the metallic elements; a drop of the working solution, brought into contact with a drop of the carbazide solution, shows a deep rose colouration when excess of ammonium molybdate is present.[514]

[513] Metzger and Zons, _J. Ind. Eng. Chem._ 1912, ~4~, 493.

[514] Vide Skinner and Ruhemann, _Trans. Chem. Soc._ 1888, ~53~, 554; also Cazeneuve, _Compt. rend._ 1900, ~131~, 346.

The iodate process of Meyer and Speter[515] has the great advantage that it is carried out in a strongly acid solution, so that here the tedious purification from phosphoric acid is no longer necessary. After decomposition of the mineral with sulphuric acid, the sulphates are extracted with water, and a suitable quantity of nitric acid added; the solution is then treated with a nitric acid solution of potassium iodate, and the thorium iodate which separates is dissolved in concentrated nitric acid, and re-precipitated to remove traces of the cerium elements. The iodate, after washing, is dissolved in hydrochloric acid, and reduced by sulphur dioxide; the hydroxide is then precipitated by ammonia. Since zirconium is also thrown down under these conditions, the hydroxide is dissolved in hydrochloric acid; pure thorium oxalate is precipitated from this solution by oxalic acid, and is ignited and weighed as oxide, in the usual manner. Since ceric iodate is also insoluble in dilute nitric acid, it is necessary to reduce any ceric compound which may be present before the iodate treatment by the usual methods.

[515] _Chem. Zeitg._ 1910, ~34~, 306. See also _Zeitsch. anorg. Chem._ 1911, ~71~, 65.

Another method which can be carried out in acid solution is based on the insolubility of the hypophosphite, ThP₂O₆,11H₂O, in dilute acids.[516] To the boiling acid solution, an aqueous solution of sodium hypophosphate, Na₂H₂P₂O₆,6H₂O, is added drop by drop. The precipitate, which contains any titanium and zirconium present in the original solution, is best treated with a mixture of sulphuric and fuming nitric acids; the phosphates produced by the oxidation are freed from nitric acid by evaporation, dissolved in water, with addition of sulphuric acid, and thorium precipitated as the oxalate, which is then ignited as usual. This method has been suggested for the technical separation of thorium from monazite (_vide_ p. 278). Since the precipitations by means of sodium hypophosphate and potassium iodate can be carried out with solutions obtained directly from the product of the action of sulphuric acid on the mineral, these two methods are probably more suitable for the rapid and accurate estimation of thorium for technical purposes than any of the others mentioned.

[516] Wirth, _Zeitsch. angew. Chem._ 1912, ~25~, 1678; see also Koss, _Chem. Zeitg._ 1912, ~36~, 686, and Rosenheim, _ibid._ p. 821.