Magnesium-EDTA chelate

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

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. 

A common form of EDTA used as a preservative is calcium disodium EDTA (CaNa2EDTA). What metals will this form of the sequestrant scavenge effectively The dissolution of the solid will yield calcium ions, sodium ions, and the EDTA anion. Any metal more effectively complexed than calcium will be readily scavenged, including all ions listed in Table 9.1 except silver (Ag+) and magnesium (Mg2+). (In the absence of the calcium counterion, as in the case of the acid form of EDTA, chelation of calcium in the body can occur. In fact, EDTA administered orally is an FDA-approved treatment for calcium deposits in the bloodstream that lead to cardiovascular disease.) Citric acid (Fig. 9.3.3) is another sequestrant of metal ions in foodstuffs. 

In the back-titration method, a measured amount of an excess standard EDTA solution is added to the sample. The analyte ion combines with EDTA. After the reaction is complete, the excess EDTA is back-titrated against a standard solution of magnesium or zinc ion. Eriochrome Black T or Calmagite is commonly used as an indicator. After all the remaining EDTA chelates with Mg2+ or Zn2+, ary extra drop of the titrant solution imparts color to the indicator signifying the end point. The cations that form stable complexes or react slowly with EDTA can also be measured by the back-titration method. 

Tetra sodium salt (Na4) of EDTA. Chelating agent for calcium, magnesium and other divalent and trivalent metal ions in acid, neutral and alkaline conditions. 

Similarly, ions such as cadmium and zinc, which form more stable EDTA chelates than does magnesium, can be determined in the presence of the latter ion by buffering the mixture to a pH of 7 before titration. Eriochrome Black T serves as an indicator for the cadmium or zinc end points without interference from magnesium because the indicator chelate with magnesium is not formed at this pH. 

In some instances, it might be necessary to supplement the 0.32M sucrose solution, pH 7.4, with 1 mAf EDTA, 0.25 mM dithiothreitol (DTT), and/or a cocktail of protease inhibitors. Membrane-bound phospholipases and proteases can be activated during cell disruption. EDTA chelates metal ions (calcium and magnesium) that activate certain phospholipases. DTT is a reducing agent that prevents oxidation of functionally important sulfhydryl groups. Cell disruption may also cause release of proteases from lysosomes. Protease attack on membrane proteins can be prevented by addition of protease inhibitors, such as phenylmethylsulfonyl fluoride (PMSF an inhibitor of serine proteases) and/or E-64 (an inhibitor of cysteine proteases), to the sucrose solution... 

Phosphonates exhibit all the properties of polyphosphates, such as threshold effect, crystal distortion, and sequestration, but are superior in their effectiveness. They provide good chelates for calcium, magnesium, iron, and copper and are commonly used where iron fouling is a problem. Their sequestering properties are generally superior to other common chelants, such as EDTA and NTA. 

EDTA sequesters calcium and magnesium from hard water, preventing them from forming insoluble soap films (scum) with soaps and detergents. Chelators are sometimes used to sequester metal ions that interfere with dyes and perfumes. 

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. 

Ethylenediamine tetraacetic acid (EDTA) was introduced originally as a water-softener and as a textile dyeing assistant because of its ability to form very stable, water soluble complexes with many metal ions, including calcium and magnesium. The equilibria involved in chelation of metal ions by EDTA and related ligands have been exhaustively studied, notably by G. Schwarzenbach and his colleagues, and provide the basis for complexometric methods of chemical analysis. EDTA and its metal complexes have also become probably the most familiar examples of agents used in chelation therapy. 

Acid cleaning agents such as hydrochloric, phosphoric, or citric acids effectively remove common scaling compounds. With cellulose acetate membranes the pH of the solution should not go below 2.0 or else hydrolysis of the membrane will occur. Oxalic acid is particularly effective for removing iron deposits. Acids such as citric acid are not very effective with calcium, magnesium, or barium sulfate scale in this case a chelatant such as ethylene diamine tetraacetic acid (EDTA) may be used. 

A common treatment for metal intoxication is the use of chelators. A chelator is a flexible molecule with two or more electronegative groups that can form stable complexes with cationic metal atoms. The complexes are then eliminated from the body. The most widely used chelator is eth-ylenediaminetetraacetic acid (EDTA). It has four binding positions (two nitrogen atoms and two oxygen atoms) that focus on the metal ion. It works very well on many metals, the most notable being calcium, magnesium, and lead, see also Enzymes Proteins Tertiary Structure Toxicity. 

HEDTA was developed specifically for chelating ferric ions in alkaline systems. It chelates with maximum effectiveness in a pH range of 6 to 12. This prevents precipitation of iron compounds (e.g., ferric hydroxide and oxide) in this alkaline range. It is not as effective as EDTA for calcium and magnesium ions. It also tends to be less stable250. 

The objective in acid degumming is to chelate the calcium and magnesium ions and render the nonhydratable phosphatide forms hydratable. In addition to phosphoric acid, citric and malic acids are effective, as well as eth-ylenediaminetetraacetic acid (EDTA). Acid-treated phosphatides are not used for production of commercial lecithins. Extensive reviews on oil degumming have been prepared.106 107 Lurgi, a German equipment manufacturer, has developed an EnzyMax process that cleaves the nonhydratable phosphatides with a phospholipase B at the triglyceride s second carbon to produce a lysophosphatide that is insoluble in oil and is removed by centrifuging.108... 

The BS I lectin requires bound calcium for activity.131 Two moles of calcium and 1.25 moles of magnesium per mole of protein were found by atomic absorption spectroscopy. Inactive, metal-free lectin, obtained by exhaustive dialysis, could be reconstituted by addition of calcium, cadmium, or strontium (magnesium restored 80% of the activity). Although bound-calcium was not removed by dialysis against EDTA, inclusion of this chelating agent in the precipitin reaction resulted in complete inhibition. 

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