A STUDY OF THE PRI-PARATION OP TELLURIC ACID A: r US APPLICATION IN ANALYTICAL CHE by . !£N HORNER B« S«, Franklin and Marshall College, 1949 A THESIS submitted in partial fulfillment of the requirements for the degree MAST Department of Chemistry KANSAS STA GB OF AGRICULTU L AND APPLIED SC 1951 &0<LUl~ ii ^1*8 T4 \<V$I Hlol TABLE OP CONTENTS I Oo i INTRODUCTION ft* 1 EXPERIMENTAL 5 Volumetric Determination of Tellurium Dioxide 5 Purification of Tellurium Dioxide . 13 Determination of the Solubility of Telluric Acid and Tellurixim Dioxide in Concentrated Ammonium Hydroxide ..... 13 Preparation of Telluric Acid ... 14 Determination of the Purity of Telluric Acid 20 The Determination of Barium by Homogeneous Precipitation as Barium Tollurate . 21 DISCUSSION 25 SUMMARY 27 ACK 29 LITERATURE CITED 30 INTRODUCTION The studies reported upon In this paper were directed towards (1) The preparation of telluric acid, and (2) the homogeneous precipitation of barium and similar Ions as tellurates. A review of the literature shows that there have been several methods of preparing telluric acid. Meyer and Franke (4) oxidized the elementary tellurium with a mixture of barium chlorate and sulfuric acid. Krepelka and Kubik (2) oxidized the elementary tellurium with 30 percent hydrogen peroxide, but their method necessitated using a 10-15 fold excess of the oxidizing agent. Staundenmaier (9) reacted a mixture of nitric and chromic acids with tellurium dioxide to effect oxidation to telluric acid. The procedure as developed by Mathers et al. (3) is the most widely used. Crude tellurium dioxide is purified and then oxidized by a slight excess of permanganate in a hot solution of about five molar nitric acid. Manganese dioxide is dissolved by the addition of 3 percent hy rogen peroxide. Upon purification the yield is found to be 75-76 percent. A controversy continues to prevail concerning the molecular structure of telluric acid. In the literature it is referred to as both the hydrate form, H Te0 2Hg0, and g 4 the hexabasic form, HgToOg. Pauling (5) has recognized the latter as being the correct formula. According to that i rvestigator, X-ray data have shown definitely that the six oxygen atoms in the molecule are equivalently related to the tellurium atom. Rep- resentative salts are AggTeOg, HggTeOg, AggHgTeOg, KgH Te0g.3Hg0. 4 Many complex crystals such as (NH ).(HP0 ) Te(0H)g and 2KI0 4 4 2 3 Te(OH)g contain telluric acid. Tellurates combine with MoO* or to form 3 compounds such as KgTe(Mo04 )g* The method as developed during this investigation involved the oxidation of the tellurium dioxide in a basic solution of ammonium hydroxide through the use of 30 percent hydrogen per- oxide as the oxidizing a,.;ont. The commercial grade tellurium dioxide was purified as suggested by Mathers et al» (3). The four possible theoretical equations for the oxidation reaction are as follows: (1) Te02+H2 2+NH40H * (NH^gTeC^.XHgO or (2) Te0 +Hg0 +NH 0H . ** 2 2 4 N^HTeO^.XHgO or (3) Te0 +H 0g+NH 0H NH HgTeOg.XHgO g 2 4 or (4) Te0 +HgO +HH OH )gH Te0g.XHg0 2 g 4 > (M4 4 Reaction of the ammoniun tellurate with dilute nitric acid converts the compound to telluric acid. Before starting the preparation it was deemed necessary to ascertain the approximate solubility of the tellurium dioxide and the telluric acid in concentrated ammonium hydroxide. These solubilities were determined only qualitatively. As a result of the preceding study it was decided to attempt the oxidation of tellurium dioxide to telluric acid as postulated. Prom qualitative solubility studies it was reasoned that the tellurium dioxide would dissolve sufficient- ly to be attacked by the hydrogen peroxide. After oxidation occurred the ammonium salt of the acid would precipitate and more of the tellurium dioxide could enter into solution, there- by permitting the reaction to go to completion. The purity of the product was determined by the method of Rosenheim and Y.einheber (7). Since it was desirable to know the purity of the tellurium dioxide used as the starting material, it was analyzed by the procedure developed by Brauner (1). This process involved the titration of an acid solution of the tellurium dioxide with standardized potassium permanganate and the back-titration with oxalic acid. After several analyses it became evident that reproducible results could not be obtained by this method. Two other methods for volumetric analysis of tellurium were pre- sented in the literature. Schrenk and Brov/ning (8) utilized potassium dichromate to titrate tellurium electrometrically in the presence of ferric ion, selenium, and copper. V.illard and Young (12) employed eerie sulfate in the presence of chromic ion as a catalyst and obtained very good results. Since manganous ion should be of IV VI capable inducing the oxidation of Te to Te , it was decided to use this ion as a catalyst in the eerie sul- fate titration. It was found to perform excellently for inducing the oxidation if it were used in a certain minimum concentra- tion. During the development of this modification, Watson (10) published a note on the use of manganous sulfate as a catalyst in the standardization of eerie sulfate against sodium oxalate. The second phase of the problem involved the precipitation of barium from a basic solution produced by the decomposition of urea at elevated temperatures. .illard and Tang (11) and "illard and Fog^ (13) have made the principal contributions to the 3tudy of homogeneous precipitation. The theory suggested by these authors is based upon the following. Urea decomposes into carbon dioxide and ammonia* when its solution is heated. Since urea is very soluble in water, and its rate of decompo- sition into ammonia and carbon dioxide, which is relatively low i is dependent upon the temperature and duration of heat- >od control of tue pH of the solution is possible, ox- pocially if it is buffered. The increase in pH can be stopped at any time by simply cooling, since no decomposition of urea takes place in the cold. The pH of the solution cannot rise very high and the more basic elements will not be precipitated, since the ammonia produced is driven from the hot solution if the pH is much above seven. The slow evolution of carbon di- oxide, produced by the decomposition of I ea, effectively prevents bumping. In such a solution the change in pH will be uniform throughout, insofar as the temperature is the same, and no local excess of the reagent will be present at any point. No undesirable foreign ion will be introduced, except in some cases when a buffer is employed, and the excess of the precip* itant can be decomposed easily. : E} A.L Volumetric Determination of Tellurium Dioxide The determination of the purity of tellurium dioxide was first attempted by the method of Brauner (1). Tim tellurium dioxide in 2N sulfuric acid solution was titrated v/ith excess potassium permanganate and the excoss permanganate was back- titrated with a standard solution of oxalic acid. Careful ad- herence to the method as described by Brauner gave results shown in Table 1. Table 1. The analysis of commercial grade tellurium dioxide utilizing the method of Brauner. Sample : Weight of s ample : eight of TeOp : Percent taken ( g) : found (g : Te0 1) 2 1 0.2753 0.1739 63.17 2 0.3617 0.2080 .54 3 0.3800 0.2775 73.02 4 0.3331 0.2979 89.45 5 0.3784 0.3271 86.41 6 0.2018 0.1820 90.17 The data recorded in the preceding table did not provide satisfactorily the desired information. It was decided to : 6 conduct further investigations to learn the reason for the wide range of results. The following series of experiments were conducted with tellurium dioxide that was purified by tho method of fathers et al. (3). Since the oxidation of the tellurium dioxide by the potas- sium permanganate is slow it is necessary to permit the solu- tion to stand for some time before conducting the back-titration. The data for a series of determinations when the amount of per- manganate added was held constant, and the time of standing before back-titratin,: wa3 varied, are recorded in Table 2. Table 2. The analysis of purified tellurium dioxide utilizing the method of Brauner. • 1 • : Per- Sample Lght of sample : Time 3tand-•: Weight of TeO,2 : c ant L taken (g) ing (min. ) : found (g) ":Te02 1 0.2020 10 0.1895 89.63 2 0.2101 10 0.1390 89.96 3 0.2100 10 0.1881 89.58 4 0.2149 10 0.1373 87.16 5 0.2020 15 0.1359 92.00 6 0.1950 15 0.1856 95.23 7 0.1970 15 0.1818 92.28 8 0.2068 30 0.2132 103.09 9 0.1986 30 0.2047 103.07 After addition of excess potassium permanganate and before back-titration with oxalic acid. A preliminary survey of the data obtained from samples 1-7 would indicate that fairly reproducible results could be obtained if the solution were permitted to stand for a prescribed length of time before attempting back-titration. However, the results shown in 8-9 reveal clearly that the potassium permanganate is reduced by something in addition to the tellurium dioxide. -en the first few drops of permanganage are added to the solution containing the tellurium dioxide, a brown coloration appears and as the process is continued a brown precipitate is formed* This precipitate is either manganese dioxide or a mix- ture of manganese dioxide and manganic sulfate.