Back titration with EDTA

The excess of manganese ion is evaluated by back-titration with EDTA. The amount of standard manganese ion solution consumed is equivalent to the EDTA liberated by the fluoride ion, which is in turn equivalent to the magnesium in the sample. 

Back titration with EDTA is used in the pharmacopoeial assays of aluminium glycinate, aluminium hydroxide, aluminium sulphate, calcium hydrogen phosphate. 

BackTitrations. In the performance of aback titration, a known, but excess quantity of EDTA or other chelon is added, the pH is now properly adjusted, and the excess of the chelon is titrated with a suitable standard metal salt solution. Back titration procedures are especially useful when the metal ion to be determined cannot be kept in solution under the titration conditions or where the reaction of the metal ion with the chelon occurs too slowly to permit a direct titration, as in the titration of chromium(III) with EDTA. Back titration procedures sometimes permit a metal ion to be determined by the use of a metal indicator that is blocked by that ion in a direct titration. Eor example, nickel, cobalt, or aluminum form such stable complexes with Eriochrome Black T that the direct titration would fail. However, if an excess of EDTA is added before the indicator, no blocking occurs in the back titration with a magnesium or zinc salt solution. These metal ion titrants are chosen because they form EDTA complexes of relatively low stability, thereby avoiding the possible titration of EDTA bound by the sample metal ion. 

Solutions containing both Le + and AF+ can be selectively analyzed for Le + by buffering to a pH of 2 and titrating with EDTA. The pH of the solution is then raised to 5 and an excess of EDTA added, resulting in the formation of the AF+-EDTA complex. The excess EDTA is back titrated using a standard solution of Le +, providing an indirect analysis for AF+. 

Only slightly less accurate ( 0.3—0.5%) and more versatile in scale are other titration techniques. Plutonium maybe oxidized in aqueous solution to PuO " 2 using AgO, and then reduced to Pu" " by a known excess of Fe", which is back-titrated with Ce" ". Pu" " may be titrated complexometricaHy with EDTA and a colorimetric indicator such as Arsenazo(I), even in the presence of a large excess of UO " 2- Solution spectrophotometry (Figs. 4 and 5) can be utilized if the plutonium oxidation state is known or controlled. The spectrophotometric method is very sensitive if a colored complex such as Arsenazo(III) is used. Analytically usehil absorption maxima and molar absorption coefficients ( s) are given in Table 10. Laser photoacoustic spectroscopy has been developed for both elemental analysis and speciation (oxidation state) at concentrations of lO " — 10 M (118). Chemical extraction can also be used to enhance this technique. 

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. 

B. Back-titration. Many metals cannot, for various reasons, be titrated directly thus they may precipitate from the solution in the pH range necessary for the titration, or they may form inert complexes, or a suitable metal indicator is not available. In such cases an excess of standard EDTA solution is added, the resulting solution is buffered to the desired pH, and the excess of the EDTA is back-titrated with a standard metal ion solution a solution of zinc chloride or sulphate or of magnesium chloride or sulphate is often used for this purpose. The end point is detected with the aid of the metal indicator which responds to the zinc or magnesium ions introduced in the back-tit ration. 

Sulphate may be determined by precipitation as barium sulphate or as lead sulphate. The precipitate is dissolved in an excess of standard EDTA solution, and the excess of EDTA is back-titrated with a standard magnesium or zinc solution using solochrome black as indicator. 

Phosphate may be determined by precipitating as Mg(NH4)P04,6H20, dissolving the precipitate in dilute hydrochloric acid, adding an excess of standard EDTA solution, buffering at pH = 10, and back-titrating with standard magnesium ion solution in the presence of solochrome black. 

Add excess of standard EDTA and back-titrate with standard Mg solution using solochrome black as indicator. This gives the sum of all the metals present. 

Palladium(II) compounds can be determined by a similar procedure, but in this case, after addition of the cyanonickelate, excess of standard (0.01 M) EDTA solution is added, and the excess is back-titrated with standard (0.01 M) manganese(II) sulphate solution using solochrome black indicator. 

Nickel may be determined in the presence of a large excess of iron(III) in weakly acidic solution by adding EDTA and triethanolamine the intense brown precipitate dissolves upon the addition of aqueous sodium hydroxide to yield a colourless solution. The iron(III) is present as the triethanolamine complex and only the nickel is complexed by the EDTA. The excess of EDTA is back-titrated with standard calcium chloride solution in the presence of thymolphthalexone indicator. The colour change is from colourless or very pale blue to an intense blue. The nickel-EDTA complex has a faint blue colour the solution should contain less than 35 mg of nickel per 100 mL. 

Chromium. 25.0 mL chromium(III) ion solution (0.02M, prepared by dilution of stock solution) + 50.0 mL 0.02 M EDTA + 50mL acetate buffer, boiled for 10 minutes, solution cooled, pH adjusted to 4.6 with hexamine, 1 drop of mercury-EDTA solution added, and then back-titrated with standard zinc ion solution. 

Aluminium. 25.0 mL aluminium ion solution, acidified with a few drops of 2.5M nitric add (to pH 1-2), boiled for 1 minute, 50.0 mL 0.05M EDTA added to hot solution, solution cooled, 50 mL acetate buffer and 1 drop of 0.0025 M mercury-EDTA added, excess of EDTA back-titrated with standard zinc ion solution. 

Back titration prevents precipitation of analyte. For example, Al(OH)3 precipitates at pH 7 in the absence of EDTA. An acidic solution of Al3+ can be treated with excess EDTA, adjusted to pH 7-8 with sodium acetate, and boiled to ensure complete formation of stable, soluble Al(EDTA)-. The solution is then cooled, Eriochrome black T indicator is added, and back titration with standard Zn2+ is performed. 

The BaS04(.s) is washed and then boiled with excess EDTA at pH 10 to bring Ba2+ back into solution as Ba(EDTA)2-. Excess EDTA is back-titrated with Mg2+. 

A 1.000-mL sample of unknown containing Co2+ and Ni2+ was treated with 25.00 mL of 0.038 72 M EDTA. Back titration with 0.021 27 M Zn2 at pH 5 required 23.54 mL to reach the xylenol orange end point. A 2.000-mL sample of unknown was passed through an ion-exchange column that retards Co2+ more than Ni2+. The Ni2+ that passed through the column was treated with 25.00 mL of 0.038 72 M EDTA and required 25.63 mL of 0.021 27 M Zn2+ for back titration. The Co2+ emerged from the column later. It, too, was treated with 25.00 mL of 0.038 72 M EDTA. How many milliliters of 0.021 27 M Zn2+ will be required for back titration ... 

Oxamyl (melhyl-N.N -dimethyl-N- [ (methyl-carbarn oyl)oxy] -1 -thio oxamidate) Plants including leaves, potato, tomato and wheat Ethyl acetate extraction of sample Addition of excess copper sulfate, back-titration with standard EDTA to 1 -(2-pyridylazo)2-naphthol [61]... 

Assay Dissolve about 1 g of sample, accurately weighed, in 50 mL of water, add 50.0 mL of 0.05 M disodium EDTA and 20 mL of pH 4.5 buffer solution (77.1 g of ammonium acetate and 57 mL of glacial acetic acid in 1000 mL of aqueous solution), and boil gently for 5 min. Cool, and add 50 mL of alcohol and 2 mL of dithizone TS. Back titrate with 0.05 M zinc sulfate to a bright rose-pink color. Perform a blank determination (see General Provisions), and make any necessary correction. The milliliters of 0.05 M disodium EDTA consumed is equivalent to 50 minus the milliliters of 0.05 M zinc sulfate used. Each milliliter of 0.05 M disodium EDTA is equivalent to 23.72 mg of A1K(S04)2T2H20. Ammonium Salts Add 1 g of sample to 10 mL of 1 N sodium hydroxide in a small beaker, and heat on a steam bath for 1 min. The odor of ammonia is not perceptible. Fluoride Determine as directed in Method V under Fluoride Limit Test, Appendix IIIB. 

Verma and Bhuchar determined copper by reducing its tartrate complex with glucose to form insoluble CujO, which was treated with an excess of standard iodine and back-titrated with standard As(III). Oxalate was added as a complexing agent to aid in the oxidation of the CU2O, and precautions were taken to avoid air oxidation. The method has the advantage of avoiding interference from V(V). For the determination of copper in alloys, Rooney and Pratt separated copper by precipitation as its diethyldithiocarbamate from EDTA solution. 

Uranium(VI) was determined by titration with EDTA using xyle-nol orange (X0) as indicator (6). The acidity of the organic phase was determined by adding 2 % sodium oxalate solution to the aqueous solution back-washed and titrating with 0.01 M sodium hydroxide... 

Back-titration is useful for the determination of cations that form stable EDTA complexes and for which a satisfactory indicator is not available. The method is also useful for cations such as Cr(III) and Co(III) that react only slowly with EDTA. A measured excess of standard EDTA solution is added to the analyte solution. After the reaction is judged complete, the excess EDTA is back-titrated with a standard magnesium or zinc ion solution to an Eriochrome Black T or Calmagite end point. For this procedure to be successful, it is necessary that the magnesium or zinc ions form an EDTA complex that is less stable than the corresponding analyte complex. 

The Cr plating on a surface that measured 3.00 X 4.00 cm was dissolved in HCl. The pH was suitably adjusted, following which 15.00 mL of 0.01768 M EDTA were introduced. The excess reagent required a 4.30-mL back-titration with... 

Chromel is an alloy composed of nickel, iron, and chromium. A 0.6472-g sample was dissolved and diluted to 250.0 mL. When a 50.00-mL aliquot of 0.05182 M EDTA was mixed with an equal volume of the diluted sample, all three ions were chelated, and a 5.11-mL back-titration with 0.06241 M copper(II) was required. The chromium in a second 50.0-mL aliquot was masked through the addition of hexamethylenetetramine titration of the Fe and Ni required 36.28 mL of 0.05182 M EDTA. Iron and chromium were masked with pyrophosphate in a third 50.0-mL aliquot, and the nickel was titrated with 25.91 mL of the EDTA solution. Calculate the percentages of nickel, chromium, and iron in the alloy. 

Chromium(ni) is slow to react with EDTA (H4Y) and is therefore determined by back-titration. A pharmaceutical preparation containing chromium(III) is analyzed by treating a 2.63-g sample with 5.00 mL of 0.0103 M EDTA. Following reaction, the unreacted EDTA is back-titrated with 1.32 mL of 0.0122 M zinc solution. What is the percent chronoiura chloride in the pharmaceutical preparation ... 

Titration with AICI3 solution Fiuoride-iSE compared with volumetric titration, with excess Ce(lll) back-titrated with ethyienediaminetetraacetic acid (EDTA). 

For an indicator to be useful in an EDTA titration, the indicator must give up its metal ion to EDTA. If a metal ion does not freely dissociate from the indicator, the metal is said to block the indicator. Calmagite is blocked by Cu, Ni, Co, Cr, Fe, and Al ". It cannot be used for the direct titration of any of these metals. However, it can be used for a back titration. For example, excess standard EDTA can be added to Cu ". Then indicator is added and excess EDTA is back titrated with Mg ". ... 

Anions that precipitate metal ions can be analyzed with EDTA by indirect titration. For example, sulfate can be analyzed by precipitation with excess Ba at pH 1. The resulting BaS04( ) is filtered, washed, and boiled with excess EDTA at pH 10 to bring Ba back into solution as Ba(EDTA). The excess EDTA is back titrated with Mg. ... 

We studied the calcium—EDTA chelation using Calcein [4] as the fluorescent indicator. This indicator forms a fluorescent complex with free calcium ions. In order to determine calcium in a sample, the excess EDTA remaining after stoichiometric chelation of calcium in the sample is "back-titrated" with a known concentration of calcium solution. The equivalence point is reached when all of the excess EDTA is chelated and is detected by fluorescence of the Calcein-calcium complex formed by a slight excess of free calcium ions. The calcium content of the sample (in micrograms) is equal to the volume (in milliliters) of titrant added when no sample was introduced, minus the volume (in milliliters) of titrant added when the sample was introduced, times the calcium content of the titrant (in micrograms per milliliter). 

In the second stage, an excess of a cyanide solution is added to another sample of the initial solution. A known quantity in excess of EDTA is then added to the obtained solution. The EDTA in excess is finally back-titrated with a standard solution of Mg +. This stage gives the Mg + concentration. 

Pd is determined according to the same principle. The liberated Ni + ions are back-titrated with an excess of EDTA in the presence of eriochrome black T. The back-titrant solution is a MnS04 solution. 

KAI(SO,).12Hp may lose crystal water on standing. The reagent should be standardised by adding an excess of EDTA with ZnSO, and back-titration with Eriochrome Black T as an indicator. 

Both ions can be titrated with EDTA, the titre gives the sum of their concenu tions. When NaF is then added, it will form Mgp2 from the Mg complexonate. When excess of Mn(II) ion solution is then added, the excess can be back-titrated against EDTA. Prepare a solution approximately 0.05 M in Mn(II) and 0.05 M in Mg. Pipette 25.0 cm of the mixed solution, treat as above and record the average titre of 0.05 M EDTA. At the end point, add 2.5 g NaF and agitate to dissolve for a minute, then introduce from a burette the standardised Mn(II) solution in 1 cm portions until a permanent red colour is obtained and then add a slight excess. After recording the volume used, titrate with standardised EDTA to the blue end point. Calculate from the titres the concentrations of Mg and of Mn(Il) in mol dm . 

---