COLLOID-CHEMICAL STUDIES ON HEAVY METAL PHOSPHATES

Abstract

CC are known to give complexes with metal ions. Their structures have, however, not been fully elucidated and uncertainties still exist. Many factors contribute to this state of uncertainty. These are (i) the dubious nature of the polyphosphate ion, e.g., its existence as R,P2oI2 or P?0^4 in solutions of pyrophosphorlc acid or as polypyrophosphate, Pn 03n+1 , with n equal to 2 in sodium pyrophosphate solution? (ii) possible polymerisation; (iii) hydrolysis and adsorption of the reacting species on the reaction products; and (iv) colloidal nature of the complexes. It is thus evident that investigation on the complexes of pyrophosphate and other polyphosphates are not as simpler as in the case of the other metal complexes. 1—2 Recently books and reviews have appeared on the composition, physico-chemical behaviour, and structural characteristics of these complexes. Before presenting the purpose and scope of the present studies, it will be worthwhile to give a brief survey of the literature relevant to the problem under investigation. The role of chain phosphates in modifying the physical behaviour of their metal complexes is recognised since long. Soon after the discovery of different types of phosphates3"*5, several reactions of metal ions with them were extensively investigated. In many cases it was found that these reactions either prevent precipitation or redissolve the reaction product when added in excess. Later investiga tions9 went to show that addition of excess of the phosphates to the precipitates resulted in the formation of soluble complexes through either ion association and covalent bonding. Rapid development in the field of metal phosphates could, however, took place in recent years after the discovery in the late thirties of the fact that these could be used for the softening of water10. More critical studies in later years finally settled the controversy regarding their structure and the chain-phosphates are now classed as typical polyelectrolytes The phosphate family of the compounds is a very large one12"13. By definition the phosphates are those structures in which the anion consists of P04 tetrahedra which may be linked together by sharing of comers. The -3 simplest phosphates correspond to the isolated P04 ion, known as the orthophosphate ion. Then there are the homologous series of chain and ring phosphates in which P04 groups are held together through P-O-P linkages. The orthophosphate is considered as the first member of the chain series, with the pyrophosphate being the second member of the series. The dissociation of the hydrogen and sodium complexes of the pyrophosphates has been a subject of detailed study by a large number of workers. Investigations carried out by Monk and Co-workers14'16 in this direction need special mention. Using conductivity data he reported the following values for the four dissociation constants15 (pKD) in the case of pyrophosphoric acid dilute solutions (1 to 25xlO~4M) i 1.0, 2.0, 6.57 and 9.6. Corresponding data on the two pKD valued for Na4P207 were given as 2.352 and 2.52. Examples are not lacking of metal complexes in which the composition determined by chemical analysis differs from that determined by the help of physico-chemical methods. This is particularly true to metal phosphate complexes in which the precipitates readily undergo dissolution by adding excess of the ligand. Early work on complex pyrophosphates takes into account the results of chemical analysis on the isolated 17 compounds dried at elevated temperatures . This naturally led to dubious conclusions in view of the change in composition due to factors like adsorption, hydrolysis etc. Bassett, Bedwell and Hutchinson18 repeated the previous work and prepared salts, which for the most part gave the same analysis 19 as the salts obtained by Rosenheim and co-workers . In reporting their results, the investigators assumed that the original metal cation was present in the precipitate as complex anion. Sodium pyrophosphate in solution changes into other phosphates and thus is unstable but pyrophosphoric acid when dissolves into a solvent, it does not change to other species and hence its solution remains stable. If the studies are made with sodium pyrophosphate solution, then conflicting results might be achieved. 9 2 Rogers and Reynolds reported conductometric titrations of 4x10*4M solutions of sodium pyrophosphate with 0.1M solutions of such metal ions as calcium, cobalt, nickel, magnesium, aluminium, ferric iron, copper and zinc. The maximum resistance for solutions of the divalent metal ions all occurred at molar ratios of metal to pyrophosphate ranging from 0.93 to 0.97, and. except for cadmium ion, precipitation was said not to occur until after the resistance peak was passed. The resistance peak shifted with concentra tion in the case of copper so that, at high concentrations, the peak corresponds to two pyrophosphates complexing the copper ion, and at low concentrations one pyrophosphate per copper ion. Three inflexion points were observed with ferric iron corresponding to mole ratios of pyrophosphate to metal ion of 2:1, ltl and 0.75tl (noticeable precipitation at this point). A similar study involving thermometric and trans ference studies as well as conductometric titrations was 23 made by Haldar on pyrophosphate-copper complexes. 22 Rogers and Reynolds observed that the cuprous and ferric reductions were reversible in sodium pyrophosphate solution. The two polarographic reduction waves attributed to the step-wise reduction of the cupric ion were later 24 shown by Laitinen and Onstott to be due to the presence of two complexes of cupric ion,the 2il and the 1:1 pyrophosphate to metal species. The pKD*S for the dissociation of the 4- - [Cu(HP 07)2~1 and the (CuHP207) ions to the free metal ion plus anion were given as 10.0 and 6.40 respectively. The manganese pyrophosphate system has been 25 studied polarographically by Kolthoff and Watters who presented evidence for a 3tl pyrophosphate to metal complex stable at pH values 4.3 and below. The molecular weight of the complex ion was estimated by application of Jander's expression26 to the polarographically determined diffusion coefficients. Titrations of pyrophosphate (sodium pyrophosphate) with copper, nickel, cobalt, iron were followed by pH measurements in a study again reported by Rogers and Reynolds . The observed change in pH are closely tied in with precipitate formation but are not directly interpretable in terms of the formation of soluble complex ions. Watters and Aaron have published an extensive spectr©photometric investigation of the copper complexes of pyrophosphate, including studies in the presence of other complexing agents such as ethylenediamine and ammonia. The concentration range of the metal ions studied was considerably higher than that used by Laitinen and Onstott in the polarographic investigation of the same system. The reactions are dependent on hydrogen ion concentration; and whereas Laitinen and Onstott24 could not interpret their data for pH values above 5, evidence for li2 and 111 complexes of metal to pyrophosphate in the pH range 5 to 10 is presented and for 2:1 and 4tl complexes in more dilute solutions.