EXTRACTION STUDIES ON SOME ANIONIC COMPLEXES OF ZINC, CADMIUM AND MERCURY WITH LIQUID ION EXCHANGERS

Abstract

The use of high molecular weight amines (hmwa) as liquid anion exchangers dates beck to 1948 when Smith and Page(l) first reported their acidvbinding properties. The extraction of acids by these materiale depends on the fact that acid salts of these bases ere essentially insoluble in water but readily soluble in water-immiscible organic solvents. The operation of liquid ion exchangers involves the principle of liquid-ilfttid extraction, in which a eoluto can distribute itself in a certain ratio between two immiscible phases, an aqueous phase and an organic phase containing the liquid ion exchanger. Thus, liquid ion. exchange is a special case of solvent extraction and refere only to formation of ion-pairs, with special reference to exchange of one ion for another between the aqueous and the organic . phase. The organic solvent containing the amine can extract 9n aqueous acid to form an amine salt in the organic phaeei en * II* ♦ A* ^ *,BK*a£__ (extraction) (1) Vorg * eq * aq ^ 3 org where Olia high sx>lecular weight amine, a*" is the anion of either a simple acid or a complex metal acid, e.g., PeCl*, HgCl* and org and aq denote the organic and the aqueous phase respectively. The reaction will require some minimum acidity to go (say) halfway to completion. This minimum acidity dep* nds not only on the particular amine (varying with its class and structure) but also on the type of diluent and on the Identity of anion. There can ale© be a maximum acidity limit aet by Increasing miscibiiity between the -bases or even by chemical 1 2 degradation of the amine. In alkaline solution, the amine salt thus formed rrverte to frec-baee form by hydrolysis. An amine salt in the organic phase can undergo anion-exchange with an anion(B*) in the aqueous phasei R.HH*AT ♦ B~ ^=^ ******£«« ♦ C <«nion»oxchange) (ii) T^ org eq^ 3 org aq uitcrnarv nmonium salts cannot H ct ciccoriiixj to equation(1) but only in analogy to equation(ii)t B4N*A* is a quaternary ammonium salt. Prom the very beginning, liquid-liquid extraction by high molecular weight amines has been compared with anion. exchange in solid resins, m its normal method of use, a liquid exchanger is closely similar to a solid resinous exchanger used in the botch analysis technique. The behaviour of analogous functional groupe in the two systems is usually similar, at least as a scoping approximation. The extraction curves have the seme general form as those for solid anion exchangers. The order of preference for anions in the amine organ c solution is similar to that in solid anlon-*ychange resins(2)i CIO4 >•"? > C1~ > m°* ^ * ' The amine salt on treatment with an alkaline solution, such as aqueous ammonia, sodium hydroxide or sodium carbonate will revert to the free amine, thua further resembling ion exchangers. Therefore, the previous studies on the resinous 3 rs can set us*lies guidelines for extraction in high Molecular weight amines. Though there 1* e fairly good similarity between th* two systems, yet soms workers in the field recommend not to over-emphasise the analogy. A resin phase containing water and salts Is physically different from an organic solution of high molecular weight amine containing little or no water, Moseover, one can find no analogy at ail simply by changing the organic diluent for the amine. Details of the extraction behaviour can markedly differ from the sorption behaviour, and the extractions can differ with individual structures within a single type of extractant. Thus, the resin sorption can give a good suggestion of the nature of the extraction, but not of the wide range of extraction levels available. However, in the theais the author has frequently used the term llcuid anion exchanger for high molecular weight amine. Liquid anion-cxchange type materials are primary, secondary and tertiary aminea and quaternary ammonium salts. For practical solvent extraction purposes, an amine has to fulfil certain basic requirements, such as compatibility with s practical diluent, sufficient extraction power, rapid phase separation, low aqueous solubility and sufficient stability. With B few exceptions, usually only saturated nitrogen empamda fulfil these reo^irements. These compounds are of high molecular weight ( r*J XSO-too), sparingly water soluble but readily soluble in many organic solvents, such as petroleum distillates, aliphatic ana aromatic hydrocarbons, chlorinated hydrocarbons and high 4 molecular weight alcohola. Beaidos these amines and ammonium salts, certain quaternary phosphoniura and arsonium ccrpounds have also been used as extractants(3-7). see* workers in the field consider high molecular weight dibasic monoalkylphosphoric and -phosphonic acids, monobasic dialkyi and dlarylphosphoric and -phoaphinic acids, alkylsulphonic acids, and alkylcarboxylic acids as liquid cation exchangers. A detailed list of various liquid Ion exchangers has been given by Coleman si al.(B), In arecent review, oiten(9) hae listed different ion exchangers used for various analytical purposes, in the recent past, a number of amines, amine salts and quaternary ammonium salts have been extensively used for various analytical purposes. This has prompted ch- mists to carry out studies on those amine properties which affect their use as extrectants. some of these important properties like solubility, basicity, dipoie moment, nature of hydrogen bonding, thermal and radiation stability and their tendency to form aggregates have been very nicely dealt with by Marcus and Kertes(lo). There are various factors which affect the extraction of metals in these aminee. since metaia are extracted from aqueous solutions preferentially when they exist SB anions <Cr2C> .VO" ,Roc£> or form anionic complexee (Feci; , nclj~), the parameters controlling the extraction of metal ions are still those which affect the formation of such extractable epecies in the aqueoue phase. These parameters Include the aqueous llgand concentration, concentration of other complexing agents that may compete 5 Cor the metal ion and the presence of other anions that compete for the bulky cation. Besides this, the hydrogen ion concentration, the concentration of metal ion itself and physical parameters, such as temperature may also affect the extraction, in addition to the parameters of the aqueoue phase, amine extraction systems sro particularly susceptible to organic phaae parameters like nature, structure and sine of the organic base, its concentration, and type of organic diluent. Each of these factors can affect the overall extraction of a metallic species to an almost unpredictable extent. However, some predictions can be made regarding the effect of these parameters on extraction, It ie apparently a general phenomenon that the extraction efficiency of alkylaminee follows the order • primary < secondary <tertiary <quaternary. However, thia effect is strongly modified by the nature of the diluent and by the extent of branching of the alkyi groups, where steric factors play an important role, Ths extraction behaviour of a branebed-chain secondary amine is closer to a stroighb-chain tertiary than to a straight-chain secondary amine. Highly branched chains generally interfere with efficient extraction, probably owing to steric effects, but ttwj branct^ed-chaln aminea may be more ccmpstible with the diluent, therefore, the net effect of chain branching on the extraction coefficient depends on the nature of the diluent. In amine extraction systems the nature of the organic diluent affects the extraction coefficient 6 substantially, a—*r*l *tte*pta ha— b«*B —4* to establish an empirical order of solvent efficiency and to understand the effect of diluent on the extractive pow r of various amines(11- 22). There seems to be a lack of correlation between the extraction coefficient of a metal and basic physical characteristics of a solvent, such as ita dipole movent and dielectric constant. This led Coleman and coworkers (2) to suggest that the amine-dlluent combination should bs considered an the effective extrectant Instead of amine only. According to Teube(17,18), three main parameters affecting tfce extraction of a metallic species are degree of aggregation of the extracted species, ita dipole moment and the dielectric constant of the solution. Xn ion-association extraction systems there exist, mm a rule, large (usually coordinativeiy unhydrated) ions with mutual dipole Interaction, If these species are not aggregated they will favour extraction in solvents cf high dielectric constant while aggregated species with low dipole moment will be better extracted into solvents of low dielectric constant. This concept is a helpful guide but too gross for any useful quantitative correlation. It ftssumss a medium of uniform and unchanged dielectric constant and a constant dipole moment for the solute Irrespective of the chenge in experimental parameters. This situation is hardly encountered in actual solvent extraction systema. Liquid anion exchangers have been extensively used for verlous separations partly because many of the advantages of the solvent extraction and ion-exchange combine in them. Compared with resinous exchangers the use of liquid ion exchangers offers a much more rapid method of separation of the required ionic species. The added advantages lie in the physical handling of the liquid-liquid system and in the extended chemical control of the extraction power and selectivity available by means of choice of reagent structure and concentration, diluent effects and additives. Moreover, amine extraction technique is particttlarly valuable because it functions efficiently in strong acid solutions and even in alkaline solutions with the quaternary ammonium salts. Xnsplte of these advantage* the use of KMNA is linited due to s number of difficulties and disadvantages associated with them. The inherent problems in their use, the tendency to form emulsions end the problem of analyeis in the organic phase are of considerable importance. Another serious limitation in their use Is the frequently occuring phenomenon of the formation of a second organic phase, called third phase. However, some of these problems can be controlled by taking due care. Scon after the ork of 3ralt^ and Page analytical applications of high molecular weight aminea were begun by Moore and coworkers (23-25) and have grown rapidly ever since reaching commercial application in 1957 in the production of uranium and related metals from ores (36,27). since then a large nasber of high molecular weight amines have been explored as successful extractants. one can have an idea about the fast expansion in the field from the varioue revlews{e,9,2S-37) which have appeared during the last fifteen years, m his two reviews(%33) Green has summarised the work done on the various applications of this class of 8 compounds in Inorganic analysis