Exchangeable Calcium and Magnesium

Where atomic absorption spectrophotometry is possible the ammonium acetate extract can be directly analysed for Ca and Mg. The spectrophotometric standards are prepared in the ammonium acetate solution and both the standard and extracts are read against ammonium acetate as blank. If AAS is not possible the calcium and magnesium are analysed by complexometric titrations using ethylene diamine tetra acetic acid (EDTA). 

The method makes use of excellent chelating properties of disodium ethylene diamine tetraacetate (versenate) which forms soluble complexes with metal cations. 

The formula II is preferred over I, since it has been shown from measurements of the dissociation constants that two hydrogen atoms are probably held in the form of zwitter ions. For the purpose of simphcity we shall assign the formula H Y to EDTA the di-sodium salt is therefore Na2H2Y and affords the complex-forming ion H2Y2 in aqueous solution it reacts with all metals in a 1 1 ratio. The reactions with cations 1VP+, can be written in the general form as  

One mole of complex forming H2Y reacts with one mole of the metal ion and in each case two moles of hydrogen are formed. It is evident from the equations above that dissociation of the complex will be governed by the pH of the solution lowering the pH will decrease the stability of metal-EDTA complex. The more stable the complex, the lower the pH at which an EDTA titration of the metal ion in question is carried out. 

The stability of a complex is characterised by the stability constant (or formation constant) K  

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Alkali compounds are used in the Surtek process to reduce the interfacial tension between the oil phase and the aqueous phase. In addition, an alkaline agent neutralizes rock and clay surfaces and reduces the amount of exchangeable calcium and magnesium ions from the soil surface. Both of these functions reduce surfactant and polymer adsorption into the soil matrix. 

Catalysis of Lignite Char Gasification by Exchangeable Calcium and Magnesium... 

When the sodium salt of sulfonated poly(styrene) is used as the cation exchanger, calcium and magnesium ions are exchanged for sodium ions ... 

Water is softened by removing calcium and magnesium ions from hard water in exchange for sodium ions at sites on cation-exchange resin. Water softeners typically use a gel polystyrene sulfonate cation-exchange resin regenerated with a 10% salt brine solution (25). 

Brine Preparation. Rock salt and solar salt (see Chemicals frombrine) can be used for preparing sodium chloride solution for electrolysis. These salts contain Ca, Mg, and other impurities that must be removed prior to electrolysis. Otherwise these impurities are deposited on electrodes and increase the energy requirements. The raw brine can be treated by addition of sodium carbonate and hydroxide to reduce calcium and magnesium levels to below 10 ppm. If further reduction in hardness is required, an ion-exchange resin can be used. A typical brine specification for the Huron chlorate ceU design is given in Table 6. 

Hardness Calcium and magnesium salts in the forms of CaCOj, Ca, Mg. This is the primary source of scaling in heat exchange equipment, boilers, pipelines/transfer lines, etc. Tends to form curds with soap and interferes with dyeing applications as well. 

Weakly acidic cation-exchange resins have carboxylic groups (COOH) as the exchange sites. When operated on the hydrogen cycle, the weakly acidic resins are capable of removing only those cations equivalent to the amount of alkalinity present in the water, and most efficiently the hardness (calcium and magnesium) associated with alkalinity, according to these reactions ... 

The regeneration of the resin bed is never complete. Some traces of calcium and magnesium remain in the bed and are present in the lower-bed level. In the service run, sodium ions exchanged from the top layers of the bed form a very dilute regenerant solution which passes through the resin bed to the lower portion of the bed. This solution tends to leach some of the hardness ions not removed by previous regeneration. 

Where hardness removal is required, the simplest pretreatment method for smaller, lower pressure boiler plants (below 200-300 psig) is to use a cation-exchange softener. This removes the calcium and magnesium at source and converts the bulk of temporary hardness salts into sodium bicarbonate (NaHC03), which decomposes to form sodium carbonate (soda ash) but does not scale under normal boiler operating conditions. 

Sodium cycle softening (base-exchange softening) is used primarily to remove the risk of calcium- and magnesium-based crystalline scale formation and deposition. The general reaction is as shown in Figure 9.2. 

In spite of the good calcium and magnesium tolerance of AOS systems, situations can arise whereby too many exchangeable hardness ions present themselves to the surfactant formulation. One way to overcome this is by blending AOS with a divalent cation-tolerant surfactant. A product which has been field-tested [87] is a blend of AOS and a disulfonated alkyldiphenyloxide (DPOS). Calcium and magnesium salts of DPOS are water-soluble and remain soluble even in solutions which are very concentrated in calcium and magnesium salts. 

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