Determination magnesium hardness

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. 

Table 12.5 lists other standard methods for the determination of major ions carbonate and bicarbonate (alkalinity)50 51 67 68 and calcium and magnesium (hardness).50 51 70-72... 

For this purpose the determination of the magnesium hardness is easily effected by treating a measured volume of the carefully... 

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. 

If the temporary, permanent, and magnesium hardnesses are known, the following formula provides a convenient method for determining the required amounts of lime and sodium carbonate ... 

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... 

This hardness, which can be removed by heating, is called temporary or carbonate hardness. Temporary hardness is derived from contact with carbonate (limestone and dolomite). Hardness which cannot be removed by boiling is called permanent or non-carbonate hardness and it is due to anions, such as chloride, nitrate, sulfate and silicate. This hardness does not contribute to scale formation. Contact with gypsum would result in permanent hardness. Calcium hardness is that due to Ca only, while magnesium hardness is due to Mg only. Magnesium hardness can be calculated from a determination of total and calcium hardness ... 

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. 

The precipitate is filtered and (he excess of NaOH (or liine) left uuconsuined is determined by titrating the filtrate against standard acid. The. amount of NaOH (or lime) consumed is a measure of the magnesium hardness of the simple. 

In devising an efficient and economical softening proce.ss, determination of total hardness and the relative amounts of CH NCH and calcium magnesium hardness is important. 

It is thus important to determine the relative amounts of calcium and magnesium, for addition of too much lime means that calcium ions are reintroduced into the water, i.e. it becomes hard again, the hardness being permanent. 

The ash content of furnace blacks is normally a few tenths of a percent but in some products may be as high as one percent. The chief sources of ash are the water used to quench the hot black from the reactors during manufacture and for wet pelletizing the black. The hardness of the water, and the amount used determines the ash content of the products. The ash consists principally of the salts and oxides of calcium, magnesium, and sodium and accounts for the basic pH (8—10) commonly found in furnace blacks. In some products potassium, in small amounts, is present in the ash content. Potassium salts are used in most carbon black manufacture to control stmcture and mbber vulcanizate modulus (22). The basic mineral salts and oxides have a slight accelerating effect on the vulcanization reaction in mbber. 

In considering the corrosion of magnesium and its alloys it is important to examine the methods available for assessing corrosion tendencies and particularly those known as accelerated tests. Tests carried out by immersion in salt water or by spraying specimens regularly with sea-water are worthless as a means of determining the resistance of magnesium alloys under any other than the particular test conditions. Extrapolation to less corrosive conditions is not valid and even the assessment of the value of protective measures by such means is hardly possible. The reason is to be found in the fact that corrosion behaviour is directly related to the formation of insoluble... 

The hardness of water is generally due to dissolved calcium and magnesium salts and may be determined by complexometric titration. 

The sum of calcium and magnesium ions in the mud determines the total hardness. These ions are analyzed with complexometric titrations using EDTA (versenate). 

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. 

A problem exists with this procedure, however, in that at basic pH values, many metal ions precipitate as the hydroxide, e.g., Mg(OH)2, and thus would be lost to the analysis. This occurs with the magnesium in the water hardness procedure alluded to earlier. Luckily, a happy medium exists. At pH =10, the reaction of the metal ion with the predominant HY - and Y4- species (Figure 5.21) is shifted sufficiently to the right for the quantitative requirement to be fulfilled, while at the same time the solution is not basic enough for the magnesium ions to precipitate appreciably. Thus, all solutions in the reaction flask in the water hardness determination are buffered at pH = 10, meaning that a conjugate acid-base pair... 

Hardness of a water sample is a measure of its capacity to precipitate soap. The presence of calcium and magnesium ions in water essentially contributes to its hardness. Other polyvalent ions, such as aluminum, also cause hardness. Their effect, however, is minimal, because these polyvalent ions occur in water often in complex forms and not as free ions. As a result, they cannot precipitate soap. Although calcium is not the only cation causing hardness, for the sake of convenience, hardness is expressed as mg CaC03/L. Similarly, anions other than carbonate, such as bicarbonate, also cause hardness in water. To distinguish the contributions of such anions from carbonates, hardness is sometimes termed as carbonate hardness and noncarbonate hardness. This can be determined from alkalinity. The relationship is as follows ... 

Not until the whole of the alkaline earth metal has been fixed as insoluble curd is the sodium soap free to yield a lather. Consequently the higher the percentage of calcium or magnesium, the larger the amount of soap required and the greater the hardness of the water. The amount of soap required to produce a lather is thus a measure of the hardness of the water and, as indicated below, is used m quantitatively determining the same. 

---