Metal nitrit komplekslerinin sulu ortamdaki ardışık oluşumunun iyon değiştirme yöntemi ile incelenmesi

Abstract

Çalışmada, geçiş metal iyonlarından bakır (II) ve ni - kel (II )katyonlarının nitrit iyonu ile verdiği kompleks lerin sulu ortamdaki dengeleri araştırılmış ve 2O C da ve sabit iyonik şiddetlerdeki perkloratlı ortamda stabilite sabitleri saptanmıştır. Bu araştırmada, oluşan komplekslerin stabilite sa bitleri katyon değişimi yöntemi ile hesaplanmıştır. Su lu ortamlarda kompleks oluşumuna ve stabilite sabitleri nin hesaplanmasında kullanılan diğer yöntemlere ait ge rekli kuramsal bilgiler ayrıca sunulmuştur. İncelenen derişiklik aralığında, her iki sistemde tek kompleks oluşmuştur. Cu(NO2)2 kompleksinin stabilite sabi ti sırasıyla I=0. 05 de log/32=2. 64 ; I=0.15 de log/32=2. 43-, I=0. 25 de log/32=2. 27 olarak ve Ni(N02)2 kompleksinin sta bilite sabiti I=0.10 da log/32=1.66 olarak bulunmuştur.

Among the numerous techniques for the determination of formation contants, potentiometric and extinctiometric methods are most applied. But the application of those presupposes that suitable electrodes can be found, or that the complexes have absorption bands within available wave length ranges. It is the aim of the present investigation to apply an i on -exchange method, the applicability of which is not limited by such conditions, and to prove it on a complex system, that has been thoroughly investigated before by potentiometric and extinctiometric methods. As models the copper-nitrite and nickel -nitrite systems were chosen and complex formations are investigated in an aqueous me dium of contstant ionic strength with sodium perchlorate as a supplementary salt. A coordination compound consists of a central metal ion and a number of groups which surround this central ion. The surrounding groups around the central metal ion are called as ligands. Central ion is an acceptor and the llgand is donor. It is known that acceptors are Le wis acids and donors are Lewis bases. Therefore it is possible to think that coordination compounds are formed as result of lewis acid-base type of a reaction. The most important way tocharacterize the complex formation in solution is to determine the equilibrium constants of complexes formed. The majority of quantita tive equilibrium measurements have been carried out in aqueous solution because of enhanced solubilities and, moreover, more is known about how ions behave in this solvent. In earlier studies, researchers were used the excess amount of l'igand and observed only formation of the last complex. But today it is known that complex formation ~ shows a stepwise formation mechanism. In recent years many different techniques have been developed to determine the stepwise complex formation. After a large amount of stability constant data had been determined, many chemists began to examine these data in order to make a generalisation on metal ligand associati on. For this purpose, some reliable classification sche mes have been proposed. In the most important classifi cation, acceptors and donors are classified as hard (or class a) and soft (or class b), but there are many cases where acceptors and donors fall on the borderline bet ween two classes. This classification enables us to un derstand why a particular metal ion shows a preference for one ligand rather than another. Much of the interest in transition metal -nitrite complexes in solution concerns the fact that the nitrite ligand can be bonded to a metal ion via either the nit rogen or the oxygen atom. The electronic structure of the metal ion has a dominant influence on this bonding. The class a or hard acceptors profer the O atom while the soft acceptors prefer the N atom. Nitrogen could there fore be classified as considerably softer donor atom than oxygen. Ion-exchange is a widely used separation technique that depends on the differing affinities of species for the resin. Ion-exchange resins come in two major class es, cation exchange resins in which negatively charged groups such as carboxylate or sulphonate built into the resin bind cations CI) and anion exchange resins in which positively charge quaternary ammonium groups are built, into the resin C2). K* _ aCR - O H+) + ML2+ - -» CR - O ) MLs++sH CD VX + _ KR zCR - N CI >+ ML2+ " t (R - N+) ^MLz++zCl C2> The resin which, of course, must be neutral overall is initially provided with suitable counter ions such as protons or chloride ions, and these are displaced by the charged metal complex ions as shown in (1 > and (2). Determination of the stability constant of negati vely-charged or uncharged complexes can be carried out- according to Schubert and co-workers by means of a cati on exchange resin on the basis of the following theoreti cal considerations. Suppose that metal ion, M, may form with multivalent ligand ion, L, complexes of different composition ML, ML2, MLa, etc. but the resin binds only the positively charged ion,M, which is not bound as a complex. Moreover, suppose that distribution coefficient is independent of the concentration, i.e. the concentrati on of ion, M, is very low compared with the accompanying electrolyte. The values of the distribution coefficient in the absence and presence of complexing agent will then be as follows: (M) K° =, (3) d CM3 K = <4> CM3+EML3+CML ]+... 2 Considering that the formation constants are CML3 [ML 3 ft = ; ft = 2 etc.. (5) CM3 EL3 CM3 £L3 the following relation can be written: vxx V- ? = l+/5 CL3+/? [L32+... C6> K 1 2 d In the case when only one complex of MLn is formed, the stability constant of the complex ftr% can be calculated from measurable data according to the following relation: Kd ft = 2 (7) CL]n The value of K d should not be determined separately ' beca use it can be obtained by graphical extrapolation as fol lows. Determine the distribution coefficent of ion, M, in the presence of various amounts of complex -forming elect rolyte. Also add to the sample solution non-complexing indifferent electrolyte in order to ensure constancy of the ionic strength of the solution. Plot 1/Kd aganist. CL] on the basis of the measured data. If the line ob tained is extrapolated, the intercept at EL3 =0 gives 1/K d. The equation of the line is namely the following according to relation (7) : a n [L3n + <8> d d d The calculation can also be carried out in the case when several complexes of different composition are formed. Naturally, this presumes that none of them are bound on the resin or at least the extent of binding is not very great. If two complexes of composition ML and MLz are formed, the formation constants can be calculated on the basis of the following relation : vxxx J ^7 EL3 ft +ft ELD (9) 1 f 2 If K d is determined in the presence of the non-complex forming electrolyte and Kd in solutions containing vari ous amounts of L ions, but of equal ionic strength, then plotting the right-hand side of the equation from a know ledge of the measured data against the concentration ELD, the intercept of the obtained dissociated acid, then the degree of dissociation K can be calculated from a know ledge of the pH and concentration. In this case KEHLDo should be written instead of EL], where EHLDo is the to tal analytical concentration of dissociated and non-dis- socciated acid. In the investigations care should always be taken that the metal ion is present in very low con centration compared with the larger amount of electrolyte because only in this way can it be ensured that the dist ribution coefficent is independent of the concentration. Determination of the distribution coefficient of traces of metals is best carried out by radiochemical methods. Schubert and co-workers applied this method to the study and determination of stability constants of comp lexes of the alkalinbe earths, manganese, cobalt, zinc, copper, nickel and uranium ions with various organic a- cids (oxalic, succinic, citric, tartaric), hydroxy acids (glycollic, ascorbic) and amino acids (glycine). Other workers have investigated by a similar method complexes of iron (III) ions with sulphate ions, of zirconium and hafnium ions with tartaric acid, of radium ions with EDTA and of cerrium(III) and rare earth ions with citrate ions Feldmann and co-workers extended the method to the deter mination of three successive formation constants during a study of the complexes formed by berylliumdl ) ions with citrate ions. Beukenkamp and Herrington investigated the behaviour of iron(II) and titanium (IV) ions in perchloric acid and sulphiric acid solutions by means of a cation-exchange resin. They took into consideration, however, the bind ing of univalent and bivalent cations in determination of the formation constants of oxy and sulfate complexes. IX In the investigation Schubert's ion exchange method was applied on the copper-nitrite and nickel -nitrite sys tems. Both investigations were carried out at a constant ionic strength with sodium perchlorate âs a supplementary salt in the solution. The measurements were carried out in the following way. To 0.1 liters of the complex solutionCCM mM Me(C104>2, Cl mM NaN02, 1=1 with NaClCMO 0.1 grams of the dried cation exchanger were added. As an exchanger Dowex SOW, a sulfonated polystrene resin, was used particularly because its capcsity is independent of pH over a wide pH range. The solution was shaken with the exchanger for four hours at20 C and the solution was seperated from the exchangerand analysed. The metal ion concentrations of the solutions were determined by the atomic absorption method. In the copper-nitrit system, experiments were per formed at three different ionic strength, i.e., 0.0B, 0.15 and 0.25 respectively. Complex formation between nickel and nitrite ions was studied at an ionic strenght of 0.10. In the concentrations range studied, Cu and Ni form complexes with the ratio 1:2. Values of constants for the formation of Cu(N02>2 are log/?2=2.64, logftz=2. 43, log/52=2.27, at 1=0.05, 1=0.15 and 1=0.25 respectively The nickel -nitrite complex, Ni (N02>2, has the stability constant value, log/"52=l.S6 at 1=0.10.