‘Manta, Vol 22, pp 1037-1040 Pergamon Press, 1975 Pnnted m Great Bntarn

ANALYSIS OF CHROMITE BY CATION-EXCHANGE USING ETHYLENEDIAMINETETRA-ACETIC

ACID

MOH.UMAD JAWAID and FOLKE INGMAN

Department of AnalytIcal Chexmstry, The Royal Institute of Technology, Stockholm, Sweden

(Recewed 25 October 1974 Accepted 21 January 1975)

Summary-A method for the separation and determmation of five major elements m chronute. ore (and chrome-bearmg refractories), based on complexatlon of the metals with EDTA IS described After removal of s&a, the cations are separated mto two groups by passmg the solution through a cation- exchange resin (Dowex 5OW-X8, m Na-form) m the presence of an excess of the complexmg agent The optimum condltlons for the separation are discussed on the basis of exchange constants that were either known or determmed The first group contains Cr and Fe, which emerge m the filtrate at pH between 1 5 and 2 1, whereas Al, Mg and Ca, which are adsorbed on the resin, form another group Complexometric titrations are used for the subsequent determination of the cations m each group The method IS simpler and more rapld and accurate for routme analysis than the current methods

The methods avadable for the analysis of chromate ore and chrome-bearmg refractones, are often de- scribed as laborious and time-consuming and yet they lack the desired accuracy Consequently, these methods have constantly been subjected to revision and improvement to make them simple, rapld and more accurate

Zagorchev et al1 were probably the first to use an ion-exchange resin to separate chromium from rron m the analysis of ferrochrome alloys Bennet and Marshall’ employed a liquid ion-exchanger to extract chrommm as dlchromate m slightly aadlc solutions and were able to separate it quantitatively from the

ent pH values These differences m the dlstrlbutlon coefficients can be utilized to choose condltlons for separatmg the catlons mto some suitable groups, there- by facllltatmg complexometrlc titration of one cation without interference from the other catlons present m the solution Thus, a direct titration of iron against EDTA can easily be performed m presence of chro- mium, which, m turn can be back-titrated at a higher pH Bmllarly, alummmm can be titrated successfully m the presence of magnesium and calcium

EXPERIMENTAL

rest of the elements present m chromate ore A complexometrlc procedure based on the usual

Apparatus and reagents

analytical separations and complexometrlc titrations Ion-exchange column Conventional ion-exchange

was developed by Sosm and Ztrez.eszewska3 and was columns with a bore of about 1 5 cm were used The resin

mod&d later by Liteanu and Crqan 4 The methods, bed was about 10cm long The resm, Dowex 5OW-X8 (50-lOOmesh), was treated Hrlth 4-5&f, hydrochloric acid

although comparatively rapid, had lmuted accuracy and washed free from acid with demmerahzed water It

The ion-exchange separation of chrommm from iron was then rinsed with ethanol and dried overmght at 110”

and alummmm, m the presence of EDTA and DCTA The exchange capacity of the resin was checked by the

has been reported by Babachev,’ and m the presence Fischer and Kunnm* method and was found to be 5 1 meq/g of dry resin The resin was converted mto the

of EDTA by Kratochvd et al 6 sodmm form by agltatmg it with sodium chloride solutron

In the present method, the separation of cations and washing it free from chloride ions

1s carried out by usmg EDTA as the complexmg agent EDTA so&on, OOlM Standardized against standard

The separations are based on theoretical calculations lead nitrate solution vvlth Methylthymol Blue as mrllcator

m accordance Arlth Rmgboms’ concept of condltlonal Alummwm and zron solutzons Prepared from pure metals

and standardlzed agamst the standard EDTA solutxon,

stability constants of metal-EDTA complexes The with Xylenol Orange as indicator

notation used throughout this work IS also that of Chromwm solutwn Prepared from potassium dlchromate

Rmgbom. and standardized agamst EDTA, with Xylenol Orange as indicator

The condltlonal dlstrlbutlon coefficients dl”‘, for Cr, Magnesmm and calcwm solutwns Prepared from analytl- Fe, Al, Mg and Ca on a strongly basic cation- cal grade carbonates and standardized agamst EDTA, with

exchange resin (Dowex SOW-X8) m the presence of Eriochrome Black T (EBT) and Murexide respectively as

an excess of the complexmg agent (EDTA), may be mdicators

calculated from known exchange constants and the Choice of condltlons for the separation

condltlonal stability constants of the metal-EDTA complexes and show considerable differences at dlffer-

The formation of metal-EDTA complexes IS greatly affected by the hydrogen-ion concentration m the solution

1037

1038 MOHAMMAD JAWAID and FOLKE INGMAN

Fig 1 CondItional stability constants of metal-EDTA complexes as a function of hydrogen Ion concentration

At higher hydrogen-ion concentrations, a greater propor- tion of the metal ions 1s free and consequently more easily adsorbed on a catlon-exchanger, whereas m more alkaline solutions most of the metal Ions may be bound m neutral or m less posltlvely-charged complexes that are not easdy adsorbed

In Fig 1, the condltlonal stability constants of meta- EDTA complexes of Cr, Fe, Al, Mg and Ca are plotted as a function of hydrogen-ion concentration It can be seen that at higher hydrogen-ion concentrations (pH less than 2 5) Cr and Fe, which have a relatively high complexmg abdlty, are transformed mto unadsorbable anionic com- plexes, whereas Al, Mg and Ca exist predominantly m the cationic form that can be selectively adsorbed on a catlon- exchange resin

In Fig 2, the condltlonal dlstrlbutlon coefficient, DM for Cr, Fe, Al, Mg and Ca m the presence of an excess of EDTA, 1s plotted as a function of hydrogen-ion con- centration The condltlonal dlstrlbution coefficients have been calculated from the stability constants of the metal- EDTA complexes and the exchange constants of the metal ions on the cation-exchange resin The values used for Cr, Mg and Ca are based on the data taken from Bonner et al ,’ whereas the exchange constants for Al and Fe were determined by the present workers lo

The conditional dlstrlbutlon coefficients have been cal- culated from the expression

D”’ =

where

[Na& = concentration of sodium ion m the resin phase = resin capacity

[Na] = concentration of sodium ion m the solution

K;;; = exchange constant of the metal ion on the resin

(KMUY 1s the stabdlty constant of the metal-EDTA complex and [Y’] IS the excess of complexmg agent not bound to the metal ion )

A satisfactory separation of two metals on a catlon- exchange resin requires that the condttlonal dlstrlbutlon coefficient must be greater than 10’ ’ for complete adsorp- tion, and less than 10’ ’ for practically no adsorption 7

From Fig 2 It can be seen that the condltlonal dlstrlbu- tlon coefficient D”’ for Al, Mg and Ca IS greater than 10’ ’ at pH below about 24, whereas for Cr and Fe It IS less than loo ’ at pH above about 14

If we choose to work m the pH range 1 S2 1, there wdl be enough margin of safety and the results should be in good agreement with the calculated values

Procedure for the analyszs of chromzte

Decomposztwn of chromzte sample A mixture of sodium carbonate and borax (3 2) 1s recommended as a universal flux for the decomposltlon of chromate ore and chrome- bearing refractories I’ The followmg procedure has been found suitable by the present workers

Weigh out about 1 g of anhydrous sodium carbonate and about 0 5 g of boric acid m a clean and freshly lgmted platinum crucible, and fuse together on a low flame Cool to room temperature and add 0 1 g (accurately weighed) of chromate sample previously ignited at about 650” Heat the contents, first gently with occasional swlrlmg of the crucible, and then place the crucible m an electric furnace at 900-950” for 45 mm Allow to cool and extract with dilute hydrochloric acid

The melt cake IS taken out m the usual manner The function of very dilute hydrochloric acid IS to neutralize the solution and to wash out any solid melt still adhering to the crucible Care should be taken to mmlmlze the con- tact of melt dissolved m hydrochloric acid with the platl- num crucible

If boron 1s not removed, precipitates will sometimes form during the ion-exchange separation process The removal of boron may be combined with the reduction of Cr(V1) which may be effected by bodmg with methanol

Transfer the solution to a conical flask and evaporate it almost to dryness on a water-bath Add a few drops of acetic acid and about 20 ml of methanol to remove boron and reduce Cr(V1) Evaporate to dryness Repeat the procedure twice with further 15-20 ml portions of methanol Dissolve the residue m a mnnmum quantity of dilute hydrochloric acid, filter off sdlca and dilute to about 100 ml

Separatzon of chromzum and zron fom alumznzum, magne- szum and calczum To the soiutlon prepared as above add an excess of 0 1M EDTA such that the concentration of the free complexmg agent is about 0 OlM The excess con- centration must not be much higher than OOlM because there IS risk of EDTA preclpltatmg Adjust the pH to 1 8 + 0 3 (ddute hydrochloric acid or sodlvm hydroxide) and boll for 20-25 mm Cool to room temperature and pass the solution through the column, pretreated with about 50 ml of 0 OlM EDTA at pH 1 8 + 0 3 Adjust the flow-rate to 1 5 + 0 5 ml/mm Wash the column with 150 ml of 0 OlM EDTA at the same pH and add the washings to the mam filtrate

Determznatzon of Fe and Cr

Heat the filtrate to reduce its volume to about 50 ml Add 15 ml of hydrochloric acid (12M) and then in order to destroy EDTA, add cautiously about 2 g of potassium chlorate m tmy portions Evaporate to dryness on a water- bath Repeat the procedure with 10 ml of hydrochloric acid and 1 g of potassmm chlorate Take up the solution with a small amount of dilute mtrlc *acid while heating on the water-bath, cool to room temperature and make up the volume to 100 ml m a standard flask

‘0 B

-50 I 2 3 4 5 6 7 8 9 IO

PH

Fig 2. Conditional dlstrlbutlon coefficients of metal ions on Dowex 5OW-X8 (sodium form) m the presence of an excess (OOlM) of EDTA as a function of hydrogen ion

concentration

Analyszs of chromate 1039

Determznatzon of Fe To a smtable alzquot (15-20 ml) add about 10 ml of ethanol and heat gently for 5 mm Add a drop of nztrzc aced to oxldtie Iron Cool to room temperature, dilute to about 100 ml and if necessary adJust the pH to 10-l 5 wzth dilute sodnzm hydroxide or nitric acid Heat to about 80” and titrate agamst OOlM EDTA, usmg Xylenol Orange as mdzcator

Determznatzon of Cr To the solution after the tztratzon of Fe, add an excess of OOlM EDTA and 10 ml of hexa- mme buffer at pH 5 Heat to bozlmg for 2&25 mm Cool to room temperature and back-titrate the excess of EDTA wzth standard lead solutzon

Elutzon and determznatzon of Al, Mg and Ca

Elute the sorbed cations with 150 ml of 4-5M hydro- chlorzc acid Wash the column wzth about 50 ml of water and add the washmgs to the eluate Heat the eluate to reduce zts volume to about 50 ml and add cautiously about 2 g of potassnzm chlorate to destroy EDTA Evaporate to dryness on a water-bath Repeat the procedure wzth 10 ml of hydrochloric acid and 1 g of potassium chlorate Take up the solutzon m a small amount of dilute hydro- chorzc aced and make up the volume to 100 ml m a standard flask

Determznatzon of AI I2 To a suitable alzquot (15-20 ml) of the solution piepared as Just descrIbedi add an excess of OOlM EDTA and 10 ml of oH 5 5 hexamme buffer Heat to bozlmg for 5 mm, cool gnd dzlute to about 100 ml Check the pH and readJust, if necessary, wzth sohd hexamme and 1M hydrochlorzc aced Back-titrate the excess of EDTA agamst standard lead solution, using Xylenol Orange as mdzcator

Determznatzon of Mg and Ca The sum of Mg and Ca 1s determmed m another alzquot (15-20 ml) of the sohztzon by the usual complexometrzc tztratzon at pH 10 (NH&- NH3 buffer) with EBT as indicator Ca 1s determmed m ls-20 ml of the sample solution by titrating complexome- trzcally with EDTA at pH > 13 (sodzum hydroxide), wzth Murexzde as mdzcator Murexlde zs the only Indicator that we have found that will work properly m the presence of a precipitate of magnesium hydroxzde Mg zs then deter- mined by difference

RESULTS AND DISCUSSION

In Table 1 some results of successive titrations of Fe3+ and Cr3+ are shown Table 2 shows the results of analysts of a synthetic mixture whereas m Table 3 are presented the results of analysis performed on a standard chromate sample (British Chemrcal Stan- dards No 308, Grecian chromate ore) The amounts found show a negative error of about 0 5%, which 1s probably due to losses during the lon-exchange procedure

The present method, m comparison to other methods based on complexometric titrations, posesses certam definite advantages The separation of cations mto groups facihtates the complexometric determma-

Table 1 Complexometrzc determmatzon of Fe3+ and Cr3 + wzth EDTA, m presence of each other

Amounts added Amounts found Fe3+, mg Cr3+, mg Fe3+, wzg Cr3+, mg

5 55 5 20 5 53 524 1110 520 1102 5 16 1110 1040 1106 10 34 1110 1560 1108 1554 1665 1560 1658 1548

Table 2 Catzon-exchange separatzon of Cr” and Fe3+ from A13+, Mg*+ and Ca’+, m presence of EDTA at pH

18 f 03

Amount found, mg Amount Filtrate Adsorbate

Cation added, mg (1) (2) (3) (1) (2) (3)

$1 25 9 252 258 256 - - - 139 138 135 135 - - -

Al3 + 68 - 65 64 67 Mg2+ 63 - 63 61 61 ca*+ 62 - 62 60 60

Table 3 Analysis of B C S Sample No 308, Greczan Chro- mite Ore

NO

1 2

3 4 5

RCUlge Mean Reported values Mean error RehtlVe

error, %

Cr,O,x% FeO, % AI,O, % MgO, %

41 26 IS 14 1923 1627 41 17 1533 IV 18 1635

41 32 1526 1942 1641 41 24 1537 1931 1625 4138 1508 1934 1621

021 029 024 020 41 27 1524 1930 1630

415 I53 194 164 -02 -01 -01 -01

-048 -065 -052 -061

tions m the same solutron by means of one direct and one back-titration This avoids the necessrty of masking and demaskmg, which 1s often mcomplete and thus reduces the accuracy

Iron (III) is titrated at pH 1 O-l 5 m the presence of chrommm(II1) In this pH range, the conditional stability constant of the Fe-EDTA complex is high enough to permit a direct trtratron, whereas Cr3+ does not enter mto reactron since It requrres a pro- longed heating (more than 10 mm)‘3 for complex for- mation However, successive titrations lead to reci- procal errors which may be signficant for materials contammg large amounts of iron and small amounts of chromnrm Slight evidence of this is shown m Table 1 Slmllarly, m the back-titration of A13+ at pH 5 5, Mg2+ and Ca2+ do not interfere, because of then low conditional stabihty constants at this pH At higher pH (> lo), the condmonal stabrhty constant of the Al-EDTA complex is low enough to permrt the direct titration of Mg2+ and CYazf

The method, which is rapid and yields better accu- racy than the current methods, will be found useful m the routme analysis of chromate ore (and chrome- bearing refractories), particularly m those laboratories that are poorly equipped

Acknowledgement-The authors wzsh to thank Professor Axe1 Johansson for hzs interest m thus work and for grant- mg the necessary faczhtzes

REFERENCES

1 B Zagorchev, L Ilcheva and E Mztropoltzska, Tekh- nol Inst, 1961,8 (2), 25, Chem Abstr 1963, 59. 9302h

2 H Bennett and K Marshall, Analyst, 1963, 88, 877 3 Z Sosm and I Ztrezeszewska, Chem Anal (Warsaw),

1964, 9, 425, Chem Abstr , 1965, 62, 3394d

1040 MOHAMMAD JAWAID and FOLKE INGMAN

4 C Llteanu and I A. Crown, St&a UW Babes-Bolyal, 9 0 D Bonner, C F Jumper and 0 C Rogers, J Phys Ser I, 1967,lZ (2), 7, Chem A&r, 1968, 68, 111@3T Chrm , 1958,62, 250

5 G N Bahachev, Ind. Chun Beige, 1967,32 (Spec No 10 M Jawiud and F Ingman, Talanta, 1975, 22, 1155. Pt l), 173, Chem Abstr, 1969, 71, 18_467h 11 C S Richards and E. C Boyman, Anal Chem, 1964,

6 V Kratochvd, P Povondra and Z Sulcek, Collectton 36,179O Czech Chem Commun, 1970, 35, 234. 12 J Kragten, Analyst, 1974, 99, 43

I A Rmgbom, Complexatron m Analytical Chemrstry, pp 13 G Schwarzenhach and H Flaschka, Complexometrlc 195-256 Interscience, New York, 1963 Stratums, 2nd Ed, pp 21&223 Methuen, London,

8 S Fischer and R Kunnm, Anal Chem, 1955,27. 1191 1969