A Study of Electrometric Methods for Determining Selenium and Tellurium

Home , Potassium dichromate, Telluric acid, Tellurous acid

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Masters Theses Student Theses and Dissertations

1926

A study of electrometric methods for determining selenium and tellurium

Bertie Lee Browning

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This thesis is brought to you by Scholars' Mine, a service of the Missouri S&T Library and Learning Resources. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact [email protected]. A STUDY OF ELECTROMETRIC METHODS FOR DETEmlINING SELENIUM AND TELLURIUM.

B. L. Browning

TJIESIS submitted to the faculty of the SCHOOL OF MINES AND METALLURGY OF THE UNIVERSITY OF MISSouRI in partial fulfillment of the work required for the

Degree of

MASTER OF SCIENCE IN CH~~ICAL ENGINEERING

Rolla,![o.

1926

Approved by 7.~ t . Associate ~rofessor of Chemistry The2ia~ A study of ~lect~o~8tric methods f~Jr detel.'mir... ing sel'3:::::_~l>:'e t ..lid teJl~)J:·iu!L. :2.rowniE~~. J.926. The anthor- wish.es: t· expres.s: apprecia.tion t,o Dr.· W. T. ScllrenIc fo~' the su.ggestion of tlli.s: research.,. and far TC;1~l.uab:le advice- and sugge·stions gi.ven whi~e the work was in progress. Table of Contents

Part 1. The electrometric determination of tellurium in the presence of ferric iron, selenium and copper. Introduction••••.••••••••.••••••••••••••••••page 1.

Tables and disoussion.•••.•••••••••••.••••••page 3. IJlethod and materials•••••...... •...••...••..page 7.

Summary••••••••••••••••••••••••••••••••••••• page B.

J:Jart 2. The electrometric determination of selenium and

tellurium by means of potassium permanganate. Introduction•••••••.••••••••••.•••••.•.•••••page 1. Methods of analysis•••••••••••••••••••••••••page 4. Titration ourve .••••••.•••••••••••••••••.•••page 5.

Tables and discussion.~•••••.•••.•••.•••••••page 8. Summary •••••••••••••••••••••••••••••••••••••page 24.

Bibliography•••••••••••••••••.••••••••••••••page 25. Ch~nlic.l fReprint from the Journal of the American Society, 4a t 1:.39 (1926).]

The Electrometric Determination of Tellurium in the Presence of Ferric Iron, Selenium and Copper

By 'w. T. Schrenk and B. L. Browning [Reprint from the Journal of the American Chemical Society, 48, 139 (1926).J

[CONTRIBUTION FROM THE CHEMICAL LABORATORY, MISSOURI SCHOOL OF MINES AND MSTALLURGY] THE ELECTROMETRIC DETERMINATION OF TELLURIUM IN THE PRESENCE OF FERRIC IRON, SELENIUM AND COPPER

By W. T. SCHR.ENK AND B. L. BROWNING

REcsrVllD SSPTEMBER 8, 1925 PUB~ISHED JANUARY 8, 1926 The quantitative oxidation of tellurous acid to telluric acid by excess of potassium dichromate has been shown by Lenher and Wakefield! to be applicable to the volumetric determination of tellurium. 'The purpose of this investigation was to study the applicability of the electrometric method to this titration. In converting Lenher and Wakefield's method, with slight modifications, to an electrometric titra-tion, to avoid the use of a spot plate. indicator, their results as a whole have been verified. The method is based on the following reaction: 3Te02 + K 2Cr207 + 4H2S04 ~ 3H2Te04 + ~S04 + Cr2(S04)3 + H 20, the excess of dichromate being titrated against ferrous sulfate. It was found that selenium dioxide is not affected by potassium dichromate under similar conditions. Method and Materials.-Samples of pure tellurium-dioxide or aliquot portions of a stock solution of the same material were used. The tellurium dioxide was dissolved in a very small amount of sodium hydroxide solu tion, and sufficient dilute acid added to bring the final acidity to the

TABLE I TITRATION OF TELLURIUM DIOXIDS

10-20 ce. of H2S04 (d. 1.84) CuS04.5H20 SeOt TeOt No. of Error of Av. deviation taken. g. taken, g. taken, g. analyses average, mg. from avo mg. Part 1 0 0 0.2527 5 -0.14 0.18 0 0.1390 .2527 5 - .06 .10 Part 2 0 .0556 .1015- .4023 7 .06 .20 Part 3 0 0 .2520 3 + .13 .13 0.062-0.500 0 .2520 4 .10 .10 .125 .0556 .2068 2 .15 .15

1 Lenher and Wakefield, THIs JOURNAL, 45, 1423 (1923). 140 w. T. SCHRENK AND B. L. BROWNING Vol. 48 proper value. Water was added so that in each case the volume was 200 cc. A small excess of 0.1 N potassium dichromate solution was added and the solution allowed to stand for 45-60 minutes. The excess of dichromate was titrated against a standard solution of ferrous iron (ferrous ammonium sulfate), using an Eppley titration apparatus. A ratio between the potassium dichromate and ferrous irQn solutions was determined at the close of each day's work. The dichromate solution was standardized against specially purified tellurium dioxide. Part 1 of the table shows that tellurium dioxide is quantitatively oxi dized to, the telluric state by potassium dichromate, and that selenium dioxide has no effect upon the determination. Selenium. dioxide in quan tities up to .2780 g. does not affect the titration of potassium dichromate by .ferrous sulfate. Part 2 indicates the accuracy of the method for various amounts of tellurium dioxide in the presence of selenium dioxide. Part 3 indicates that tellurium dioxide may be accurately determined in the presence of both selenium and copper. Summary 1. Tellurium in large or small amounts may be rapidly and accurately determined by oxidizing with an excess of potassium dichromate and titrating the latter electrometrically with ferrous sulfate. 2. Small amounts of selenium and copper do not interfere with the determination. RO~~AJ MISSOURI • 1'l-Ij£ :3L2~CTI~OIviETRIC DETERI\'~INATION O:B' TELLURImr Il~ THE PRESENCE 0]' FERRIC IRON, SELENIUM, AND COPPER.

Introduction. The quantitative oxidation of tellurious aoid to telluric acid by potassium dichromate has been shown by Lanher and Wakefield* to be applicable to the volumetric determination of tellurium. The purpose of this investigation was to study the applicability of the electrometric method to this titration. Hildebrana** and many others have discussed in recent literature the applicability of oxidation potentials to the electrometric method of titration. G. S. Forbes and E. P. Bartlett*** demonstrated the possibility of using the electrometric method for ti trating potassium dichromate with ferrous sulfate. Eppley and Vosburgh**** showed that this titration could be carried on successfully in hydrochloric and sulfuric aci,d solutions. and that, the concentrations of these acids could vary within wide limits. * J. Am. Chern. Soo •• 45, ~423 (1923) ** J. Am. Chern. Soc •• 35,' 847 (1913) *** J. Am. Chem. Soc •• 35, 1527 f 1913) **** J. Am • Cham • Soo •• ' 44, 2148 (.1922) In converting Lenhar and 'Wakefield's method, with slight modifications, to an electrometric titra tion, to avoid the use of a spot plate indicator, their results as a whole have been verified. In addi tion, it was discovered that selenium dioxide or se lenious acid is not oxidized in sulfuric acid solution by potassium dichromate. rhe method is based on the following reaction: 3Te02+K2Cr207+4H2S04~3H2Te04+K2S04+CrZ{S04)3+H20 Selenium dioxide is not affected by potassium dichromate under similar conditions. page 3

T'ab-le I Titration of Te02

1.7 ae H2:S04 sp. gr. 1.84. 200 cc volume Te02 taken 8e02 taken K2Crz0 7 used TeOa found Error

gm gm 00 gm gil .2527 0 31.42 .2526 -.0001 .2527 0 31.40 .. 2,525 -.0002 .25'27 0 31.44 .25'28 +.0001 .2527 0 31.• 40 .2525 --.0002 ~2,52.7 0 31.~8 .2524 -.0003 .2.52,7 .1390 31.45 .2529 +.0002 .252.7 .1390 31.45 .2529 +.0002 .25'2.7 .1390 31.43 .25'27 .0000 .2,52,7 .1390 31.43 .252,7 .0000 .2,52,7 .139·0 31.42 .2526 --.0001

Tab1e I shows that tellurium dioxide. is quan~ titativelY oxidized to the telluric state by potassium dichromate in sulfurie acid solution and that the

presence of quantities of selenium dioxide up to .1Z~O gm has no effeot upon the determination. page 4

Table II Varying amounts of 8802

10 cc R2S04 sp. gr. 1.84~ 200 co volume.

8e02 taken, FeS04 required gm oc ce

.0 20.00; 24.38 .0278 20.00 24.~O .0555 20.00 24.37 .0836 20.00 24.39 .111,2 20.00' 24.32 .13,90 20.00' 24.44 .27'80 20.00 24.34

Table II shows that selenium dioxide is not

oxidized b~ potassium diahromate in sulfurio said of the above coneentration. The amount of selenium diox~ ide pres,ant; up to .2780 gm. has no effeot upon the tl~ tration. Table III

Varying amounts of Ta02

15 cc H2S04 sp. gr. 1.84, 200 co volume

1}.1 eO 2 taken SeOZ taken 'TeOZ found. Error gm gm gm gm

.1015 .0556 .1013 -.0002 .1521 .0556 .1521 .0000 .2018 .0556 .2020 +.0002 .2517 .0556 .2517 .0000 .3026 .0556 .3026 .0000 .3531 .0556 .3530 -.0001 .4023 .0556 .4013 -.0010

Very pure 1'602 was prepared. Varying amounts were weighed out and determined in the presence of 8e02- The results tabulated in Table III give evidence of the accuracy of the method for varying amounts of Te02 in the presence of small amounts of Se02. {a} 20 ec: H~04 sp. gr'. 1 ..84" 200 ee vol'llme

TeO 2 t:a.ken TeO' 2', f Ol~U1d Error

gm gm gm

.2.520 o .2:.52l +.QOO'1 .2520 o .2523· +.0003 .2520. o ..2.,52-0 .0000 .2520, .062:, .. 2'52:'0 .0000 .2520 .125' .2520 .0000 .2.520 .250 .Z5·~9 -.0.001 .2,520 .500 .2,517 -.00:03'

{be} 1.5~ ee: RZS04 sp. U. 1 ..84~, 200 cc voltune

5H2~ '~Q;r' Te.02' taken SeOZ taken CUS04.• taken Te02 -round grrl gm. gm

...2.047 .0555 .·125," .:a044~ ~.O-OQ3 .2089 ..0550" ..125 .2089 .OO·OQ·

.01.58 gm to .1270 gIn' llave n.o e'f'fec·t upon. the determina-

tiO'Il Q'£ tell.tlrium' by t~.is metho-d. Table IV (b') indic:a.tes

tha.t telluri.um Ina¥ b·e determined ver~r aecu.ra.t.e:~y in tile

pres:e·nce Qf' small amount.s o·f bet!! s:e'lenium and copper.. page 7

~~THOD AND MATERIALS:-

Samples of pure ~e02 or aliquot portions of a stock solution of the same material were .used.

The ~e02 was dissolved in a very small amount of sodium hyd.roxide solution, and sufficient dilute acid added to bring the final acidity to the proper value. Water was added so that in each case the volume was 200 cc. A sma.ll excess of NllO .K2Cr207 solution was added and the solution allowed to stana 45 minutes to one hour. The excess dichromate was titrated with a standard solution of ferrous iron,

(ferrous BJPJIlonium sUlfate) t using an Eppley titrat10n appara.tus. A ratio between the potassium dichromate and ferrous iron solutions was determined at the close of each day'S work. The dichromate solution was stan dardized on specially purified Te02• page 8

SUMMARY:- 1. Tellurium may be determined rapidly and aocurately electrometrically. 2. The method is accurate for both large and small amounts of tellurium. 3. Small amounts of selenium and copper do not interfere with the determination. THE ELECTROThtETRIC DETER]:!INATION OF SELE~rIUM

AND TELLURIU~~ BY IVIEMTS OF POTASSIUl[ PER~WTGL~.A.TE.

INTRODUCTION

Previous investigators have discovered that selenious and tellurious acids may be quantitatively oxi dized by potassirum permanganate in aoid solution. Brauner* reported that in the reaotion with tellurious aoid permanganate is not rednaed to its lowest state of valenoe. but to higher hydrated oxidase He also showed that the reaotion is not quantitative in fairly aonoen trated sulfurio acid solation, due to loss of oxygen, Gooah and Danner ** reduced the error to a negligible figure by carefully limiting the oonoentration of Bnl- furio aoid. Goooh and alemons*** suocessfully determ1n~ ed selenious acid by oxidation with potassium permanganate. The present investigation was to study the electrometrio determination of both selenium and tellnr- ium by the use of potassium permanganate. The chemioal

* Qhem.Soc. 59 .238 (1891) **jm. lel. 44, 301 (189a) ***tm. Sci. 50,51,.(1895) the manganese dioxide whioh precipitated was reduaed very slowly by the ferrous sulfate. In many oases it was doubtful whether the manganese dioxide was all reduoed at the apparent endpoint, since a small amount remained as a stain on the glass. The results obtained in the first determinations varied greatly, and the odor of chlorine was observed during the titration. Since no hydrochloric aoid or ohlorides were added. the question ar'ose as to the souroe of the ahlorine. Systematic examination of the chemicals used revealed that the sodium hydroxide which was used to dissolve the tellurium dioxide oontained a considerable amount of ohlorides. On adopting the procedure of dis solving' the tellurium dioxide directly in sulfuric acid, the presenae of chlorides was avoided and no more d1~fi cnlty was experienoed from this souroe. Manganese sul. fate and phosphoria acid in varying amounts, both sepa rately and together, were added in an attempt to prevent the aetion of the permanganate on the chlorides present. This prooedure was unsuooessful, chlorine being liberated as before. ~he addition of 41sodlum phosphate, and of suffiolen~ 804ium sulfate to oonvert all of the sulfuric aoid present to sodium acid su1fate. faile4 to prevent th liberation of ohlorine. The conolusion was, therefore, that the titration cO'll1d be carried out suceessfully only in the absence of ahlorides. It was found that the addition of phosphoric aald or disodium phosphate prevents the precipitation of manganese dioxide·. »isodium phosphate was used since ita addition does not inorease the acid concentration. On adding potassium permanganate to a selenium or tell~ urium solution oontaining disodium phosphate, the sol~ nt10n beoomes deep brown in oolor until an exoess of per manganate 1s present when it beoomes purple oolored. The data obtained shows oonclusively that the method is very accurate for the determination of either selenium or tellurium. If both are present in the same sample, they may be accurately determined by a combination of the permanganate and dichromate methods. l.m'TH ODS OF ANALYS IS -

The fo11owing prooedure for the determination of either selenium or tel1urinm was used and is recommend ed. A sample containing from .14 to .20 sa. of the'Gx1de

1s weighed and dissolved in 25 00 of 4~ sulfurio aoid.

In the case of tel:Lur1UJD dl xii. t the sample must be .arm • ~tl1 solatiQD 1s complete. The solution is iil-tel 'reactions involved are as follows:

KgMn208~5Se02+3R2S04+2H20~ K2S0,+2MnS04+5E2Se04 KZMn208+5Te02+3H2S04+2R20~ K2S04+2MnS04+5K2!e04

The reactions are oomplete in a short time, and manganese dioxide preoipitates. The precipitation of manganese dioxide necessitates the addition of an excess of potassium permanganata, the excess being destroyed by a reducing agent. Gooch and @lemons added an exoess of standard oxalic aaid solution and then titrated the 8X- cess with standard permanganate solution.

In the electrometric method, the exoess of po~ tassium permanganate was titrated with ferrous salfate solution. The ohange in potential is moat marked when passing from oxidation to reduction potential. A titration Ollrvs between po,tass1um permang'snate and ferrous sulfate was determined. The setting of the potentio~ meter at the steepest port~on of the ourve indicates the greatest ohange of potential with the addition of ferrous sulfate, and was taken as the endpoint of the titration in subsequent determinations. !he investigation showed that two difficultie8 must be overoom.e. !he permanganate reaated with hydrochloric aoid or ohlorides present during titration, and

--2- to 150 CC 9 and 12 gm of disodium phosphate added. A standara solution of potassium permanganate is added to about 10 co in ex.cess. and the solution allowed to sta.nd 10-30 minutes. The excess of permanganate is then titrated eleotrometrically with ferrous sulfate solution. If selenium and tellurium are to be determined in the same sample, two methods are available.

(1). The tellurium m~v be determined by oxidizing with an excess of potassium dichromate, the excess of diohromate being titrated eleatrometrioally with ferrous

Bulphate by the method of Sohrenk and Browning*. An excess of potassium permanganate is then added, and the selenium determined. (2). The selenium and tellurium may be determined

together by oxidation with potassium permanganate. ~he

tellurium is then determined on a separate portion by the use of potassium dichromate, and the selenium found by differenoe.

*Am. Ohem. So. 48, 139 (1926).

Figure I. This curve shows the change in potential obtained on titrating potassium pennanganate with ferrous sulfate in sulfuric acid solution.

-,- TABLE" I

Titration ourve o:f FeS0 a.nd KMn0 4 4

00 FeS04 Potentiometer reading

o 148.4 5 148.3 10 148.3 15 148.3 20 148.3 25 148.0 30 148.1 35 148.5 40 148.0 41 147.4 42 146.8 43 146.2 43.2 1~6.8 43.3 11'7.6 43.4 112.4 44 108.0 45 102.4 46 99.0 48 9'7.8

Table I shows the titra.tion of 30 co of perman.. ganate solution by ferrous Bulfate solution. ~ABLE II

Gravimetrio analysis of 8902 and fe0 2

8e0 taken 5e found SeO found SeQ found 2 gm gm gm 2(oalo) ~ 2

1. .2rt75 .1959 .2750 99.10 2. .2'175 .1959 .2150· 99.10 3 ••2'175 .1961 .2753 99.21 average purity 99.14%

TeO 2 TeO taken Te found TeO found ieo2 found gm 2 gm gm 2( oalc} 'I••2490 .1993 .2493 100.12 2. .2473 .1973 .2468 99.'19 3 ••2502 .1996 .249'1 99.80 4 ••2489 .1982 .24'78 99.66

average puri ty 99.8~

!he S8Q and Te0 used were analysed gravimet 2 2 rioally by the method of Lenherand Kao* with the

~esult. shown in Table II.

* • Chem. oa. 47. ?69, (1925) Table III. Time reaction of Se02 and Te02 J?a.rt (a)-

Time Se02 taken KMn04 used Se02 found erl"or min .. gm cc gm gm l. 5 .1372 23 .. 6~ .J.368 --.0004 2. 10 .1372 23.60 .1:367 -.0005 3. 15 .~~72 23.64 .13'70 -.0002 4. 20 .~372 23.65 .].370 -.0002 5. 25 .13?2 23.66 .1371 -.0001

Part (b)-

Time Te02 tal'Cen KMn04 used Te,02 found av. dey. lnila. cc cc gm from avg.

1.. 5 75 2.8.oa .2329 -.0001 2. 10 75 28.0'1 .2328 -.0002. 3. 15 75 . 28.09 .2330· .OOOC 4.. 20 75 28.10 .2331 +.0001 5.. 25 7'5 28.~2 .2332 +.0002

Tab1e III shows that the oxidati.on is cOIrij?~ete within five minutes. Table IV

Influence of Na~04 1~a·2HP04 Se02 precip- KMn°4 Se02 error taken itate used found gm gIn co gin. gIn l. 1 .~400 yes 25.52 .~~97 --.0003 2. 2· .~4.00 yes· 25.56 .~3·99 -.0001 3. 4. .1.400· yes 25.6~ .1402 ~.OOO2 4. 8 .1400 none 25.64. .1404 +.0004 5. ~2 .].400 nOlle 25.66 .1405 +-.0005

The inf~uence of the amount of disodium phos-· phate present is shown in table IV. In genera~~ it may be s.tated that 12 gm. of l~a2HP04 in 200 ce of solution will

prevent the precipitation of manganes.e dioxide formed by

the reduction of 25 ee, of N/l.O permanganate for at period of one hour. The amount necessary to prevent precipi-

tatiQn increases viith the time of standing and with the amount of permanganate reduced•

....1.1.. Tab~e V

L'1.f~uenc.e of K2SO'4 SeQ 2 H2S0 KMnO',4 SeQ2 error taken a.dde~ used :found gm cc cc gm gIn

~. .l4.Ql 1 25.60. .~40~ .0000 2. .1.401 3 25.6~ .1402 ~.OOO~ 3'. .1.401 8. 25.64. .1404 +.0003 4.• .~401 ~4 25.67 .1405 '+.0004 5. .J..401 20 25.74. ..1409 +.0008

The ~imits within which the co'ncentration of the

sulfuric acid may vary without impairing the accuracy of the determination are indicated in tab1e V: Thia

sho~vs that the titration may be successful~y carried out in, concentrations of sulfuric acid of from .9% to

1.3% by we·ight (.5% to· 7% by vo~ume). Tab:le· VI

Ratio Na~04 of and H2SO 4 SeQ2 H SO Na2HPO'4 2 4 KYn

Table VI shows tha,t the ratio of disodium phosphate to sulfuric acid was found to be without effect. within the limits of acid concentration described above. Table VII Influence of excess of YJm04 Se02 ' KMnO& EMn04 seOZ error taken adcte used gm co co gm gIn

1. .1371 Z4.94 25.20 .1367 - •. 0004 2 • •1371 34.94 25.22 .1368 - .0003 3. .1371 34.94 25.18 .1366 - .0005 4. .1371 34.94 25.18 .1366 -.0005 5. .1371 39.93 25.25 .1369 -.0002 6. .1371 40.93 25.19 .1366· -.0005 7. ~1371 39.93 25.20 .1367 -.0004 8. .1371 39.93 25.22 .1368 -.OOO~ 9. .1371 49.91 25.27 .1370 -.0001 10. .1371 49.91 25.29 .1371 .0000 11. .1371 49.91 25.27' .1370 -.0001 12. .1371 49.91 25.32 .1373 +.0002

Table VII shows the influence of the amount of excess of potassium per~Bnganate added. The amount of excess present can vary from 40 to lOO~ of tha a rrtount used. without influencing the results of the titration.

-14- Titra:tion. of SeOa.

S'e()2 taken. IQ1n04 uaecl Se02 found Err~r' gnl ce gm gm Par"t fa}

1.. .J~3?6 25~~ ..26: ..1.377 +..,0001 2. ..l~76·· 2,5 ..2,5 ...1-3.';6;. ...0000 S. ..1376' 2.5 ...2-5 ..1.376 .0'000.' 4.. ...1.376 25 ..25 .IZ~rz6' .000.. 0· Far··t, (b)

5. 0-1375 2.5.34 .l~?4 -.ooo·!- 6.. .1.3'75' 25.32.. ...13,73 - .. 0.00'2 '1.. ..·~374 25 ...3,~ ... 1.~7~ ~.QQO'2, 8., ..I375 25 ..3l ~1.::;~:3~ -.0001 S. .lS75 25~.~4. ..1.3'74.

F'art {el

10••1375' Z5'.32, ~1.374 ~.ooo·~ 11.••.1.3'75 2~5.2,9 ..1372 .....0.00.;;: 12••13"75, 2,5.2'1 ..l3?1. .... O·004~

Tab.le VIrr re~o:r·d.s:, a. t J"1'ical. dat.a.. sheet. Qf" a seriea o·f' d..e"terrr.Linatio.na b~"- tIli.S: me',tl1o,d. and. in.dicat.••

..15- Titr&i.,i.an of TeOZ Error TeOz. taken Kllh04 us'ed Te.OZ :round.. gm cc gm gm

1.. ~:L9'al., 26.(1(1: .2QSS ~..OQ'52 2-.. ..197rz: 25'.9"Z .2028 "'-.005,1 3. .1983 2.,5: ..9:'1. .2026' ....{),04$ 4. •.1.9·7.4 25~65 ..2000: -f-.OOS2. 5. ..~9!'l~ 2.5..SO· .1994 T.OOZl 6. '..19:75 25~ ..6Z .2004 +.002.9 ? .1.986 25.6,8 .2008 +...002.2, 8. .. I9~'13 25 .. 6~ .2003 +.oo~o 9 .. ..1.99:7 2.5 ..'1'6:, .2.0I~4, -r:.0017 10• • 1.984. 2:5" ..54 ~1997 +..aC.13 1, • • 2.0,02 2,5.?9 ~2~O,17' +.OO;~5· 12.... .1.9.90 2.5 ..6-3 .200,4: +..0014 13. ..19,82, 2.5.54 ..2Q06: +.0024 14-:. .1-98& 25 ..6"6: ..2JQQ'l +.002:5

Tah1e lX records a. series' of' deternlinat,io·ns of

~e()~ in which chlorides: I'er·e known t,o b,e present and the

Q:dar o-f escap'i.n.g' free ef'.ll.orine was no,ticed. The 'Varianee

in re-sults obta.ined. YlaJ3 therefore t.o be EaI1ec"tec.. Table X

Te0 Ti.trati.o.n of 2

TeO:2 taken KMn° 4 used Te02 fecund erl"or gm C.C gm gm Pa,rt ('a}

~. ..19.87 25.60 .l992 +.0005 2. .1988 25 .. 62 .1.994 +.0006 3. .1987 25.59 .1992 +.000·5 4. .1990 25.62 .199~. +-.0004.. 5~. .~990 25.64 .~996 +-.0006' 6. .1987 25.57 .1.990 +.OOO~ '1. .1.993 2.5.59 .i992 .... OOO~ 8. .1989 25.53 .1987 ~.OOO2 9. .1983 25.42 .1978 ... 0005 10. .1998 25.69 .~999 +.OOO~ 11. .1989 25.58 ..199~ +-.0002 12. .1989 25.55 .1989 .0000 1:5. .1984 25.51. .1986 +.,0002 1.4. .1984 25.50 .1985 .....0001

PaJ!t (b )

1.5. .1984. 25.36 .1983 .... OOO~ 16:. .1984 25.36 .1983 -.0001, 1'7. .1984- 25.S5 .1.982 -.0002 18. .1984 25.35 .1982 --.0002

The resulta· l.isted in table X sho,w the accuracy of tlle method for tel~urium in titrati,o.ns in which. chlorides are known to be absent. Tab1e XI

Titratio·n of varying amounts of TeO'2

Te02 talc.en KMn° 4 used Te02 found error gm cc gin gm

1. .0052 .69 .0054 +-.0002 2. .. O~96 2.56 .O~99 +.0003 3. .050'4 6.58 .0512 +.0008 4. .1006: ~2.95 .1007 +.OOO~ 5. .2,000 25.76 • 2004 +'.0004. 6. .2997 38.52 .2997 .0000 7. .3994. 51,.3~. .3994 .0000

Tab~.e XI shows the· ac·c.uracy of the method :Cor varying amounts of Te.02t. from .0050 gm to .4000 gm.. Tabl.e X.II

Titration D·f SeO'2 and TeO'2 Part (a )

Te.02 Se02 KMnO'4 Te.02 error taken taken used found g.m gm cc gm gm

1. .O~OO .1.375 26.70 .O~O5 +.0005 2. .0696 .~375 34.38 .0702 +.0006 3. .1.501. .1375 44.55 .~494. -.000'1 4. .2.002 .~.S75 51.02 .~997 -.0005 5. .2509 .~:575 57.04. .2l1.9 -.0300 Part (b: )

Te02 Se.02 KMh04 SeO'2 erro.r taken taken used found gm gm cc gIn gIn

6. .1997 .0055 26.90 .0064 .... 0009 7'. .. 2002 .0269 30.66 .0265 -.0004 8. .2007 .0545 35.83 .0542- -.0003 9. .1991 .0814. 40.60 .0910 -.0004 ~O. .l983 .1,091 45.59 .. 108? -.0004

~1.9.. In ord.er to determine whether the method could be applie.d to the determination of varying amounts of se~enium and te~lurium together, the data shown in tab~e XII \vas determined.. The results were calcu1a ted as fol~ows: In part (al a constant amount of se

~cnium was added, the amount of perr.aanganate required for i t calculated from the standa,rdization. and the re.. mainder of the potassium permanganate used to calculate the tel~urium found. In part (b} a constant amount of tel~urium was added, the amount of permanganate re quired for it calculated from the standardization. and the remainder of the potassium permanganate used to calculate the selenium found. Tl1e a.ccuracy of this procedure is sho~vn by the results recorded. -21- In order to find whether selenium and tellur ium could be determined in the same sample, the data of table XIII was taken. ~he selenium and tellurium were oxidized together by potassium permanganata. the tellurium determined in a separate sample by potassium dichromate, and the amount of permanganate required by the tellurium in the first case calculated. The amount of permanganate remaining was used to oalculate the se lenium found. This procedure is shown to be qUite ac curate for the determination of selenium, and the a mount of tellurium may be readily determined from the dichromate titration.

-22- Tabl.e XIV

!itratioD. o~ Te02. and SeOa: with K2~ra97'" then wit.h KMn07.

T'e(l2 taken Se02.' -t·aken. DJ104 used. Se02,: :round hro-r gm grll ec gm wn 1.. ..0701. .13'74. ~·.50' .I3'l9 ".0005 2:.. ..O:?·O~ ..~;3·'l4 23 ..'78 .};5·77 ~,..0003 ~.. .0701 .1374 23.'13 .13?fi +.. 0001

In a mixtur'e' whi,ch c~ont,ai.ns both s'ele,nium and. t.el.ltlr:ium~ the t.eI.luritml madi be oxi.dized. t-o the tellur'ie sta.te by pota.s:s.i:l..1m di..c·bromate and. the· excess. of the la.t.ter de:s.troyed, by el,eetrome·tri.e titrat,ion with f'erroua B'tb1fa..t.e.

Tl-re s-e'l.eni-um. mq- b'e then determined by oxidat,ion with pQ,tassium permanganate in the usual mann~er;. rypieel. re·mtl'ts"; are shown in tabIe XIV.

-2Z- SUMlvlARY: - (1). Selenious and tellurious acids may be very accurately determined by oxidation with potassium permanganate and titration of the excess of the latter electrometrically with ferrous sulfate solution. \2). Chlorides must be absent. (3). 1he addition of disodium phosphate pre vents the precipitation of manganese dioxide. (4). Selen.ions and tellurious acids in the

same samples may be determined by using a combination

of the dichroma~e and permanganate methods.

-24- Bibliography

1. Potentiometric Titrations. Kolthoff and Furman. Wiley. (1926).

2. The volumetric determination of tellurium by

the dichromate method. Lenher and rrfakefield. J. Am. Chern. Soo. 45,1423, (1923).

3. The electrometric titration of dichromate with ferrous sulfate. Eppley and Vosburgh. J. Am. Chem.

Soc. 44~ 2148, (1922).

4. Some applications of the hydrogen electrode in . analysis, research and teaching. Hildebrand. J. Am. Chem. Soc. 35, 847, (1913).

5. The increase in oxidizing potential of the di chromate ion on platinum caused by certain reducing

agents. An improved method for the electrometria ti tration of ferrous salts. Forbes and Bartlett. J. Am. Chem. Soc. 35, 1527, (1913).

6. Volumetric estimation of telluriu~. Brauner. J.

Am. Cham. Soc. 59, 2~8, (1891).

-25- 7. On certain points in the interaction of po~ tassium permanganate and sulfuric acid. Goo-ch a.rld Danner. .An1 .. J. Sci. 44~ 301. .. (1.892.).

8 ~ The separation. of selenium and tellurium.

Lenher and Kao. J. Am. Chem. Soc. 47, 769 a (~925).

9. The determination of selenious acid by potassium permanganate.. Gooch and C·~emons. .Am •. J" .. Sci 50,

5~. (1895).

~o. The electrometric determination of tellurium

in the presence of ferri.c iron. se~enium and copper.. Schrenk and Browning.. J. Am. Chem. Soc .. 48. 139,

(192.6 ).

-25-