Magnesium EDTA titration

Procedure. To a 50 mL sample of the water to be tested add 1 mL buffer solution (ammonium hydroxide/ammonium chloride, pH 10, Section 10.54) and 30-40 mg solochrome black indicator mixture. Titrate with standard EDTA solution (0.01 M) until the colour changes from red to pure blue. Should there be no magnesium present in the sample of water it is necessary to add 0.1 mL magnesium-EDTA solution (0.1 M) before adding the indicator (see Section 10.54). The total hardness is expressed in parts of CaC03 per million of water. 

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 ... 

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. 

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. 

EDTA titrations are routinely used to determine water hardness in a laboratory. Raw well water samples can have a significant quantity of dissolved minerals that contribute to a variety of problems associated with the use of such water. These minerals consist chiefly of calcium and magnesium carbonates, sulfates, etc. The problems that arise are mostly a result of heating or boiling the water over a period of time such that the water is evaporated, and the calcium and magnesium salts become concentrated and precipitate in the form of a scale on the walls of the container, hence the term hardness. This kind of problem is evident in boilers, domestic and commercial water heaters, humidifiers, tea kettles, and the like. 

The murexide method measures Ca2+ only Mg2+, at the concentration in milk, does not affect the indicator appreciably. Calculation of Mg2 + concentration is possible when the total calcium and magnesium (obtained by EDTA titration) is known. This is based on the assumption that the same proportion of each cation is present in the ionic form, which is justifiable since the dissociation constants of their citrate and phosphate salts are virtually identical. 

Magnesium, formerly determined by precipitation as magnesium ammonium phosphate and determining P in the latter, can be analyzed readily by EDTA titrations. It can be obtained either as the difference between titrations for (Ca and Mg) and Ca alone or by titrating the supernatant after Ca is precipitated as oxalate (White and Davies 1962). 

Hardness can be measured by either (1) calculation from the concentration of calcium and magnesium ions in the ample, or (2) EDTA titration. 

An EDTA titration on the tap w ater will indicate the presence ot calcium and magnesium ion.s in the tap water, the eri(x hroine black I indicator changing colour from red to blue as these ions are complexed out of soiutitm. In soft water, the criochrome black T will l)e blue, since calcium and magnesium ions are absent. 

Water hardness is ordinarily determined by an EDTA titration after the sample has been buffered to pH 10. Magnesium, which forms the least stable EDTA complex of ail of the common multivalent cations in typical water samples, is not... 

The second method is by ethylene diamine tetra acetic acid (EDTA) titration, in which an indicator, Eriochrome Black T is added to the water sample, which develops a red color due to the presence of ( k. When EDTA titer is added, Ca is complexed and yields an end point at which the color changes to blue. To ensure a sharp end point, a small amount of magnesium salt of EDTA is added. The titration is performed at room temperature and at a pH of 10. 

A strong acid cation exchange resin is converted to the H form. This then is used to remove the cations from a liquid sample such as mineral water, pond water, or the juices from low acid foods and exchange them with H. An aliquot of the sample containing H" will be titrated with standard base to determine the total amount of ions present. A second aliquot will be titrated with EDTA to determine the total calcium and magnesium ions present, and a third aliquot will be used to determine magnesium, again with an EDTA titration. 

The formation constants of the EDTA complexes of calcium and magnesium are too close to differentiate between them in an EDTA titration, even by adjusting pH (see Figure 9.4). So they will titrate together, and the Eriochrome Black T end point can be used as above. This titration is used to determine total hardness of water, (Ca " plus Mg " —see Experiment 9). Eriochrome Black T cannot be... 

Several analytical methods will differentiate the "free" (hydrated) metal ions from dissolved complexed metal ions. These methods include specific ion electrodes, polarographic, and other amperometric and voltammetric methods and various types of spectroscopy (see Section 7-10). Specific ion electrodes only respond to the free metal ion for which they are "specific." To determine the relative amounts of complexed and uncomplexed metal ion in a solution, we can use a "wet chemical" method to measure the total concentration of "free + complexed" ions, and then an ion-specific electrode to determine the free metal ion concentration (activity). Care must be taken to eliminate interferences that may affect these measurements. We deduce the concentration of the "complexed ions" by the difference between these two measurements. For example, in the EDTA titration method for hardness, free and complexed calcium and magnesium ion s are measured. 

Alkaline-earth metal oxides Calcium and magnesium oxides are constituents of nearly all glasses and may be determined using EDTA titration. Neither determination is free of interference. 

The method described here is based on the difference between measurements of total alkaline earths by complexometric titration with EDTA (ethylenediamine-N,N,N, N -tetra-acetic acid) and selective measurement of calcium described in Section 11.2.1. The simultaneous EDTA titration of calcium, strontium and magnesium involves Eriochrome Black T (EBT) as indicator and was originally applied to seawater analysis by Voipio (1959) and Pate and Robinson (1961). To eliminate subjective errors in the determination of the endpoint, Culkin and Cox (1966) used photometric endpoint detection. A slight modification of this procedure, including the standardization of EDTA by magnesium is reported here. 

A method for the isolation of magnesium from the bulk of sea salts by ion exchange and subsequent EDTA titration (Greenhalgh etai, 1966) is outlined in Section 11.2.2.7. 

The following method may be used for determination of the calcium. The total calcium and magnesium is determined in an aliquot by EDTA titration, the calcium precipitated as oxalate from a further aliquot and the residual magnesium titrated to give the calcium by difference. 

Why is magnesium-EDTA chelate added to a magnesium-free water sample before it is to be titrated with EDTA for Ca ... 

Replacement titration When a particular metal ion does not form a sufficiently stable complex with E.D.T.A., magnesium EDTA complex is added and the ions set free are titrated with E.D.T.A. ... 

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. 

These results imply that at the specified pH the magnesium complex is appreciably dissociated, whereas the lead complex is stable, and clearly titration of an Mg( II) solution with EDTA at this pH will be unsatisfactory, but titration of the lead solution under the same conditions will be quite feasible. In practice, for a metal ion to be titrated with EDTA at a stipulated pH the value of log K H should be greater than 8 when a metallochromic indicator is used. 

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