Water hardness EDTA determination

Ca and Mg in water (water hardness) Ca determination Add 2 cm 3 of NaOH solution (0.1 mol dm 3) to 50 cm3 of sample and titrate with EDTA using murexide indicator (table 5.8). Mg determination Destroy murexide colour with 1 cm3 concentrated HQ add 3 cm3 of NH3—NH4C1 buffer and titrate with EDTA using eriochrome black T. 

Description of the Method. The operational definition of water hardness is the total concentration of cations in a sample capable of forming insoluble complexes with soap. Although most divalent and trivalent metal ions contribute to hardness, the most important are Ca + and Mg +. Hardness is determined by titrating with EDTA at a buffered pH of 10. Eriochrome Black T or calmagite is used as a visual indicator. Hardness is reported in parts per million CaCOs. 

A very important ligand (or chelating agent) for titrimetric analysis is the ethylenediaminetetraacetate (EDTA) ligand. It is especially useful in reacting with calcium and magnesium ions in hard water such that water hardness can be determined. The next section is devoted to this subject. 

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. 

At present, the complexonometric method of determining water hardness is being widely used. This method is very sensitive and simple to use. It is based on the ability of the disodium salt of ethylenedinitrilotetraacetic acid (EDTA or complexon)... 

Yappert, M. C. DuPre, D. B. Complexometric Titrations Competition of Complexing Agents in the Determination of Water Hardness with EDTA, J. Chem. Educ. 1997, 74, 1422-1423. 

Numerous tertiary amines that also contain carboxylic acid groups form remarkably stable chelates with many metal ions. Ethylenediamine tetra-acetic acid (EDTA) can be used for determination of 40 elements by direct titration using metal-ion indicators for endpoint detection. Direct titration procedures are limited to metal ions that react rapidly with EDTA. Back titration procedures are useful for the analysis of cations that form very stable EDTA complexes and for which a satisfactory indicator is not available. EDTA is also used for determining water hardness the total concentration of calcium and magnesium expressed in terms of the calcium carbonate equivalent. 

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 total hardness of water can be determined either by using standard soap solution or by using EDTA reagent. 

FIGURE 14-13 Photometric titration curves (a) tola hardness of water, (b) determination of sulfate. In (a), total water hardness is obtained by titration with 0.10 M EDTA at 610 nm lor 100 ml of a solution that contained 2.82 mmol/L total hardness. Eriochrome Black T was the indicator, fn (b). 10.0 ml of a solution containing sulfale was titrated with 0.050 M BaCI using Thorin as an indicator and a wavelength of 523 nm. The response shown is proportional to transmittance. (From A. L, Underwood. Ana/. Chom.. 1954, 26. 1322. Figure t, p. 1323. Copyright 1954 American Chemical Society.)... 

Complex formation is used extensively in water analysis. The determination of hardness, Ca and Mg concentration, employs the com-plexation of these metals with the chelating agent ethylenediaminetetra-acetic acid (EDTA) the titrimetric finish of the chemical oxygen demand (COD) test uses the complexing agent 1,10-phenanthroline to detect the presence of Fe and so indicate the endpoint chloride analysis by. the mercurimetric method depends on the formation of the complex HgCl taq) between Hg " and the chloride ion. 

The chelating agent EDTA " can attach at six sites, since each of the acetate groups and the two nitrogen atoms have free electron pairs necessary for coordinate bond formation. For example, the EDTA complex with Ca that forms during the titrimetric determination of water hardness... 

Several different kinds of reactions other than acid-base reactions can be used for titrations. Oxidation reactions can be used, for example, in detennining dissolved oxygen levels in water the precipitation of CF ion with a standard solntion of Ag is one of the oldest titration procedures, and chelation with the anion of the strong chelating agent ethylenediantinetetraacetic acid (EDTA) can be used for determining Ca ion concentration in water (water hardness) (Chapter 3, Section 3.10). 

The hardness of water is a function of its Ca + and Mg + contents. The separated titrations allow us to determine the hardness due to Mg + and that due to Ca . To clarify what has just been said, the total hardness is determined by a titration carried out at 40 °C (for kinetic reasons) with complexone(III) at pH 9-10 in the presence of eriochrome black T and of magnesium complexonate [Mg(EDTA)] . In an alternative way to determine Mg +, it proceeds as before but after Ca + has been removed by precipitation as oxalate. 

Inorganic Analysis Complexation titrimetry continues to be listed as a standard method for the determination of hardness, Ca +, CN , and Ch in water and waste-water analysis. The evaluation of hardness was described earlier in Method 9.2. The determination of Ca + is complicated by the presence of Mg +, which also reacts with EDTA. To prevent an interference from Mg +, the pH is adjusted to 12-13, precipitating any Mg + as Mg(OH)2. Titrating with EDTA using murexide or Eri-ochrome Blue Black R as a visual indicator gives the concentration of Ca +. 

Livneh, M. Determination of the Hardness of Water by a Microscaled EDTA Titration, J. Sci. Educ. (Rev. Educ. Cien., Colombia), 2005,6(1), 51-52. 

A scientist determines how hard the tap water is in the laboratory, using an EDTA titration. If the pipes in the building are old and some rust dissolves into the tap water, how will the results of the test change ... 

Walter Biedermann and Schwarzenbach published the first titrimetric determination for the hardness of water using Erio-T as an indicator in January of 1948. In the next 10 years, over 1200 papers would be published on the uses of EDTA. 

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... 

Many complexing agents are hardly degradable and therefore can be observed in environmental waters. Since the complexing agents such as ethylenediamino tetra-acetic acid (EDTA), nitrilo triace tic acid or aminophosphonic acids are extremely polar compounds, they were determined preferentially by ESl-LC-MS. IC interfaced by ESI to the MS allowed the determination of EDTA in pg L quantities without any pre-concentration [397]. Metal complexes of EDTA were very stable and could be observed after ESI-LC-MS as [M-i-metal] ions in positive ionisation mode [398]. ESI CE-MS was applied to separate and to quantify the stabile Ni-EDTA complexes [399]. 

The total concentration of Ca and Mg in a sample of hard water was determined by titrating a 0.100-L sample of the water with a solution of EDTA. The EDTA chelates the two cations ... 

To measure hardness, water is treated with ascorbic acid to reduce Fe to Fe and with cyanide to mask Fe, Cu, and several other minor metal ions. Titration with EDTA at pH 10 in ammonia buffer gives [Ca ] + [Mg ]. [Ca ] can be determined separately if the titration is carried out at pH 13 without ammonia. At this pH, Mg(OH)2 precipitates and is inaccessible to EDTA. 

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