Calcium-murexide

Murexide forms complexes with many metal ions only those with Cu, Ni, Co, Ca and the lanthanides are sufficiently stable to find application in analysis. Their colours in alkaline solution are orange (copper), yellow (nickel and cobalt), and red (calcium) the colours vary somewhat with the pH of the solution. 

Murexide may be employed for the direct EDTA titration of calcium at pH =11 the colour change at the end-point is from red to blue-violet, but is far from ideal. The colour change in the direct titration of nickel at pH 10-11 is from yellow to blue-violet. 

Fiir die Magnesiumbestimmung wurde auch das Differenzverfahren angewandt. (Calcium gegen Murexid und Summe Calcium + Magnesium gegen Erio T.)... 

Ionized calcium may be determined spectrophotometrically after reaction with murexide or using a Ca2 +-specific electrode. 

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. 

Fig. 10. The effect of different concentrations of the ionophore X 537 A on calcium release, by sarcoplasmic reticulum vesicles11S. The reaction mixture contained 20 mM Tris-maleate pH 6-8, 50 mM KC1, 10 mM MgClj, 0.1 mM CaCl2, 0.1 mM murexide and 0.27 mg protein/ml. Calcium uptake and release were followed by monitoring the changes in the absorbance undergone by murexide. The measurements were performed with a filter dual wave length (540—507 nm) double beam spectrophotometer...

Figure 3.24. Calcium binding in 3% /3-casein dispersion at TI2 = 0.14, pH 7.0 and 2°C, as determined by the resin contact time and murexide methods. Apparent maximum number of sites = 11.2 moles of calcium per mole. Apparent intrinsic binding constant = 76.62 liters/mole. (From Jaynes and Whitney 1982. Reprinted with permission of the American Dairy Science Association.)...

Sundararajan, N. R. and Whitney, R. McL. 1975. Murexide for determination of free and protein-bound calcium in model systems. J. Dairy Sci. 58, 1595-1608. 

Although changes in pM can readily be followed by physical means (e.g. potentiometrically), following colour changes associated with the formation and dissociation of metal coordination complexes visually or spectrophotometrically is a more versatile and convenient procedure. The serendipitous discovery of so-called metallochromic indicators made by G. Schwarzenbach (1945) led immediately to the introduction of murexide (50) as an indicator in calcium titrations and initiated the search for indicators for other metal-EDTA systems. It will be realized that the chosen metal indicator must be considerably less stable than the metal-EDTA complex, but not so weak as to dissociate appreciably in the vicinity of the end-point when the concentration of free metal... 

Calcium chloride solutions (pH =6.2) labeled with Ca or 36ci were displaced vertically downward through columns of homogeneous, repacked, water-saturated sandy soil by a chemically identical solution labeled with Cl or Ca, respectively. Constant water fluxes, and solution activities of 1 to 15 pCi/dm, were used. Calcium solutions were analyzed by titration with disodium dihydrogen ethylenediamine tetraacetate to a murexide end point (11). The activity of radioactively labeled solutions was obtained by liquid scintillation techniques. Concentrations of adsorbed calcium were calculated from isotope dilution. The concentration of calcium chloride in the influent solution was 0.08 N. Because exchange of calcium for itself was the only chemical process affecting transport, the calcium chloride concentration remained constant at 0.08 N throughout each experiment, both within the column and in the effluent. 

The. composition of murexide is expressed by the formula CgH NsO N H4. By double decomposition with potassium nitrate, the potassium salt (CgH NgOgK) is obtained. The salts with calcium, barium, tin, and mercury are sparingly soluble red or violet precipitates. 

Various organic reagents are used for direct determination of calcium, such as murexide (ammonium purpurate) (e = 1.4-10 at 500 nm) [2,49], Metalphthalein [50], Calcein [51,52], Chrome Azurol S (in the presence of 1,10-phenanthroline) [53], Alizarin S [54], 8-hydroxyquinoline (extraction into CHCI3 in the presence of n-butylamine or butoxyethanol) [55], and Emodine (l,3,8-trihydroxy-6-methylanthraquinone) [56]. Calcium has been determined as a complex with Emodine, in the presence of Be and Mg, by the derivative spectrophotometry technique. The anionic complexes of calcium with bromo-oxine [57] or HTTA [58] have been extracted into benzene as ion associates with Rhodamine B. Calcium was also determined as a complex with o-cresolphthalein [59-63], or thymolphthalein [64]. 

Calcium and Mg in mineral water were simultaneously determined with Methylthymol Blue by both batch and flow-injection techniques [3], FIA method using murexide as a colour agent was employed to the determination of Ca in ores and Ca-containing pharmaceutical formulations [4]. 

One practical alternative to the radiometric methods is provided by murexide, which forms a complex with free calcium (46). This complex can be determined spectrophotometrically and used for the calculation of unbound calcium. Bound calcium calculated by difference, enables one to construct binding plots. This method has two advantages in special cases it is very rapid and therefore can be used for labile binding proteins, and since no dialysis or ultrafiltration is needed it can be used with low molecular weight peptides. About 1 mg of protein is required for the murexide method. 

Masking can be achieved by precipitation, complex formation, oxidation-reduction, and kinetically. A combination of these techniques may be employed. For example, Cu " can be masked by reduction to Cu(I) with ascorbic acid and by complexation with I . Lead can be precipitated with sulfate when bismuth is to be titrated. Most masking is accomplished by selectively forming a stable, soluble complex. Hydroxide ion complexes aluminum ion [Al(OH)4 or AlOa"] so calcium can be titrated. Fluoride masks Sn(IV) in the titration of Sn(II). Ammonia complexes copper so it cannot be titrated with EDTA using murexide indicator. Metals can be titrated in the presence of Cr(III) because its EDTA chelate, although very stable, forms only slowly. 

Heparinoids and mucopolysaccharides react with, and modify, many of the plasma proteins. Heparin combines with fibrinogen, globulins and albumin. As judged by electrophoresis and various types of analysis and staining, the particular plasma protein components with which heparin combines are dependent upon the concentration of protein, concentration of heparin, pH value, and salts present. This explains the somewhat contradictory statements in literature about combinations of heparin with plasma proteins. The combination may result in change of solubility of the protein and reverse protein tests . Heparin can modify the murexide reaction for calcium in serum by affecting the calcium-protein-heparin complex. Many heparinoids... 

Chelatometric determination of calcium itself is carried out with chela-tone at pH 12-13 using murexide as the indicator (from red to violet), or a mixture of murexide and fluoroexone and thymolphthalexone (from green to violet-pink). By this method it is possible to determine calcium at concentrations of 10-200 mg 1 [13, 14). 

Titration methods. A diluted sample of the fluid is titrated with EDTA in the presence of an indicator dye (e.g. calcein, murexide, Eriochrome Black T) which is bound as a complex to calcium. When all the calcium present has been chelated by EDTA the spectral characteristics of the dye are changed and this can be detected fluorimetrically or colorimetrically. Magnesium interference in some of these methods can be eliminated by titration at an alkaline pH at which magnesium is precipitated as magnesium hydroxide. 

One of the most significant advances in chemical analysis as applied to pharmaceuticals during the last decade is undoubtedly the introduction and development of the complexometric titration. The ability of certain amino-polycarboxylic acids to react stoichiometrically and instantaneously with certain metal ions was first recognised and described by Schwarzenbach in 1945. Later, the same author, together with co-workers, described the first metal indicator, murexide, and then, perhaps the most important of all, Eriochrome Black T (usually referred to in this country as solo-chrome black). This was followed quite shortly by the first description of the now classic use of the complexometric titration for the determination of temporary and permanent hardness in water. It was some time, however, before metal indicators capable of functioning at an acid pH were developed with the availability of such indicators a rapid increase in the application of complexometric titrations took place and there are now few metal ions that are not capable of determination by this means. In the present book reference will be found to the use of complexometric methods for determination of aluminium, bismuth, calcium, copper, iron, lead, magnesium, manganese, mercury and zinc. In addition, indirect methods are described for the determination of certain anions such as fluoride, phosphate and sulphate. 

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