Applications involving oxidation with potassium bromate

MICROCHEMICAL

JOURNAL

26, 481-486

(1981)

Applications Involving Oxidation Potassium Bromate II. Potentiometric

Titration of Chromium in Mixtures

with Alone or

INTRODUCTION Various metal ions can be oxidized by bromate; the subsequent sulfite reduction of the excess of bromate yields bromide. This bromide can be titrated potentiometrically with mercury(I); the approach has been applied to the determination of manganese (2). The bromate oxidation of chromium(II1) to dichromate, on the basis of electrode potentials, proceeds to completion; it was found, however, that a catalyst such as cobalt(H) must be present. Sulfite reduces chromium(W) to chromium(II1) and unreacted bromate to bromide. Potentiometric titration of the bromide with mercury(I) using a silver amalgam indicator electrode allows the determination of chromium alone or certain binary or ternary metal mixtures. Iron(II1) is reduced by the sulfite to iron(I1) and does not interfere. By means of the oxidation of iodide, some metal ions can be determined by potentiometric titration of unreacted iodide with mercury(I1). EXPERIMENTAL All solutions were prepared from high purity chemicals and deionized water and were standardized by recommended methods. Potassium hromc~tr, 0.05 M, prepared by dissolving and diluting the calculated amount of the salt with water and standardizing by either a potentiometric back-titration under slightly acidic conditions of an excess of potassium iodide with mercury (II) (I) or by direct potentiometric titration with mercury(I) of bromide produced by sulfite reduction under slightly acidic conditions and boiling 5 min. Mercury(I) nitrote, 0.05 M, prepared by reducing the calculated amount of mercury(H) nitrate, dissolved in the least amount of water, with mercury metal, separating the excess mercury by filtration, and diluting to 481 0026-265)(181/040481-06$01.00/O Copyright p 1981 hy Academtc Prec5. Inc. All rights of reproductwn I” any form reserved.

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volume with water. The solution was standardized against iron(N) solution in the presence of ammonium thiocyanate (3). Other solutions, prepared to 0.05 M and standardized by recommended methods include CyDTA, potassium chromium(II1) sulfate, potassium dichromate, manganese(I1) sulfate, nickel sulfate, copper(I1) sulfate, aluminum, iron, and potassium iodide. A 0.1 M cobalt sulfate solution was used as the catalyst, 10% sulfuric acid for acidification, and 20% sodium sulfite solution as the source of sulfite. Apparatus

The titration assembly consisted of a 150-ml beaker fitted with a silver amalgam indicator electrode and a saturat.ed calomel electrode (with salt bridge), a j-ml buret, a magnetic stirrer, and a potentiometer (E512 Metrohm Herisau). The silver electrode was fashioned from spectrographitally pure silver rod (2-mm diameter; 30-mm length) fitted to glass tube by polyethylene and paraffin wax. Procedures

In the following procedures, the silver amalgam electrode is used as the indicator electrode when either bromide is titrated with mercury(I) or iodide or CyDTA with mercury(I1). Procedure A, determination of chromium alone. Transfer to a 150-ml beaker a solution containing 0.9 to 6.1 mg of chromium(III), add a known, excess volume of the 0.05 M bromate solution (I-3.8 ml) and 2 ml of 0.1 M cobalt sulfate. Boil 15 min to oxidize chromium completely and expel liberated bromine. Add 2 ml of the sodium sulfite solution and a few drops of sulfuric acid to reduce chromium(W) to chromium(II1) and unreacted bromate to bromide, boil to expel sulfur dioxide, cool, and titrate bromide with mercury(I). Express the result as chromium(II1). Procedure B, analysis of binary mixture of chromium and iron. For a mixture of chromium(II1) and iron(III), dilute the sample (l-2.5 mg of each metal) to 20 ml with water, add a known, excess volume of 0.05 M bromate (3 ml) and 2 ml of 0.1 M cobalt sulfate. Continue as in Procedure A. The result corresponds to chromium(II1). Acidify slightly an identical diluted sample with a few drops of sulfuric acid, add a known, excess volume of 0.05 M potassium iodide (4 ml), and boil 10 min to expel liberated iodine, maintaining the volume by addition of portions of water. Cool in ice water (YC) and titrate the unreacted iodide with mercury(I1). Express the result as iron. Procedure C, analysis of binary mixture of chromium

and manganese.

Dilute the sample (l-2.5 mg of each metal) to 20 ml with water, add a known, excess amount of 0.05 M bromate (3 ml) and 2 ml of 0.1 M cobalt sulfate, and boil thereby forming chromium(V1) and manganese(IV).

DETERMINATION

483

OF CHROMIUM

Continue as in Procedure A. The result corresponds to the sum of chromium(II1) and manganese(I1). Make an identical, diluted sample alkaline with sodium peroxide and boil 20 min. Chromium(II1) is thereby oxidized to chromium(W) and manganese(I1) to hydrated manganese(IV) oxide. Cool, acidify slightly with sulfuric acid, add 2 ml of 1:3 hydrochloric acid, and boil 10 min to yield manganese(II) and to expel liberated chlorine. Cool, add a known, excess volume of 0.05 M potassium iodide (5 ml) to reduce chromium(W) to chromium(III), and titrate the excess of iodide with mercury(I1). Express the result as chromium. Manganese is calculated by difference. Procedure D, crttrrlysis of chromitrm

\clith nickl,

copper. or ultrminrrm.

Dilute the sample (1- 2.5 mg of chromium with 1- 2.5 mg of one of the other metals) to 20 ml with water, acidify with a few drops of sulfuric acid, and add a known, excess volume of 0.05 M bromate (3 ml) and 2 ml of 0.1 M cobalt sulfate. Continue as in Procedure A. The result corresponds to chromium(II1). Add to an identical, diluted sample a known, excess volume of 0.05 M CyDTA, boil 15 min, cool, buffer with hexamine, and titrate unreacted CyDTA with mercury(I1) at either pH 7.0 (nickel or copper) or pH 6.5 (aluminum). The result corresponds to the sum of chromium and the second metal. The second metal is obtained by difference. Procedure E, annlysis c?fchromium(III) crnd c~hrotnirrm(V1) presetlt together. Dilute the sample (l-2.5 mg each of the two oxidation states) to 20 ml with water, add 2 ml of 0.1 M cobalt sulfate. and a known, excess volume of 0.05 M bromate (3 ml), and continue as in Procedure A. The

result corresponds to chromium(II1). Acidify an identical, diluted sample with a few drops of sulfuric acid; add a known, excess volume of 0.05 M potassium iodide (3 ml): allow to stand 2 min for complete reduction of chromium(W) to chromium(III), and titrate the unreacted iodide with mercury(II). The result corresponds to chromium( VI). Procedure

F. antrlysis of‘ termtry tnixtrrres contnining

chromitrm (III).

For a mixture with iron and either nickel or aluminum, dilute the sample (l-2.5 mg of each of the three metals) to 20 ml with water; add 2 ml of 0.1 M cobalt sulfate and a known, excess volume of 0.05 M bromate (3 ml), and continue as in Procedure A to obtain a value for chromium(II1). Acidify an identical, diluted sample with a few drops of sulfuric acid, add a known, excess volume of 0.05 M potassium iodide (3 ml), and continue as in the second paragraph of Procedure B to obtain a value for iron. Add to a third identical, diluted sample, a known, excess volume of 0.05 M CyDTA and continue as in the second paragraph of Procedure D. The

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TABLE 1 DETERMINATION OF CHROMIUM(III) Chromium

(mg)

Taken

Found

Percentage error

End point break (mV/O. 1 ml)

0.95 1.42 1.90 2.37 2.84 3.32 3.79 4.27 4.74 5.22 5.69 6.16

0.95 1.42 1.91 2.39 2.85 3.31 3.80 4.27 4.75 5.21 5.69 6.17

0.00 0.00 +0.53 +0.84 +0.35 PO.30 +0.26 0.00 +0.21 -0.19 0.00 +0.16

174 171 170 166 157 169 156 176 161 165 182 171

result corresponds to the sum of chromium(III), iron(III), and the third metal (aluminum or nickel), which is then found by difference. For a mixture of chromium(III), chromium(VI), and iron(III), dilute the sample (l-2.5 mg of each of the metals) to 20 ml with water and proceed as in Procedure A to obtain a result for chromium(II1). Acidify a similar, diluted sample, with a few drops of sulfuric acid; add a known, excess volume of 0.05 M potassium iodide, and continue as in the second paragraph of Procedure E to establish a value for chromium(V1). Acidify a third similar, diluted sample with a few drops of sulfuric acid; add a TABLE 2 ANALYSIS OF BINARY MIXTURES Chromium

(mg)

Second metal (mg)

Taken

Found

2.37 1.19 2.32 1.16 2.32 1.16 2.32 1.16 2.32 1.16 2.32 1.16

2.37 1.18 2.32 1.16 2.32 1.16 2.32 1.16 2.32 1.16 2.31 1.17

Taken 1.20 Fe 2.40 Fe 1.32 Mn 2.64 Mn 1.39 Ni 2.79 Ni 0.63 Al 1.26Al 1.58 Cu 3.16Cu 1.26 Cr(VI) 2.53 Cr(VI)

Found 1.22 2.39 1.33 2.65 1.39 2.78 0.63 1.26 1.58 3.16 1.27 2.52

DETERMINATION TABLE ANALYSIS

485

OF CHROMIUM 3

OF TERNARY

MIXIWRES

Third metal (mg)

(mg)

Second metal (mg)

Taken

Found

Taken

Found

Taken

Found

2.32 1.16 2.32 1.16 2.32 1.16 2.32 1.16

2.32 1.16 2.32 1.16 2.32 1.16 2.31 1.16

1.20Fe 2.40 Fe 1.32 Mn 2.64 Mn 1.20 Fe 2.40 Fe I .20 Fe 2.40 Fe

I.19 2.40 1.32 2.63 I.19 2.41 I.21 2.41

1.26 Al 0.63 Al 2.79 Ni I .39 Ni 2.79 Ni 1.39Ni 1.26 Cr(VI) 2.53 Cr(V1)

I .27 0.63 2.78 1.40 2.79 1.38 1.26 2.52

Chromium

known, excess volume of 0.05 M potassium iodide (4 ml), and proceed as in the second paragraph of Procedure B to obtain a value for the sum of chromium(VI) and iron(II1). Iron is found by difference. RESULTS

AND DISCUSSION

The results of determining 1 to 6 mg of chromium alone or in certain binary and ternary mixtures are summarized in Tables 1, 2, and 3. The values, which are the average of duplicates, are in good agreement with the amounts taken. The end point potential breaks are sharp and large for the titration of bromide with mercury(I), or CyDTA or iodide with mercury(I1) (171, 172, and 307 mV per 0.1 ml of 0.05 M titrant, respectively). Under weakly acidic conditions, the bromate oxidation of chromium(II1) goes to completion only when a metal ion such as cobalt(H), nickel(II), or manganese(I1) is present. Cobalt is more effective than nickel in promoting this oxidation and equally effective as manganese, which, however, tends to be oxidized to manganese(IV) by bromate. The procedures presented should be applicable to the rapid and reliable determination of chromium(II1) alone or with a variety of metals in diverse practical samples. SUMMARY A rapid and reliable determination of chromium was developed based on bromate oxidation of chromium(II1) to chromium(V1). The reaction is complete under weakly acidic conditions and with cobalt(I1) present as a catalyst. Unreacted bromate and chromium(Vl) are then reduced with sulfite to bromide and chromium(II1). The bromide is titrated potentiometrically with mercury(I) using a silver amalgam indicator electrode. Iron(II1) if present is reduced by sulfite to iron(I1) and does not interfere. Some binary and ternary metal mixtures containing chromium can be resolved by the determination of chromium, alone or with another metal, by the above procedure coupled with procedures for further sample portions involving the potentiometric titration of unreacted CyDTA or iodide, or both. with mercury(I1).

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REFERENCES I.

Khalifa, H., and Ateya, B., Microdetermination of oxidizing agents, use of potassium iodate and periodate as primary standards for mercury(I1). Microchc,rn. J. 13, 147- 154 (1968). 2. Khalifa, H., and Ismail. I. A., Applications involving oxidation with KBrO:,. I. Rapid potentiometric method for manganese alone or in steel and some ores. Microc~hc~m. .I. 23, 220~22.5 (1978). 3. Kolthoff, I. M., and Belcher, R.. “Volumetric Analysis,” Vol. 3, p. 623. Wiley (Interscience). New York, 1957.