Silver EDTA titration

The nickel ion freed may then be determined by an EDTA titration. Note that two moles of silver are equivalent to one mole of nickel and thus to one mole of EDTA. 

The equivalent amount of cadmium ion exchanged for the silver ion can readily be determined by EDTA titration procedures. 

A silver indicating electrode can be used in a manner analogous to the mercury electrode for EDTA titrations. In contrast to the mercury electrode, the success of the silver electrode depends on the formation of a silver-EDTA complex that is weak compared with virtually all other metal ions. A trace of silver(I) is added, and after the EDTA has reacted with other metal ions, the first excess of EDTA reacts with silver and sharply decreases the potential of the silver electrode. Owing to the low stability constant of the silver-EDTA complex, titrations must be carried out at high pH. 

Many metal ions (e.g., calcium, cadmium, aluminum, lead) can be titrated with standard ethylenediamine-tetraacetic acid (EDTA) or other compleximetric titrants, using an appropriate indicator electrode. When no direct appropriate indicator electrode exists, the addition of indicator metal ions can permit a determination. For example, barium may be titrated with EDTA in the presence of silver-EDTA complex as an indicator reagent using a silver electrode. Examples of compleximetric titrations are given in Table 3. 

The method may also be applied to the analysis of silver halides by dissolution in excess of cyanide solution and back-titration with standard silver nitrate. It can also be utilised indirectly for the determination of several metals, notably nickel, cobalt, and zinc, which form stable stoichiometric complexes with cyanide ion. Thus if a Ni(II) salt in ammoniacal solution is heated with excess of cyanide ion, the [Ni(CN)4]2 ion is formed quantitatively since it is more stable than the [Ag(CN)2] ion, the excess of cyanide may be determined by the Liebig-Deniges method. The metal ion determinations are, however, more conveniently made by titration with EDTA see the following sections. 

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). 

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. 

Treat an aqueous suspension of about 0.072g (accurately weighed) silver chloride with a mixture of 10 mL of concentrated ammonia solution and 10 mL of 1M ammonium chloride solution, then add about 0.2 g of potassium cyanonickelate and warm gently. Dilute to 100 mL with de-ionised water, add 50 mg of the indicator mixture and titrate with standard (0.01 M) EDTA solution, adding the reagent dropwise in the neighbourhood of the end point, until the colour changes from yellow to violet. 

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 ... 

Pipette 25.0 mL of the bromide ion solution (0.01-0.02M) into a 400 mL beaker, add excess of dilute silver nitrate solution, filter off the precipitated silver bromide on a sintered glass filtering crucible, and wash it with cold water. Dissolve the precipitate in a warm solution prepared from 15 mL of concentrated ammonia solution, 15 mL of 1M ammonium chloride, and 0.3 g of potassium tetracyanonickelate. Dilute to 100-200 mL, add three drops of murexide indicator, and titrate with standard EDTA (0.01 M) (slowly near the end point) until the colour changes from yellow to violet. 

Bromides, D. of as silver bromide, (g) 491 by EDTA, (ti) 339 by mercury(I), (cm) 542 by oxygen flask, 113 by silver ion, (cm) 546 by silver nitrate, (ti) 351 by Volhard s method, (ti) 356 with iodide, (ti) 352 4-Bromomandelic acid 473 Bromophenol blue 265, 267 Bromopyrogallol red 182, 319 Bronsted-Lowry bases titration with strong acids, 277... 

Lebel [224] has described an automated chelometric method for the determination of sulfate in seawater. This method utilises the potentiometric end-point method for back titration of excess barium against EDTA following precipitation of sulfate as barium sulfate. An amalgamated silver electrode was used in conjunction with a calomel reference electrode in an automatic titration assembly consisting of a 2.5 ml autoburette and a pH meter coupled to a recorder. Recovery of added sulfate was between 99 and 101%, and standard deviations of successive analyses were less than 0.5 of the mean. 

Other typical reagents generated for coulometric titrations are hydrogen and hydroxyl ions, redox reagents such as ceric, cuprous, ferrous, chromate, ferric, manganic, stannous, and titanous ions, precipitation reagents such as silver, mercurous, mercuric, and sulfate ions, and complex-formation reagents such as cyanide ion and EDTA [8-10]. 

Issa et al. [9] used various metal ions for the volumetric determination of mefenamic acid. Mefenamic acid was precipitated from its neutral alcoholic solution by a standard solution of either silver nitrate, mercurous acetate, or potassium aluminum sulfate. In the argentimetric procedure, residual Ag(I) was titrated with standard NH4SCN. With Hg(OAc)2 or potash alum, the residual metal was determined by adding EDTA and conducting back titration of excess of EDTA with standard Pb(N03)2 using xylenol orange indicator. The applied methods were used for the determination in bulk drug substance, and in its formulations. 

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. 

The liberated nickel ion required 26.73 mL of 0.02089 M EDTA. The bromate in a 10.00-mL aliquot was reduced to bromide with arsenic(III) prior to the addition of silver nitrate. The same procedure was followed, and the released nickel ion was titrated with 21.94 mL of the EDTA solution. Calculate the percentages of NaBr and NaBr03 in the sample. 

The term log in Equation 13.44 is equal to — plon, and so ion-selective electrodes (ISE) can be used to monitor changes in pM during a titration. For example, a cation-selective glass electrode that is sensitive to silver ion can be used to follow changes in pAg in titrations with silver nitrate. A calcium-sensitive electrode can be used for the titration of calcium with EDTA. The electrode should not respond to sodium ion since the disodium salt of EDTA is usually used. If the electrode responds to a second ion in the solution whose activity remains approximately... 

The major serum electrolytes—sodium, potassium, calcium, magnesium, chloride, and bicarbonate (CO2)—are fairly easy to determine. The metals are most readily determined by the use of fiame-spectrophotometiic or atomic absorption methods, although colorimetric methods exist for calcium and magnesium. Calcium and, less frequently, magnesium are also titrated with EDTA. Ion-selective electrodes are used for the routine analysis of sodium, potassium, and calcium. Bicarbonate is analyzed also by titration against standard acid (see Experiment 8) in addition to a manometric method. Chloride is widely determined by automatic coulometric titration with electrogenerated silver ion. 

The solubility of the complex anion Ag(CN)2 in contrast to the high insolubility of neutral silver cyanide (AgCN) was exploited for the determination of silver (and/or cyanide) by Justus von Liebig as long ago as 1851 this procedure may well be cited as the best established complexometric titration preceding the great developments brought about by G. Schwarzenbach in 1940 when he pioneered the use of ethylenediaminetetraacetic acid, EDTA, and other aminopolycarboxylic acids as titrants" and opened up new vistas in titrimetry (see Section 10.6). 

Practical considerations call for a liquid of high viscosity and low vapour pressure and a low solubility in water. A solution of a calcium complex with a dialkylphosphoric acid (e.g. ((RO)2POO)2Ca, where R = CgH to C16H33) in a very polar solvent such as di-n-octylphenyl phosphate can be used in an ion-selective electrode that gives a linear response over 10 [Ca +] 5 X 10. Solutions of Ni(phen)3 nitrate (or perchlorate, or tetrafluoroborate) in nitrobenzene of p-nitrocymene have been used as specific ion electrodes for nitrate (or perchlorate, or tetrafluoroborate). A mercury dithizonate electrode can be prepared which is ion-specific for mercury (II) and replaces the mercury drop electrode for the titration of Al, Ca, Cu, Hg , Ni and Zn with EDTA. Silver, lead and palladium dithizonates have also been used in titrimetry. ... 

Chromium oxide (Cr203). Up to 0.1% colorimetrically with diphenylcarbizide at 540 nm. Above 0.1% but as a minor constituent, colorimetrically with EDTA at 550 nm. As a major constituent by oxidation to dichromate by peroxodisulfuric acid using a silver nitrate catalyst, destruction of permanganate with HCl and titration against ferrous ammonium sulfate using diphenylamine-4-sulfonate indicator. 

The second approach is much like the preceding method. The precipitate in which the anion to be determined is engaged is treated with a reagent that stoichiometri-cally liberates a cation that is titrated with a standard solution of EDTA. Hence, a halide (except the fluoride) or the thiocyanate ion is precipitated as the corresponding silver salt. The latter is treated with a solution of potassium tetracyanonickelate K2 [Ni(CN)4]. The silver salt dissolves into this solution with liberation of one nickel ion Ni + for two ions X . Ni + is titrated with EDTA in the presence of murexide. The reaction constituting the basis of the process is... 

The most widely used titrant for such determinations is diamino-ethane-tetra-acetic acid (ethylenediamine tetra-acetic acid) which is conveniently employed as the disodium salt, referred to throughout this book as EDTA. (The titrant is referred to in the B,P, and B,P,C, as sodium edetate, but this synonym has not gained universal acceptance.) Many other amino-polycarboxylic acids have been used and in certain special applications they may have some advantage for routine pharmaceutical work, however, it has not been found necessary to use any titrant other than EDTA. This substance reacts stoichiometrically with most metals to form a 1 1 complex and, usually, the reaction is instantaneous (but see Aluminium, p. 32). pH has a marked effect on the stability of the complexes formed the alkaline earth metals form complexes that are stable in alkaline solution but decompose in neutral and acid solution aluminium, copper, lead and mercury all complex under mildly acid conditions while bismuth and ferric iron form stable complexes in a solution as acid as pH 1. The monovalent ions of sodium, potassium and silver form complexes that are too weak to be used for titration purposes whilst mercurous mercury forms no complex,... 

A titration is the process of determining the quantity of a substance by adding measured increments of another substance, the titrant. The latter is almost always added as a standardised solution (or by electrolyte generation, as in a coulometric titration). The end-point of the titration, which should indicate the addition of an exact chemical equivalence, is recognized by a visual indicator or instrumentally. Titrations are based on acid-base reactions (for determination of acids or bases), redox reactions (for determining oxidants or reductants), chelating reactions (usually with EDTA-type compounds, for determination of metal ions) or precipitations (usually of halides or pseudohalides with silver ions). 

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