Mercury EDTA titration

Procedures have also been devised for the determination of metallic constituents. Thus, mercury is absorbed in nitric acid and titrated with sodium diethyldithiocarbamate, whilst zinc is absorbed in hydrochloric acid and determined by an EDTA titration (see Section 10.65). 

POTENTIOMETRIG EDTA TITRATIONS WITH THE MERCURY ELECTRODE... 

Mercury-EDTA solution. Mix small equal volumes of 0.05M mercury(II) nitrate and 0.05 M EDTA neutralise the liberated acid by the addition of a few drops of 3M ammonia solution. (In acid solution an insoluble precipitate, probably HgH2Y, forms after a few days.) Dilute 10.0 mL of this solution to 100 mL with distilled water. The resulting ca 0.0025 M mercury-EDTA solution is used for most titrations. 

Chromium. 25.0 mL chromium(III) ion solution (0.02M, prepared by dilution of stock solution) + 50.0 mL 0.02 M EDTA + 50mL acetate buffer, boiled for 10 minutes, solution cooled, pH adjusted to 4.6 with hexamine, 1 drop of mercury-EDTA solution added, and then back-titrated with standard zinc ion solution. 

Aluminium. 25.0 mL aluminium ion solution, acidified with a few drops of 2.5M nitric add (to pH 1-2), boiled for 1 minute, 50.0 mL 0.05M EDTA added to hot solution, solution cooled, 50 mL acetate buffer and 1 drop of 0.0025 M mercury-EDTA added, excess of EDTA back-titrated with standard zinc ion solution. 

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). 

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. 

The number of reversible metal-metal ion electrodes is limited so that the accurate direct potentiometric measurement of the activity of a metal ion with an electrode of the same metal usually is not feasible, except perhaps with the Ag/Ag,(OH2)4 system. However, a number of metal ion-metal half-reactions are sufficiently reversible to give a satisfactory potentiometric titration with a precipitation ion or complexing agent. These couples include Cuu(OH2>6+/Cu, Pbn(OH2>4+/Pb, Cdu(OH2)l+/Cd, and Znn(OH2)i+/Zn. However, all these metals can be determined by EDTA titration and the mercury electrode that is described in the preceding section. 

Figure 5.6. A J-type mercury pool electrode EDTA Titration Procedures...

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. 

Potentiometric Methods Potential measurements can be used for end point detection in the EDTA titration of those metal ions for which specific ion electrodes are available. Electrodes of this type are described in Section 21D-1. In addition, a mercury electrode can be made sensitive to EDTA ions and used in titrations with this reagent. 

The mercury electrode is thus a valuable electrode of the second kind for EDTA titrations, as discussed in Section 21G-2. 

Both metallic and membrane electrodes have been used to detect end points in potentiometric titrations involving complex formation. Mercury electrodes are useful for EDTA titrations of cations that form complexes that are less stable than HgY- . See Section 21D-1 for the half-reactions involved and Equation 21-5 for the Nernst expression describing the behavior of the electrode. Hanging mercury drop and thin mercury film electrodes appropriate for EDTA titrations are available from a number of manufacturers. As always, whenever mercury is used in experiments like these, we must take every precaution to avoid spilling it, and it must be stored in a well-ventilated hood or a special cabinet to remove the toxic vapors of the liquid metal. Before working with mercury, be sure to read its Materials Safety Data Sheet (MSDS), and follow all appropriate safety procedures. 

The most sensitive - and perhaps for the radiochemist, the most useful volumetric procedure is complexlmetrlc titration utilizing the lead EDTA complex. A number of Indicators have been used for the direct EDTA titration (W5) The most popular of these are Erlochrome Black T (Cl) (W5)j Eriochrome Red B and X-ylenol orange (W5)- The direct titration with the sodium salt of EDTA is carried out in a pH 10 buffer solution (P5)(p6). Iron, the alkaline earths and the earths interfere but bismuth, aluminum and antimony do not. Cyanide can be used to mask cobalt, nickel, copper, zinc, cadmium, mercury and platinum (Cl),... 

The official method for mercury is the modified Rupp method described earlier but gravimetric estimation as sulphide (HgS X T167 = HgClg) or the EDTA titration may be used. 

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). 

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 ... 

Bromides, D. of as silver bromide, (g) 491 by EDTA, (ti) 339 by mercury(I), (cm) 542 by oxygen flask, 113 by silver ion, (cm) 546 by silver nitrate, (ti) 351 by Volhard s method, (ti) 356 with iodide, (ti) 352 4-Bromomandelic acid 473 Bromophenol blue 265, 267 Bromopyrogallol red 182, 319 Bronsted-Lowry bases titration with strong acids, 277... 

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... 

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. 

Figure 12-9 Titration of Ca2+ with EDTA as a function of pH. As the pH is lowered, the end point becomes less distinct. The potential was measured with mercury and calomel electrodes, as described in Exercise 15-6 in Chapter 15. [C. N. Reilley and R. W. Schmid, "Chelometric Titration with Potentiometric End Point Detection Mercury as a pM Indicator Electrode Anal. Chem. 1958, 30. 947.]...

EDTA). In this application the M(OH2)l+ cation that is to be titrated forms a soluble EDTA complex that is appreciably less stable than the HgEDTA complex.141 The mercury electrode is constructed in a form like that of Figure 5.40a or b, and is used in conjunction with a saturated calomel reference electrode. The electrode is immersed in a solution that contains the ion to be titrated [M(OH2) +], and a few drops of 0.01 M HgY2- are added (where Y4-represents the EDTA anion). This establishes the potential of the mercury electrode according to the half-cell reaction... 

During the course of the titration the concentration of HgY2- remains essentially constant because it is so much more stable than the MY2- complex. The is determined mainly by the ratio [Afu(OH2)2+]/[MY2 ], which changes slowly in the middle of the titration but rapidly near the equivalence point as the concentration of Mn(OH2)2+ drops to a small value. This gives a sharp potential change that signals the endpoint. The method is general and can be applied to most cations that form soluble EDTA complexes that are appreciably less stable than the mercury(II)-EDTA complex. 

Reilley and Schmid made the important observation that the mercury-mercury (II)-EDTA electrode can be used indirectly as an indicator electrode for various metal ions, and this method was applied to the determination of 29 different metal ions by either direct- or back-titration procedures. Under some conditions,such as when a high concentration of buffer is present, formation of a mercury(I) precipitate or a complex with mercury(II) may result in incorrect end points. 

---