Displacement titrations, with EDTA

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. 

The displaced metallic ion Mn is titrated with EDTA. Figure 29.4 schematizes the principle. 

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. 

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. 

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. 

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. 

Hg2+ does not have a satisfactory indicator, but a displacement titration is feasible. Hg2+ is treated with excess Mg(EDTA)2- to displace Mg2+, which is titrated with standard EDTA. 

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. 

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. 

Eriochrome Black T is a typical indicator. It contains three ionizable protons, so we will represent it by Hsin. This indicator can be used for the titration of Mg + with EDTA. A small amount of indicator is added to the sample solution, and it forms a red complex with pait. pf the Mg " the color of the uncomplexed indicator is blue. As soon as all the free Mg. is titrated, the EDTA displaces the iudicator from the magnesium, causing a change in the color from red to blue ... 

An alternative method is to add a small amount of Mg " to the EDTA solution. This immediately reacts with EDTA to form MgY with very little free Mg " in equilibrium. This, in effect, reduces the molarity of the EDTA. So the EDTA is standardized after adding the Mg " by titrating primary standard calcium carbonate (dissolved in HCl and pH adjusted). When the indicator is added to the calcium solution, it is pale red. But as soon as the titration is started, the indicator is complexed by the magnesiiim and turns wine red. At the end point, it changes to blue, as the indicator is displaced from the magnesium. No correction is required for the Mg added because it is accounted for in the standardization. This solution should not be used to titrate metals other than calcium. 

A mixture of Mn, Mg, and Zn was analyzed as follows The 25.00-mL sample was treated with 0.25 g of NH30H Cr (hydroxylammonium chloride, a reducing agent that maintains manganese in the +2 state), 10 mL of ammonia buffer (pH 10), and a few drops of Calmagite indicator and then diluted to 100 mL. It was warmed to 40°C and titrated with 39.98 mL of 0.045 00 M EDTA to the blue end point. Then 2.5 g of NaF were added to displace Mg " from its EDTA complex. The liberated EDTA required 10.26 mL of standard 0.020 65 M Mn for complete titration. After this second end point was reached, 5 mL of 15 wt% aqueous KCN were added to displace Zn " from its EDTA complex. This time the liberated EDTA required 15.47 mL of standard 0.020 65 M Mn. Calculate the number of milligrams of each metal (Mn, Zn, and Mg " ) in the 25.00-mL sample of unknown. 

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. 

According to the principle of these titrations, the metallic ion to be titrated Mi displaces a metallic ion Mu from its complex already formed with EDTA, namely, [MiiY] ... 

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). 

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... 

Antiserum 187 inhibited photophosphorylation with PMS in control thylakoids completely in a hyperbolic manner. The inhibition was not even partially neutralized by preincubation of the antiserum with an excess of isolated CF. The inhibition was not accompanied by displacement of CF from its binding site, since no CF could be detected in the supernatants by rocket electroimmunodiffusion analysis. The antiserum retarded the decay of the light induced pH rise after illumination in control thylakoids, and stimulated the extent of H uptake in EDTA-treated thylakoids almost up to the control, whereas H uptake in control thylakoids was uneffected.The photophosphorylation in EDTA-treated thylakoids with a high amount of residual CF ( about 75%) was inhibited too, but in contrast to control thylakoids the titration curve was biphasic. 

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