EDTA titrations displacement

If the analyte metal ion forms a stable EDTA complex rapidly, and an end point can be readily detected, a direct titration procedure may be employed. More than thirty metal ions may be so determined. Where the analyte is partially precipitated under the reaction conditions thereby leading to a slow reaction, or where a suitable indicator cannot be found, back titration procedures are used. A measured excess of EDTA is added and the unreacted EDTA titrated with a standard magnesium or calcium solution. Provided the analyte complex is stronger than the Ca-EDTA or Mg-EDTA complex a satisfactory end point may be obtained with eriochrome black T as indicator. An alternative procedure, where end points are difficult to observe, is to use a displacement reaction. In this case, a measured excess of EDTA is added as its zinc or magnesium complex. Provided the analyte complex is the stronger, the analyte will displace the zinc or magnesium. 

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 binding of citrate by 4.28 has been investigated by the indicator displacement assay (IDA) method (cf. EDTA titrations, Section 3.1.3). Binding of an organic dye indicator with comparable structure to citrate competes with the substrate for the same binding site. The binding of the indicator... 

While displacement is the more correct term, the best known application is that of softening.. A cation resin in the sodium form is used to soften tap water. The residual hardness is tested by EDTA titration or standard soap solution. 

For some metal ions, there is no satisfactory indicator, but a displacement titration is feasible. In this procedure, analyte is usually treated with excess Mg(EDTA) to displace Mg, which is later titrated with standard EDTA. 

Three general methods for performing EDTA titrations are (1) direct titration, (2) back titration, and (3) displacement titration. Method (1) is simple, rapid, but requires one standard reagent. Method (2) is advantageous for those metals that react so slowly with EDTA as to make direct titration inconvenient. In addition, this procedure is useful for cations for which satisfactory indicators are not available. Finally, it is useful for analyzing samples that contain anions that form sparingly soluble precipitates with the analyte under analytical conditions. Method (3) is particularly useful in situations where no satisfactory indicators are available for direct titration. 

Ca + if a small amount of Mg -EDTA is added to the solution containing the analyte. The Mg + is displaced from the EDTA by Ca +, freeing the Mg + to form the red Mg -indicator complex. After all the Ca + has been titrated, Mg + is displaced from the Mg -indicator complex by EDTA, signaling the end point by the presence of the uncomplexed indicator s blue form. 

An interesting application is the titration of calcium. In the direct titration of calcium ions, solochrome black gives a poor end point if magnesium is present, it is displaced from its EDTA complex by calcium and an improved end point results (compare Section 10.51). 

This reaction will proceed if the metal indicator complex M-In is less stable than the metal-EDTA complex M EDTA. The former dissociates to a limited extent, and during the titration the free metal ions are progressively complexed by the EDTA until ultimately the metal is displaced from the complex M-In to leave the free indicator (In). The stability of the metal-indicator complex may be expressed in terms of the formation constant (or indicator constant) Ku ... 

Mixtures of manganese, magnesium, and zinc can be similarly analysed. The first EDTA end point gives the sum of the three ions. Fluoride ion is added and the EDTA liberated from the magnesium-EDTA complex is titrated with manganese ion as detailed above. Following the second end point cyanide ion is added to displace zinc from its EDTA chelate and to form the stable cyanozincate complex [Zn(CN)4]2- the liberated EDTA (equivalent to the zinc) is titrated with standard manganese-ion solution. 

In the back-titration small amounts of copper and zinc and trace amounts of manganese are quantitatively displaced from the EDTA and are complexed by the triethanolamine small quantities of cobalt are converted into a triethanolamine complex during the titration. Relatively high concentrations of copper can be masked in the alkaline medium by the addition of thioglycollic acid until colourless. Manganese, if present in quantities of more than 1 mg, may be oxidised by air and forms a manganese(III)-triethanolamine complex, which is intensely green in colour this does not occur if a little hydroxylammonium chloride solution is added. 

A mixture of tin(IV) and lead(II) ions may be complexed by adding an excess of standard EDTA solution, the excess EDTA being determined by titration with a standard solution of lead nitrate the total lead-plus-tin content of the solution is thus determined. Sodium fluoride is then added and this displaces the EDTA from the tin(IV)-EDTA complex the liberated EDTA is determined by titration with a standard lead solution. 

At the start of the experiment, a small amount of indicator (In) is added to the colorless solution of Mg2+ to form a red complex. As EDTA is added, it reacts first with free, colorless Mg2+. When free Mg2+ is used up, the last EDTA added before the equivalence point displaces indicator from the red Mgln complex. The change from the red Mgln to blue unbound In signals the end point of the titration (Demonstration 12-1). 

In a back titration, a known excess of EDTA is added to the analyte. Excess EDTA is then titrated with a standard solution of a second metal ion. A back titration is necessary if analyte precipitates in the absence of EDTA, if it reacts too slowly with EDTA, or if it blocks the indicator. The metal ion for the back titration must not displace analyte from EDTA. 

M C11SO4 before titration (left). Color of Cu(II)-ammonia complex after adding ammonia buffer, pH 10 teenier). End-point color when all ammonia ligands have been displaced by EDTA (right). 

In these cases a back titration is required. This involves addition of a known excess of EDTA to the metal ion (buffered to an appropriate pH). Then, the excess EDTA is titrated with a standard solution of a different metal ion. The choice of a second metal ion is important as it must not displace the analyte metal ion from its EDTA complex. 

Puschel and Stefanac ° use alkaline hydrogen peroxide in the oxygen flask method to oxidize arsenic to arsenate. The arsenate is titrated directly with standard lead nitrate solution with 4-(2-pyridylazo) resorcinol or 8-hydroxy-7-(4-sulpho-l-naphthylazo) quino-line-5-sulphonic acid as indicator. Phosphorus interferes in this method. The precision at the 99% confidence limit is within 0.67% for a 3-mg sample. In another variation, Stefanac used sodium acetate as the absorbing liquid, and arsenite and arsenate are precipitated with silver nitrate. The precipitate is dissolved in potassium nickel cyanide (K2Ni(CN)4) solution and the displaced nickel is titrated with EDTA solution, with murexide as indicator. The average error is within + 0.19% for a 3-mg sample. Halogens and phosphate interfere in the procedure. 

In displacement titrations, an unmeasured excess of a solution containing the magnesium or zinc complex of EDTA is introduced into the analyte solution. If the analyte forms a more stable complex than that of magnesium or zinc, the following displacement reaction occurs ... 

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