Murexide as indicator

Sometimes the metal may be transformed into a different oxidation state thus copper(II) may be reduced in acid solution by hydroxylamine or ascorbic acid. After rendering ammoniacal, nickel or cobalt can be titrated using, for example, murexide as indicator without interference from the copper, which is now present as Cu(I). Iron(III) can often be similarly masked by reduction with ascorbic acid. 

Silver halides can be dissolved in a solution of potassium tetracyanonickelate(II) in the presence of an ammonia-ammonium chloride buffer, and the nickel ion set free may be titrated with standard EDTA using murexide as indicator. 

The procedure involved in the determination of these anions is virtually that discussed in Section 10.58 for the indirect determination of silver. The anion to be determined is precipitated as the silver salt the precipitate is collected and dissolved in a solution of potassium tetracyanonickelate(II) in the presence of an ammonia/ammonium chloride buffer. Nickel ions are liberated and titrated with standard EDTA solution using murexide as indicator ... 

Figure 13 Colour changes in the titration of Ca2+ with EDTA using murexide as indicator. The effect of pH on pCa at...

Puschel and Stefanac ° use alkaline hydrogen peroxide in the oxygen flask method to oxidize arsenic to arsenate. The arsenate is titrated directly with standard lead nitrate solution with 4-(2-pyridylazo) resorcinol or 8-hydroxy-7-(4-sulpho-l-naphthylazo) quino-line-5-sulphonic acid as indicator. Phosphorus interferes in this method. The precision at the 99% confidence limit is within 0.67% for a 3-mg sample. In another variation, Stefanac used sodium acetate as the absorbing liquid, and arsenite and arsenate are precipitated with silver nitrate. The precipitate is dissolved in potassium nickel cyanide (K2Ni(CN)4) solution and the displaced nickel is titrated with EDTA solution, with murexide as indicator. The average error is within + 0.19% for a 3-mg sample. Halogens and phosphate interfere in the procedure. 

Method Nuclear magnetic resonance (NMR) or visible absorption spectrometry with murexide as indicator (Spectr.). 

These complexes, unlike the crown ether complexes but similar to the aza-crown and phthalocyanine complexes, are fairly stable in water. Their dissociation kinetics have been studied and not surprisingly they showed marked acid catalysis.504 Association constant values for lanthanide cryptates have been determined.505,506 A study in dimethyl sulfoxide solution by visible spectroscopy using murexide as a lanthanide indicator showed that there was little lanthanide specificity (but surprisingly the K values for Yb are higher than those of the other lanthanides). The values are set out in Table 9.507... 

The drug has been determined by complexometric titration of the Cu" 1" liberated by passing the solution through a column of cation exchange resin in the cupper form. A solution of 0.005 or 0.0025 M of disodium EDTA is the titrant and using murexide as an indicator (88). 

Figure 6. Time course of change in catalytic specificity (upper panel) and Ca2+ dissociation from extracytoplasmic low affinity sites (lower panel) following phosphorylation of the SR Ca2+-ATPase with ATP. The amount of ADP-insensitive phosphoen-zyme (E2P) was measured in two ways (I) [y-32P]ATP was included in the reaction mixture and the radioactivity incorporated into the enzyme was determined after acid quenching at various time intervals. To remove the ADP-sensitive phosphoenzyme so that only ADP-insensitive phosphoenzyme was measured, ADP was added 4 sec before the quench (upper panel, right scale) (2) by the enhancement of fluorescence from a trinitrophenyl-derivative of ADP bound in the catalytic site in exchange with ADP after the phosphorylation (upper panel, left scale). The change in Ca2+ binding was measured indirectly by use of murexide as an indicator of free Ca2+ in the medium. The data show that Ca2+ dissociates simultaneously with formation of E2P. The data points were taken from Andersen et al., 1985.

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). 

There are several methods available to determine the amount of carbon-bonded fluorine. One of the earlier methods employed the Schoniger Combustion technique followed by thorium nitrate or cerous chloride titration using sodium alizarin sulfonate or murexide as the indicator (21). 

The titration of Ca + in the presence of Mg + is of practical importance. The best results are obtained with complexone(III) as the titrant and with calcein or calcon as the indicator. Murexide, as an indicator, is obsolete since both preceding indicators have been introduced. In any case, titrand solutions must be alkaline for the titration reaction to be sufficiently quantitative. Pb + and Zn + must be masked with 2,3-dimercaptopropanol. Some other heavy metal ions must also be masked with potassium cyanide. Fe + and Mn + can be masked by eomplexation with triethanolamine. Here, again, we find what we have already stressed several times about an interest of the eomplexation phenomenon that can be used to dissimulate disturbing ions. Ba +, Sr +, and Ca + are titrated simultaneously. 

CuSO,.5HjO may lose crystal water on standing. The reagent should be standardised by titration with EDTA at pH 10 with murexide as an indicator. 

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 +. 

These reactions take place with sparingly soluble silver salts, and hence provide a method for the determination of the halide ions Cl", Br, I-, and the thiocyanate ion SCN ". The anion is first precipitated as the silver salt, the latter dissolved in a solution of [Ni(CN)4]2", and the equivalent amount of nickel thereby set free is determined by rapid titration with EDTA using an appropriate indicator (murexide, bromopyrogallol red). 

Procedure. Prepare the murexide indicator as described in Section 10.57(a), and an ammonium chloride solution (1M) by dissolving 26.75 g ammonium chloride in de-ionised water in a 500 mL graduated flask. 

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... 

As soon as contents of beakers are completely dissolved, the solns are diluted to ca 500ml and titrated, in presence of murexide indicator, with 0.1M disodium ethylene-diaminetetraacetate(EDTA) to chge of color from yel to purple. Each ml of 0.100M EDTA corresponds to I6.691mg CIO and 19.891mg CIO ... 

Complex formation of the alkali ions with murexide in methanol was studied quantitatively by spectrophotometric titration with Li+, Na+, and K+. (For Rb+ and Cs+ only qualitative measurements could be obtained since these complexes tend to precipitate). Fig. 8 shows the shift of the absorption maximum upon titration with Na+. The well defined isosbestic point is a good indication for a simple 1 1 complexation equilibrium. In so much as the spectral shift (upon complexation) is a criterion of the strength of the complexes. Fig. 9 indicates that the absolute complex stability parallels monotonically the sequence of ionic sizes. (Both /lAmax and Ae are largest for the smallest ion). In the alkali ion series Li+ forms the strongest and Cs+ the weakest complexes. This monotonic size dependence of the charge density is also expressed in the energy values for the desolvation (—zlHuydr. for Li+= 120 kcal and for Cs+ 60 kcal) (77). 

If murexide is to be used as an indicator in studies of fast processes it is important that not only pronounced spectral shifts are present, but also that they occur rapidly compared to the reaction under study. The indicator may still be used if its rate of complex formation is comparable to that to be followed. In this situation, however, it is necessary to know accurately the rate constants of the indicator reaction for the evaluation of the resulting relaxation spectrum of the coupled reactions. 

Murexide, the ammonium salt of purpuric acid, was known for long to be a good indicator for Ca2+ in aqueous solution (74). The chemical composition of the dye is as follows ... 

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