EDTA titrations direct

Manganese(II) can be titrated directly to Mn(III) using hexacyanoferrate(III) as the oxidant. Alternatively, Mn(III), prepared by oxidation of the Mn(II)-EDTA complex with lead dioxide, can be determined by titration with standard iron(II) sulfate. 

A. Direct titration. The solution containing the metal ion to be determined is buffered to the desired pH (e.g. to PH = 10 with NH4-aq. NH3) and titrated directly with the standard EDTA solution. It may be necessary to prevent precipitation of the hydroxide of the metal (or a basic salt) by the addition of some auxiliary complexing agent, such as tartrate or citrate or triethanolamine. At the equivalence point the magnitude of the concentration of the metal ion being determined decreases abruptly. This is generally determined by the change in colour of a metal indicator or by amperometric, spectrophotometric, or potentiometric methods. 

B. Back-titration. Many metals cannot, for various reasons, be titrated directly thus they may precipitate from the solution in the pH range necessary for the titration, or they may form inert complexes, or a suitable metal indicator is not available. In such cases an excess of standard EDTA solution is added, the resulting solution is buffered to the desired pH, and the excess of the EDTA is back-titrated with a standard metal ion solution a solution of zinc chloride or sulphate or of magnesium chloride or sulphate is often used for this purpose. The end point is detected with the aid of the metal indicator which responds to the zinc or magnesium ions introduced in the back-tit ration. 

Murexide may be employed for the direct EDTA titration of calcium at pH =11 the colour change at the end-point is from red to blue-violet, but is far from ideal. The colour change in the direct titration of nickel at pH 10-11 is from yellow to blue-violet. 

Cu, Ni, Co, Cr, Fe, or Al, even in traces, must be absent when conducting a direct titration of the other metals listed above if the metal ion to be titrated does not react with the cyanide ion or with triethanolamine, these substances can be used as masking reagents. It has been stated that the addition of 0.5-1 mL of 0.001 M o-phenanthroline prior to the EDTA titration eliminates the blocking effect of these metals with solochrome black and also with xylenol orange (see below). 

Direct EDTA titrations of Bi, Th, Zn, Cd, Pb, Co, etc., are readily carried out and the colour change is sharp. Iron(III) and, to a lesser extent, aluminium interfere. By appropriate pH adjustment certain pairs of metals may be titrated successfully in a single sample solution. Thus bismuth may be titrated at pH =1-2, and zinc or lead after adjustment to pH = 5 by addition of hexamine. 

Discussion. In mixtures of magnesium and manganese the sum of both ion concentrations may be determined by direct EDTA titration. Fluoride ion will demask magnesium selectively from its EDTA complex, and if excess of a standard solution of manganese ion is also added, the following reaction occurs at room temperature ... 

Potentiometric titrations - continued EDTA titrations, 586 neutralisation reactions, 578, 580 non-aqueous titrations, 589, (T) 590 oxidation-reduction reactions, 579, 581, 584 precipitation reactions, 579, 582 Potentiometry 548 direct, 548, 567 fluoride, D. of, 570 Potentiostats 510, 607 Precipitants organic, 437 Precipitate ageing of, 423 digestion of, 423... 

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. 

Chlorpromazine formed an insoluble 1 1 complex with lead picrate, and 5 3 complexes with the picrates of cadmium, copper, and zinc [70]. The sample (0.1 g) was dissolved in 15 mL of 95% ethanol, and the solution adjusted to pH 9 with 0.1 N NaOH. After adding 25 mL of a 0.02 M picrate reagent (30 mL of Pb), the solution was set aside for 2 hours. The precipitate was collected on a sintered glass fuimel, and the unconsumed metal in the filtrate was titrated directly with 0.02M EDTA at pH 10.4 (after adding 0.5 g of potassium sodium tartrate for Pb). Eriochrome black T was used as the indicator. 

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. 

It might at first seem that the direct measurement of electrode potential would, by analogy to pH measurement, afford a simple means for determining pM during the course of EDTA titrations. Unfortunately, many metal electrodes do not behave reversibly, particularly at the extremely low metal ion concentrations involved near... 

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. 

Alkaline Earths. 1.0 M stock solutions of the alkaline earth metal chlorides were prepared by dissolving the chloride salt in D2O (99.9% D). Standardization was either by direct EDTA titration (for CaCl2) or back titration of the metal-EDTA complex with standard M (C104)2 using Eriochrome Black T indicator. The solutions were buffered to pH 10 with NH3/NH4CI and the determinations were done in triplicate. 

Another advantage of potentiometric titrations is that substances to which the electrode does not respond can be determined, if the electrode responds to the titrant or to some low level of an indicator substance that has been added to the solution. For example, low levels of Al can be determined by titration with standard fluoride solution, using a fluoride electrode [22]. EDTA and other chelates can be determined by titration with standard calcium or copper solution. Manganese(II), vanadium(II), or cobalt(II) can be determined via EDTA titration if a small amount of CuEDTA indicator is added to the solution and a copper electrode is used. The electrode responds directly to the Cu activity which, however, is dependent on the activities of the EDTA and the other metal ion in solution. 

For an indicator to be useful in an EDTA titration, the indicator must give up its metal ion to EDTA. If a metal ion does not freely dissociate from the indicator, the metal is said to block the indicator. Calmagite is blocked by Cu, Ni, Co, Cr, Fe, and Al ". It cannot be used for the direct titration of any of these metals. However, it can be used for a back titration. For example, excess standard EDTA can be added to Cu ". Then indicator is added and excess EDTA is back titrated with Mg ". ... 

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),... 

Here, x is the part of the complex that dissociates at equilibrium. The pH dependence is contained in the aoy term (Figure 9-5). We can calculate x, and from it, the fraction of complex formed (Table 9-3). Figure 9-6 is plotted in this way. One can see the reason that EDTA titration of Zn is directed at pH above 5, where the reaction is quantitative. 

By using this equation the uptake of various metal ion by resin can be calculated and expressed in terms of milli equivalents per gram of the copolymer. Table 3. Data of experimental procedure for direct EDTA titration. ... 

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. 

The success of EDTA titrations must undoubtedly be ascribed to the stability of the complexes it gives with metallic ions, to the versatility of the reaction, and also to the use of metal indicators to detect their equivalence point. In this chapter, we describe some of these indicators briefly. From another standpoint, there are other kinds of EDTA titfations than the direct ones. We examine these possibilities here. We are also interested in the possibility they offer to determine some anions. Finally, we ll examine the properties of other chelating agents whose structures are close to that of EDTA. 

Injection of Calcium Gluconate, B.P. Contains about 10 per cent of calcium gluconate but up to 5 per cent of it may be replaced by a suitable calcium salt as stabiliser. The calcium content can be determined by the direct EDTA titration method given above. 

Chelating Titrants. [k) EDTA. Ferric iron forms a very stable complex with EDTA and may be titrated directly at a pH of about 2 and a temperature of about 50". Tiron (blue to colourless), salicylic acid (violet to colourless) or thiocyanate (red to colourless) are all suitable indicators, although in the presence of a high concentration of iron the iron-EDTA complex is itself strongly coloured and will modify the end-point. In practice, so many other excellent titrants exist for iron that this method is unlikely to find wide application. 

Lead may be titrated directly with EDTA at a pH of about 5 or 6 using xylenol orange as indicator. Under suitable conditions the end-point is very sharp but with materials such as lead monoxide where an excess of acid must be used to bring about solution, a rather sluggish end-point is obtained. Examples of the use of EDTA for titration of lead are given in the discussion of lead salts below. 

Some combination between zinc oxide and salicylic acid may occur in the presence of moisture and low results are obtained if the salicylic acid is titrated directly. Mitchell recommends extraction of the salicylic acid with ether after acidification and titration after evaporation of the solvent this is the method employed by the B.P.C, The zinc oxide may be determined by titration of the aqueous solutions with standard acid in the presence of ammonium chloride or by EDTA (see p. 696). 

We determined the cation ratio by EDTA titration using Xylenol Orange as the indicator. The pH was buffered to the 5.0-6.5 range using sodium acetate/acetic acid to optimize indicator effectiveness. Each crystal was weighed and dissolved in dilute HCl from which two aliquots were taken. One was treated directly with EDTA to obtain the total metal concentration according to the reaction... 

BackTitrations. In the performance of aback titration, a known, but excess quantity of EDTA or other chelon is added, the pH is now properly adjusted, and the excess of the chelon is titrated with a suitable standard metal salt solution. Back titration procedures are especially useful when the metal ion to be determined cannot be kept in solution under the titration conditions or where the reaction of the metal ion with the chelon occurs too slowly to permit a direct titration, as in the titration of chromium(III) with EDTA. Back titration procedures sometimes permit a metal ion to be determined by the use of a metal indicator that is blocked by that ion in a direct titration. Eor example, nickel, cobalt, or aluminum form such stable complexes with Eriochrome Black T that the direct titration would fail. However, if an excess of EDTA is added before the indicator, no blocking occurs in the back titration with a magnesium or zinc salt solution. These metal ion titrants are chosen because they form EDTA complexes of relatively low stability, thereby avoiding the possible titration of EDTA bound by the sample metal ion. 

Standard EDTA Solutions. Disodium dihydrogen ethylenediaminetetraacetate dihydrate is available commercially of analytical reagent purity. After drying at 80°C for at least 24 hr, its composition agrees exactly with the dihydrate formula (molecular weight 372.25). It may be weighed directly. If an additional check on the concentration is required, it may be standardized by titration with nearly neutralized zinc chloride or zinc sulfate solution. 

Unfortunately, it often happens that there is no suitable indicator for this direct titration. Reacting Ca + with an excess of the Mg -EDTA complex... 

Selection and Standardization of Titrants EDTA is a versatile titrant that can be used for the analysis of virtually all metal ions. Although EDTA is the most commonly employed titrant for complexation titrations involving metal ions, it cannot be used for the direct analysis of anions or neutral ligands. In the latter case, standard solutions of Ag+ or Hg + are used as the titrant. 

EDTA is one member of a class of aminocarboxylate ligands that form very stable 1 1 complexes with metal ions. The following table shows log Kf values for several ligands with Ca + and Mg +. Which ligand is the best choice for the direct titration of Ca + in the presence of Mg + ... 

CrP" -selective and Ni " -selective electrodes have been used to detenuine the copper and nickel ions in aqueous solutions, both by direct potentiometry and by potentiometric titration with EDTA. They have also been used for detenuining the CiT and Ni " ions in indushial waters by direct potentiomehy. 

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