Standard EDTA solutions

The almost colourless leuco form of the base passes upon oxidation into the strongly coloured indamine. When titrating iron(III) at a pH of about 3 and the colourless hydrochloride of the leuco base is added, oxidation to the violet-blue indamine occurs with the formation of an equivalent amount of iron(II). At the end point of the EDTA titration, the small amount of iron(II) formed when the indicator was introduced is also transformed into the Fe(III)-EDTA complex FeY-, whereupon the blue indamine is reduced back to the leuco base. 

The indicator solution is a 1 per cent solution of the base in water. 

Solutions of EDTA of the following concentrations are suitable for most experimental work 0.1M, 0.05M, and 0.01 M. These contain respectively 37.224 g, 18.612g, and 3.7224 g of the dihydrate per litre of solution. As already indicated, the dry analytical grade salt cannot be regarded as a primary standard and the solution must be standardised this can be done by titration of nearly neutralised zinc chloride or zinc sulphate solution prepared from a known weight of zinc pellets, or by titration with a solution made from specially dried lead nitrate. 

Water purified or prepared as described above should be used for the preparation of all solutions required for EDTA or similar titrations. 

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Standard EDTA Solutions. Disodium dihydrogen ethylenediaminetetraacetate dihydrate is available commercially of analytical reagent purity. After drying at 80°C for at least 24 hr, its composition agrees exactly with the dihydrate formula (molecular weight 372.25). It may be weighed directly. If an additional check on the concentration is required, it may be standardized by titration with nearly neutralized zinc chloride or zinc sulfate solution. 

A. Direct titration. The solution containing the metal ion to be determined is buffered to the desired pH (e.g. to PH = 10 with NH4-aq. NH3) and titrated directly with the standard EDTA solution. It may be necessary to prevent precipitation of the hydroxide of the metal (or a basic salt) by the addition of some auxiliary complexing agent, such as tartrate or citrate or triethanolamine. At the equivalence point the magnitude of the concentration of the metal ion being determined decreases abruptly. This is generally determined by the change in colour of a metal indicator or by amperometric, spectrophotometric, or potentiometric methods. 

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. 

Thymolphthalein complexone (thymolphthalexone). This is thymolphthalein di(methyliminediacetic acid) it contains a stable lactone ring and reacts only in an alkaline medium. The indicator may be used for the titration of calcium the colour change is from blue to colourless (or a slight pink). Manganese and also nickel may be determined by adding an excess of standard EDTA solution,... 

Pipette 25 mL of the copper solution (0.01 M) into a conical flask, add 100 mL de-ionised water, 5 mL concentrated ammonia solution and 5 drops of the indicator solution. Titrate with standard EDTA solution (0.01 M) until the colour changes from purple to dark green. 

Calculate the volume of standard EDTA solution equivalent to the magnesium by subtracting the total volume required for the calcium from the volume required for the total calcium and magnesium for equal amounts of the test sample. 

Procedure. To a 50 mL sample of the water to be tested add 1 mL buffer solution (ammonium hydroxide/ammonium chloride, pH 10, Section 10.54) and 30-40 mg solochrome black indicator mixture. Titrate with standard EDTA solution (0.01 M) until the colour changes from red to pure blue. Should there be no magnesium present in the sample of water it is necessary to add 0.1 mL magnesium-EDTA solution (0.1 M) before adding the indicator (see Section 10.54). The total hardness is expressed in parts of CaC03 per million of water. 

Pipette 25 mL of the solution containing magnesium, manganese and zinc ions (each approx. 0.02M), into a 250 mL conical flask and dilute to 100 mL with de-ionised water. Add 0.25 g hydroxylammonium chloride [this is to prevent oxidation of Mn(II) ions], followed by 10 mL of the buffer solution and 30-40 mg of the indicator/potassium nitrate mixture. Warm to 40 °C and titrate (preferably stirring magnetically) with the standard EDTA solution to a pure blue colour. 

Iron (and nickel, if present) can be determined by adding an excess of standard EDTA to the cold solution, and then back-titrating the solution with lead nitrate solution using xylenol orange as indicator provided the solution is kept cold, chromium does not react. The solution from the back-titration is then acidified, excess of standard EDTA solution added and the solution boiled for 15 minutes when the red-violet Cr(III)-EDTA complex is produced. After cooling and buffering to pH 6, the excess EDTA is then titrated with the lead nitrate solution. 

To the resulting solution now add a further 20 mL portion of the standard EDTA solution, add nitric acid (1M) to adjust the pH to 1-2, and then boil the solution for 15 minutes. Cool, dilute to 400 mL by the addition of de-ionised water, add hexamine to bring the pH to 5-6, add more of the indicator solution, and titrate the excess EDTA with the standard lead nitrate solution. 

After dissolution of the alloy in a mixture of concentrated nitric and hydrochloric acids the iron is masked with triethanolamine in an alkaline medium, and the manganese titrated with standard EDTA solution using thymolphthalexone as indicator. The amount of iron(III) present must not exceed 25 mg per 100 mL of solution, otherwise the colour of the iron(III)-triethanolamine complex is so intense that the colour change of the indicator is obscured. Consequently, the procedure can only be used for samples of ferro-manganese containing more than about 40 per cent manganese. 

A mixture of tin(IV) and lead(II) ions may be complexed by adding an excess of standard EDTA solution, the excess EDTA being determined by titration with a standard solution of lead nitrate the total lead-plus-tin content of the solution is thus determined. Sodium fluoride is then added and this displaces the EDTA from the tin(IV)-EDTA complex the liberated EDTA is determined by titration with a standard lead solution. 

Procedure. Prepare a standard EDTA solution (0.2M), a standard lead solution (0.01 M), a 30 per cent aqueous solution of hexamine, and a 0.2 per cent aqueous solution of xylenol orange. 

The analysis of low-melting alloys such as Wood s metal is greatly simplified by complexometric titration, and tedious gravimetric separations are avoided. The alloy is treated with concentrated nitric acid, evaporated to a small volume, and after dilution the precipitated tin(IV) oxide is filtered off heavy metals adsorbed by the precipitate are removed by washing with a known volume of standard EDTA solution previously made slightly alkaline with aqueous... 

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 ... 

The sulphate is precipitated as barium sulphate from acid solution, the precipitate is filtered off and dissolved in a measured excess of standard EDTA solution in the presence of aqueous ammonia. The excess of EDTA is then titrated with standard magnesium chloride solution using solochrome black as indicator. 

The cobalt content may be rapidly checked by titration with standard EDTA solution in the presence of xylenol orange as indicator (see Section 10.59). 

Reagents required. Standard EDTA solution, 0.05M. See Section 10.49. 

Plot the titration curve (potential in millivolts vs S.C.E. against volume of standard EDTA solution) and evaluate the end point. In general, results accurate to better than 0.1 per cent are obtained. Brief notes on determinations with various metal ion solutions follow. 

Discussion. Salicylic acid and iron(III) ions form a deep-coloured complex with a maximum absorption at about 525 nm this complex is used as the basis for the photometric titration of iron(III) ion with standard EDTA solution. At a pH of ca 2.4 the EDTA-iron complex is much more stable (higher stability constant) than the iron-salicylic acid complex. In the titration of an iron-salicylic acid solution with EDTA the iron-salicylic acid colour will therefore gradually disappear as the end point is approached. The spectrophotometric end point at 525 nm is very sharp. 

The magnesium will be liberated quantitatively and may then be titrated with a standard EDTA solution. Where mixtures of metal ions are analysed, the masking procedures already discussed can be utilized or the pH effect exploited. A mixture containing bismuth, cadmium and calcium might be analysed by first titrating the bismuth at pH = 1-2 followed by the titration of cadmium at an adjusted pH = 4 and finally calcium at pH = 8. Titrations of this complexity would be most conveniently carried out potentiometrically using the mercury pool electrode. 

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. 

The number of moles of hydrogen gas evolved is measured from standard PVT relationships. The aluminum trichloride remaining in solution is analyzed for aluminum by using standard EDTA+ solution and dithizone indicator. The active hydride/aluminum ratio was found to be 3.0 1.0 in all preparations. 

Standard EDTA Solution Dissolve 4.0 g of disodium EDTA (CioHi4N2Na208-2H20) in sufficient water to make 1000 mL. 

Final Preparation Pipet 50.0 mL of Magnesium Sulfate Solution into the flask, add exactly the volume, / , in milliliters, of Standard EDTA Solution, determined as directed under Titer Determination (below), then dilute to volume with water, and mix. 

Procedure Add 5 mL of Buffer Solution Final Preparation, 1 mL of a 1 20 potassium cyanide solution, and 5 drops of eriochrome black TS or another suitable indicator to the Sample Preparation. Begin stirring with a magnetic stirrer, and titrate with Standard EDTA Solution to a true blue endpoint, recording the volume, in milliliters, required as V. Calculate the milligrams per kilogramof total calcium and magnesium (both expressed as Ca) in the sample by the formula... 

Standard EDTA solutions are ordinarily prepared by dissolving weighed quantities of Na2H2Y 2H2O and diluting to the mark in a volumetric flask. 

Many of the metals in the periodic table can be determined by titration with standard EDTA solutions. Some methods are based on indicators that respond to the analyte itself, while others are based on an added metal ion. 

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. 

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