Mercury/EDTA electrode

Mercury(II) chloranilate 700 Mercury(II) nitrate standard soln. of, 359 Mercury/mercury( II )-EDTA electrode (mercury electrode) 586 potentiometric titration of metallic ions with EDTA and, 588 prepn. of, 587 Mercury thiocyanate 700 Metaphosphoric acid in homogeneous precipitation, 426 Metal apparatus 93 Metal ion buffer 53... 

Oxidation of [Ruin (edta)(OH2)] by chlorate ion gives the green mixed valence, fi-oxo dimer [(edta)RuivORuin(edta)]3- which can be reduced at a mercury electrode to [(edta)RuHIORum (edta)]4- facile decomposition of the latter to regenerate [RuIU(edta)(OH2)]" then occurs.2542 A preliminary report on the synthesis of (Et4N)3 [LRu( -OH)(p-02)RuL] (L = edta, Hedta) has also been published.2543 The binuclear compound rra s-[Ru(NH3)4(NCS)LRu(edta)] (L = 4-vinylpy-ridine) has recently been synthesized.254215... 

Next consider in more detail a specific example of the use of the mercury-EDTA electrode in the titration of calcium with standard EDTA. In such a titration a small amount of a solution containing mercury-EDTA complex HgY" is added to the calcium solution, and the indicating electrode is a mercury-coated gold wire. The following equilibria are involved ... 

A silver indicating electrode can be used in a manner analogous to the mercury electrode for EDTA titrations. In contrast to the mercury electrode, the success of the silver electrode depends on the formation of a silver-EDTA complex that is weak compared with virtually all other metal ions. A trace of silver(I) is added, and after the EDTA has reacted with other metal ions, the first excess of EDTA reacts with silver and sharply decreases the potential of the silver electrode. Owing to the low stability constant of the silver-EDTA complex, titrations must be carried out at high pH. 

Figure 13-8 Titration of with EDTA as a function of pH. The experimental ordinate is the voltage difference between two electrodes (mercury and calomel) immersed in the titration solution. This voltage is a measure of log[Ca "]. [From C. N. Reilley and R. w. Schmid, Anal. Chem. 1958. 30, 947.1...

In a similar manner, in a solution containing the species Hg2+, HgY2-, MY,n 4)+ and M"+, where Y is the complexing agent EDTA and M"+ is a metallic ion which forms complexes with it, the concentration of the mercury ion is controlled by the stability constants of the complex ions MYhigh stability constant), and the concentration of the metal ions M"+. Hence, a mercury electrode placed in this solution will acquire a potential which is determined by the concentration of the ion M"+. 

For complexation titrations involving the use of EDTA, an indicator electrode can be set up by using a mercury electrode in the presence of mercury (II) EDT A complex (see Section 15.24). 

POTENTIOMETRIG EDTA TITRATIONS WITH THE MERCURY ELECTRODE... 

Discussion. The indicator electrode employed is a mercury-mercury(II)-EDTA complex electrode. A mercury electrode in contact with a solution containing metal ions M"+ (to be titrated) and a small added quantity of a mercury(II)-EDTA complex HgY2- (EDTA = Na2H2 Y) exhibits a potential corresponding to the half-cell ... 

Muzzarelli and Sipos [622] showed that a column of chitosan (15 x 10 mm) can be used to concentrate zinc from 3 litres of seawater before determination by anodic-stripping voltammetry with a composite mercury-graphite electrode. Zinc (and lead) are eluted from the column by 2 M ammonium acetate (50 ml), copper by 0.01 M EDTA (10 ml), and cadmium by 0.1 M potassium cyanide (3 ml). 

Potentiometric EDTA titrations are best carried out with a mercury pool electrode (Figure 5.6) or a gold amalgam electrode. When this electrode dips into a solution containing the analyte together with a small amount of added Hg-EDTA complex, three interdependent reactions occur. For example, at pH = 8 the half cell reaction (a) which determines the electrode potential is related to the solution equilibrium by (b) and (c). 

In overall form this equation resembles that for the glass electrode (Chapter 6) and a pM-EDTA curve resembles an acid-base titration curve. The mercury electrode is most usefully employed when coloured or turbid solutions are being titrated, or when dilute solutions and weak complexes lead to poor colour changes. 

The magnesium will be liberated quantitatively and may then be titrated with a standard EDTA solution. Where mixtures of metal ions are analysed, the masking procedures already discussed can be utilized or the pH effect exploited. A mixture containing bismuth, cadmium and calcium might be analysed by first titrating the bismuth at pH = 1-2 followed by the titration of cadmium at an adjusted pH = 4 and finally calcium at pH = 8. Titrations of this complexity would be most conveniently carried out potentiometrically using the mercury pool electrode. 

The effect of ligands on the character and degree of the inner-sphere reorganization during electroreduction of aqua-, aquahydroxy-, hydroxy-, and ethylene-diamine tetraacetic acid (EDTA) complexes of Zn(II) [95] and electrochemical process of Zn(II) complexed by different ligands - glycinate [96], ethanol amine [97], azinyl methyl ketoximes [98], aspartame [99], glutathione [100, 101] and several cephalosporin antibiotics [102] -were studied at mercury electrodes in aqueous solutions. 

The most common technique to detect the end point in EDTA titrations is to use a metal ion indicator. Alternatives include a mercury electrode (Figure 12-9 and Exercise 15-B) and an ion-selective electrode (Section 15-6). A pH electrode will follow the course of the titration in unbuffered solution, because H2Y2 releases 2H+ when it forms a metal complex. 

For end-point detection, we commonly use metal ion indicators, a glass electrode, an ion-selective electrode, or a mercury electrode. When a direct titration is not suitable, because the analyte is unstable, reacts slowly with EDTA, or has no suitable indicator, a back titration of excess EDTA or a displacement titration of Mg(EDTA)2- may be feasible. Masking prevents interference by unwanted species. Indirect EDTA titrations are available for many anions and other species that do not react directly with the reagent. 

Again, we deal with the simple redox reaction 0 + ne = R. In the present discussion we suppose that 0 is a metal ion and R is the corresponding metal soluble as an amalgam in a mercury electrode or present as the pure metal electrode. It is highly probable that 0 forms several complexes e.g. with the anions of the supporting electrolyte, additives like NH3, EDTA, etc. or OH" ions. For the sake of simplicity, we confine ourselves here to considering the equilibrium between hydrated 0 and only one complex... 

Both Rum(EDTA) and Rum(HEDTA) (HEDTA = 7V-2-hydroxyethylenediaminetriacetato-) are adsorbed at mercury electrodes, but if a ligand is present which can bind the labile sixth coordination position, adsorption is reduced. Thiocyanate is effective in reducing the quantity of complex adsorbed, possibly due to the formation of a thiocyanato complex.93... 

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