This paper deals with a characteristic property of tungstic acid and its reaction with hydrochloric acid solution. The authors' experiments prove that tungstic acid freshly formed from tungstate by decomposition with hydrochloric acid, dissolves in its concentrated acid solution forming a chemical compound, provided that the produced tungstic acid is brought into contact with concentrated hydrochloric acid immediately after its formation. It was found also that if the degree of its concentration 1s reduced, this chemical compound gives the insoluble tungstic acid as a precipitate in the solution. Details are given for lixiviating the tungstic acid from tungstate · and for preparing it

Nakazawa, Yoshio

Okada, Shinzo

English

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RIGHT:URL:CITATION:AUTHOR(S):ISSUE DATE:TITLE:A New Method for the Production ofTungstic Acid from TungstatesNakazawa, Yoshio; Okada, ShinzoNakazawa, Yoshio ...[et al]. A New Method for the Production of Tungstic Acid fromTungstates. Memoirs of the College of Engineering, Kyoto Imperial University 1929, 5(4):153-1631929-03-15http://hdl.handle.net/2433/280093A New Method for the Production of T ungstic Acid from T ungstates. llv Yoshio Nak~zawa and Shinzo Okada. ABSTRACT. This paper deals with a characteristic property of tungstic acid and its reaction with hydrochloric acid solution. The authors' experiments prove that tungstic acid freshly formed from tungstate by decomposition with hydrochloric acid, dissolves in its concentrated acid solution forming a chemical compound, provided that the produced tungstic acid is brought into contac·t with concen·rated hydro-chloric acid immediately after its formation. It was found also that if the degree of its concentration 1s reduced, this chemical compound gives the insoluble tungstic acid as a precipitate in the solution. Details are given for lixiviating the tungstic acid from tungstate · and for preparing it. INTRODUCTION. It has been known for some time that strong acids usually form salts of comparatively simple comi-:osition, while weak acids such as chromic acid, silicic acid and stannic acid form more complex salts. Tungstic acid, being a weak acid, has the property of forming various salts of complex comi:osition, and also has that of forming acids having conspicuously large molecules by combining with various other acids : such salts with the oxides of silicon, boron, and phcsphorous are given below : B203• 14 WO:i· XMO ; Si02· I 2 W03• XMO ; 154 Y. Nakazawa. and S. Okada. However, very little quantitative work has been done on the forma-tion of chemical compounds by tungstic acid and the ordinary mineral acids such as hydrochloric acid and sulphuric acid. It is only known that tungstic acid is slightly soluble in cone. hydrochloric acid. Ctl OlsonC2> produced hydrochloric acid solution containing 4% tungstic acid by adding potassium tungstate solution, drop by drop, to cone. hydrochloric acid solution. In this article, we explain and discuss in detail the reaction be-tween tungstic acid and hydrochloric acid, and we deal with a new method of producing tungstic acid as the result of our experiments. Article 1. Solubility of Tungstic Acid in Concentrated Hydrochloric Acid Solution. Tungstic acid is said to be insoluble in any inorganic acid, except hydrofluoric acid, and to be very slightly soluble in cone. hydrochloric acid. This fact is only a matter of concern in analytical manipulation, the degree of solubility not exceeding one percent. The writers found by experiment that the solubility of tungstic acid in cone. hydrochloric acid could be greatly increased by properly modifying the degree of concentration of the solvent used and also the conditions under which the tungstic acid is produced. vVhen tungstic salts are deco:nposed by cone. hydrochloric acid and the whole is agitated frequently, the hydrochloric acid dissolves the tungstic acid immediately and leaves a solution containing as much as I 2 % of tungstic trioxide. Among tungstic salts, calcium tungstate is most easily decomposed by hydrochloric acid : even at 20° C, tungstic acid is produced. So far as easmess of decomposition is concerned, ammonium salts come next to calcium tungstate. Decomposition of natrium and potassium salts takes place very slowly at normal temperatures. This is the reason why we adopted calcium tungstate in our experiments. As we explain in detail in the following chapters, a chloride such as calcium chloride, the by-product 1. Treadwell, Analytische chemie, I lld. S. 5u. 2. Olson, Zs. anorg. chem. 88, 49, (1914). " A New Metlzodfor The Production 4 Tungstic Acid from Tu11gstates. 155 of decomposition of tungstic salts, remaining in the solution, does not greatly affect the solubility of tungstic acid. In our experiments, pure powdered calcium tungstate ( Ca W04) is prepared from calcium chloride and sodium tungstate (Cahlbaum C. P. reagents): this calcium tungstate is added continuously to hydrochloric acid solution, o.5gm. at a time, while the solution is constantly agitated, until it is saturated with tungstic acid, and a turbid state is obtained. Then the saturated solution is kept undisturbed and filtered : a certain quantity of the filtrate is analysed. The results obtained in the above mentioned experiments : the rela-tion between the solubility of tungstic acid and the degree of concentration of the solvent may be tabulated as follows :-Table No 1. Solubility of tungstic acid in hydrochloric acid solution at 20°C under ordinary pressure Normality of HCl Sp. gr. of hydrochloric Percentage of TV03 in solution used acid sol. of tungstic acid the solution 13 N r.37 12.67 % 12 N 1.32 9.61% II N 1.2 I 5.29% 10N I. 17 2·74% 9N 1.15 050% SN 1.14 0,05% 7N 1.12 --As clearly described above, we obtain a solution containing as much as I 2 ~~ of tu_ngsten trioxide by using hydrochloric acid solution of I 3 normality, when the tungstic acid produced by virtue of decomposition 156 12 IO 'fil > ] 8 "O §! .... .... 0 ., ~ i:i ~ ~ 6 4 2 7 Y. Nakawwa. and S. Okada. Fig. I. Solubility curve of tungstic aci<l at 20° C. ~ V 8 9 ) I/ / V / V ,/ 10 Normality of HL'l I ' ) I I ' II / V ' I I/ ) 12 13 of calcium tungstate in hydrochloric acid is treated with concentrate<l hydrochloric acid immediatel_y after its form'ltion. However, the newly formed tungstic acid does not dissolve in the solvent even when treated as mentioned above, if the solvent has the lower concentration than seven normal. It is evident that the solubility of tungstic acid depends greatly upon and is influenced by the concentration of the hydrochloric acid solu-tion; when the normality becomes below 10, the percentage of solubility decreases a great deal. By taking advantage of the property of the newly formed tungstic acid to dissolve in concentrated hydrochloric acid, by A New 111ethod for The Production {if Timgstic Acid from Tzmgstates. · I 57 adjusting the degree of concentration of the solvent used, and also by em-ploying agitation in order to prevent any sudden decrease in the concentra-tion of the solvent on account of the local chemical reaction occurring between the solute and the solvent, we succeeded in making the solubility of tungstic acid as much as. 12'.1/o tungstic trioxide, while it had been said to be almost insoluble in ordinary mineral acids. Article 2. Prope1ties of Hydrochloric Acid Solution of Tungstic Acid. Olson<?) obtained hydrochloric acid solution containing 4% W03 by adding, drop by drop, a solution of potassium tungstate and a small quantity of potassium carbonate to concentrated hydrochloric acid, while the solution was agitated. In discussing the properties of the solution, he stated that tungstic acid simply dissolved in the solvent. According to Gmelins' inorganic chemistry, tungstic acid dissolves in cone. hydrochloric acid slightly : " by the addition of water to this solution, a creamy sediment is obtained, and the sediment may be a chemical compound of tungstic acid and hydrochloric acid" [Bd III1 p 76r ]. Recently Collenberg and GutheC~l have produced a crystalline precipitate by adding chinoline hydrochloric solution to concentrated hydrochloric acid solution of tungstic acid : after analysing the precipitate, they asserted that iV02Cl2 or anion TV02C1:/ might exist in the solution. It should be noted that· there are two kinds of soluble tungstic acids, metatungstic acid and colloidal tungstic acid. When sulphuric acid is added to metatungstic acid solution and there-by the water is removed, a precipitate appears which disappears again if water is added. But if the hydrochloric acid solution of tungstic acid mentioned previously is used, an entirely different reaction takes place : tungstic acid is immediately precipitated. This shows that the above mentioned hydrochloric acid solution of tungstic acid contains no metatungstic acid. Our observation of this solution with an ultra-microscoi:e also confirms that it is not of colloidal constitution. There remains the problem whether this solution is (3) Collenberg and Giithe, Zs. Anorg. chem. 134, 317 (1924). Y. Nakazawa. and S. Okada. a simple tungstic acid solution, or whether tungstic acid exists in the solu-tion forming another chemical compound with hydrochloric acid. V.'e compare the reaction between hydrochloric acid and chromic acid or molybdic acid etc, which have nearly the same properties as tungstic acid. The following formula gives the chemical reaction between hydrochloric acid and chromic acid :-by adding H 2S04 to the above reaction, the reaction proceeds, Thus chromylchloride is formed, which is a stable compound, being also produced from chromic oxide and hydrochloric acid gas. Ch!orochromic acid is so unstable th1.t it can not exist isolated. It is a well-known fact that Mo03,2HCl is formed from hydrochloric acid gas and molybdenum trioxide. Mo03 + 2/-ICl ~ MoOi•2HCl ............ .................. (3) When hydrochloric acid gas acts on tungsten trioxide at red heat, the analogous reaction takes place : The writers produced a solution conta_ining 8 9~ of tungstic acid by introducing calcium tungstate into cone. hydrochloric acid (r.2). To this solution, the calculated amount of cone. sulphuric acid is added in order to separate from the solution, the calcium chloride, which is precipitated as calcium sulphate : the remaining clear solution is almost pure concentrated hydrochloric acid solution of tungstic acid. Then the same volume of cone. sulphuric acid ( r .84) is gradually added to this solution, and by leaving it overnight undisturbed, small crystalline precipitates of needle shape are obtained in the solution. These transparent needle-shaped crystals (see Fig 2) decompose easily with water and even moisture in air reacts on the crystals, setting free hydrochloric acid gas. They are soluble in concentrated hydrochloric A New JJ1ctltod for Tiu· Production of T1111/:-stic Acid from Tlllll{S!ates. I 59 acid ancl anhydrous alcohol, a nd inso luble in co nce ntrated sulphuric ac id. The colour becomes blue in the su nlig ht , a nd is ve ry unstable against heat; complete decompositi on takes place eve n in hydrochloric acid gas a t 300° C, hyd roc hl ori c acid gas a nd tungste n trioxide being the products of the decompositio n. This property is qt1 ite different fro m that o f WO~C!~ . which begins to sublimate at reel heat. A fter carefully washing off the hyd rochl oric acid adhering to the crystals with concentrated sul phuric acid, we estimated the ratios of the we ig hts of tungst ic o:-;ic\e ( WO~) and hyd roc hl o ric acid in the crysta ls . Fig . 2. ' .. ---:--:o. I No. 2 -No. 3 Table No. 2 JV(.'~ H Cl 1-V('~ : //Ct o.256o 0.0651 l: 1.92 0.491 1 0.1249 I I : 1.99 0.5015 0.1276 I l : '-97 In o rde r to remove s t1 lphuric acid fr om the c rysta ls, they arc washed with anhyclrot1s e the r and kept in a vacuu m for drying. The ratios of // '{}, and //0 hy 11·e ig hts in the samples arc g ive n in Table No 3. Table No. 3 S:i.mple No. I S:i.mple No. Sample No. Calculated value 2 3 WOc, 2/fCI as IVOs 74-9 7% 76.36;10 75.01 % 76.09% HCI 2 3·95 ?b 23.24% 24 .ooJb 23.91% 160 Y. Naka:oawa. aud S. Okada. According to the above results, it seems that the experimental formula of the crystal is ],fl03•2HCt or [ vVO (~i{)2J. Judging from the components of the crystal, the reaction between tungstic acid and concentrated hydrochloric acid solution may be shown as follows :-The white tun5stic acid having more than one molecule of water, which is a product of decomposition of tungstic salts, forms a soluble chemical compound with concentrated hydrochloric acid solution. If yellow tungstic acid having a single molecule of water is formed in the solution, the above mentioned reaction does not take place, but the yellow tungstic acid is precipitated, as it is insoluble in concentrated hydrochloric acid. The white tungstic acid produced right after decomposition has the property of turning immediately into yellow tungstic acid. To insure that the reac-tion in formula (S) will take place, the property of the newly formed tungstic acid of dissolving in concentrated hydrochloric acid immediately after its formation must be utilized. By adding concentrated sulphuric acid, water produced as the result of the reaction (5) is removed, thus stimulating further formation of vV0:,•2HCl. This compound being insoluble in cone. sulphuric acid is thrown out of solution as crystalline precipitates (Fig 2.). A,ticle 3. Separation of Tungstic Acid from Hydrochlo1 ic T ungstic Acid Solution. As tungstic acid exists in the solution in a very unstable condition, as stated in Article 2, it is very simple to separate it out of the solution as a precipitate : the desired result can be obtai'l.ed by simply reducing the concentration of the hydrochloric acid in the solution. There are two methods for reduction of concentration-( I) dilution with water and (2) dilution by distilling off hydrochloric acid from the solution. The comparative results of these two methods are shown 111 Table No. 4. and Fig 3. (A & B). A New 111ethod for The Production of Tung-site Acid from Tung-statts. 161 Concentration of HC! in Solution Amount of separated TVOs 12 N 0% Table No. 4-A Dilution with water. 11.5 N ll N 10.5 N ------3% 27% 73% Table No. 4-B roN 9N 87% 93% Dilution by hydrochloric acid distillation. SN 98% Concentration of 12N 11.5 N 10.8 NI 10.4 N 9·5 N 8.7 /V 7.8 N HCI in Solution Amount of 0% 3~0 16% I 66% 85% 95~6 separated TV03 Fig. 3-A Dilution with water. Relation between hydrochloric acid concentration and amount of 1V03 precipitated. 100 .,--...---.--c V So / I ~· :::: '10 6o Q) E ·5. ·;:; j I Q) .. p.. '-0 "i: 40 5 8 I I < 20 I I ~ V 12 II IO 9 8 Normality of HCI 98,% 7 7N 100% 7N JOO% 162 Y. Naka:::awa. and S. Okada. Fig. 3--B Dilution by hydrochloric acid distillation. Relation between hydrochloric acid concentration and amount of WO3 precipitated. IO:> ~ ~ -- -,,,, 80 /' /~ I I , 20 J J ... -./ 12 I I IO 9 8 Nonmlity of HCl 7 Dy adding water to the solution, hydrochloric acid is diluted, causing separation of tungstic acid from the solution: the relation between the degree of concentration of the hydrochloric acid and the amount of the tungstic acid precipitated as trioxide (/'f/O3) is shown in Table No. 4-A. If the same solution is heated, distillation of hydrochloric acid takes place causing decrease of its concentration, making tungstic acid precipitate. The results are presented in Table No. 4-B. In Table No. 4. and Fig. 3. A & B, the quantity of tungstic acid p:·ecipitated due to reduction of hydrochloric acid concentration is presented in percentage, taking the amount of tungsten trioxide in I 2 normal hydro-chloric acid solution as roo '/o. Precipitation is completed when normality of solution is reduced to 7. It is to be noted that at ro.5 normal the A New Method/or The Production of Tungstic Acidfrom Tungs:ates. 163 amount of the precipitate starts to increase rapidly : the characteristic of the curve resembles that of the solubility curve of tungstic acid in Fig I. This characteristic shows also that the solution is not a simple hydrochloric acid solution of tungstic acid. The size of the individual particles of the precipitate produced by hydrochloric acid distillation is larger than that obtained by dilution with water and it may be washed more easily. It is almost pure, occluding ·no injurious impurity. CONCLUSION. I. Tungstic acid, separated from its salts, is soluble in concentrated hydrochloric acid. The percentage of solubility may be increased as high as I 2 'Yo ( calculated as tungstic trioxide), provided that the freshly separat-ed tungstic acid comes in contact with cone. hydrochloric acid immediately after its separation : solubility depends greatly upon the degree of concentration of the solvent, a sudden increase in solubility taking place at IO. 5 normal. 2. Forming a chemical compound such as TVO3•2HCl, tungstic acid dissolves in cone. hydrochloric acid solution. So, the former is again easily separated from the solvent by merely reducing the concentration of the latter. 

A New Method for the Production of Tungstic Acid from Tungstates

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A New Method for the Production of Tungstic Acid from Tungstates

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