Copper EDTA titration

Discussion. The titration of a copper ion solution with EDTA may be carried out photometrically at a wavelength of 745 nm. At this wavelength the copper-EDTA complex has a considerably greater molar absorption coefficient than the copper solution alone. The pH of the solution should be about 2.4. 

Thiourea masks Cu2+ by reducing it to Cu+ and complexing the Cu+. Copper can be liberated from thiourea by oxidation with H202. Selectivity afforded by masking, demasking, and pH control allows individual components of complex mixtures of metal ions to be analyzed by EDTA titration. 

Another advantage of potentiometric titrations is that substances to which the electrode does not respond can be determined, if the electrode responds to the titrant or to some low level of an indicator substance that has been added to the solution. For example, low levels of Al can be determined by titration with standard fluoride solution, using a fluoride electrode [22]. EDTA and other chelates can be determined by titration with standard calcium or copper solution. Manganese(II), vanadium(II), or cobalt(II) can be determined via EDTA titration if a small amount of CuEDTA indicator is added to the solution and a copper electrode is used. The electrode responds directly to the Cu activity which, however, is dependent on the activities of the EDTA and the other metal ion in solution. 

The copper content of the complex will be determined by a disodium ethylenediaminetetraacetic acid (EDTA) titration. Disodium EDTA or "EDTA," in short, is a hexadentate chelating ligand, a ligand which can potentially bond one copper ion at a maximum of six coordination sites. 

The "copper-EDTA" complex will only form when the hydrogen ion concentration of the solution is carefully controlled. Hence, it is important to carefully adjust and buffer the hydrogen ion concentration or pH of the solution before doing the EDTA titration. 

The concentration of copper(II) is a solution can be determined by titrating with ethylenediaminetetra-acetic acid (EDTA). At a wavelength of 745 nm, the copper-EDTA complex has a much higher molar absorption coefficient than Cu(II), which means that a plot similar to Figure 1C would be obtained. 

The most sensitive - and perhaps for the radiochemist, the most useful volumetric procedure is complexlmetrlc titration utilizing the lead EDTA complex. A number of Indicators have been used for the direct EDTA titration (W5) The most popular of these are Erlochrome Black T (Cl) (W5)j Eriochrome Red B and X-ylenol orange (W5)- The direct titration with the sodium salt of EDTA is carried out in a pH 10 buffer solution (P5)(p6). Iron, the alkaline earths and the earths interfere but bismuth, aluminum and antimony do not. Cyanide can be used to mask cobalt, nickel, copper, zinc, cadmium, mercury and platinum (Cl),... 

Copper may be titrated with EDTA (see p. 786) but for general purposes this is not necessary since the cuprous iodide method given above is satisfactory, moreover there is a limit to the amount of copper which may be titrated using a visual indicator because the colour of the copper-EDTA complex is so intense that it masks the indicator change. 

Determination. The most accurate (68) method for the deterrnination of copper in its compounds is by electrogravimetry from a sulfuric and nitric acid solution (45). Pure copper compounds can be readily titrated using ethylene diamine tetracetic acid (EDTA) to a SNAZOXS or Murexide endpoint. lodometric titration using sodium thiosulfate to a starch—iodide endpoint is one of the most common methods used industrially. This latter titration is quicker than electrolysis, almost as accurate, and much more tolerant of impurities than is the titration with EDTA. Gravimetry as the thiocyanate has also been used (68). 

CrP" -selective and Ni " -selective electrodes have been used to detenuine the copper and nickel ions in aqueous solutions, both by direct potentiometry and by potentiometric titration with EDTA. They have also been used for detenuining the CiT and Ni " ions in indushial waters by direct potentiomehy. 

Zn is determined by direct titration with EDTA with xelenol indicator after iron elimination with acetate ions and copper - with sulfide ions. 

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. 

In the back-titration small amounts of copper and zinc and trace amounts of manganese are quantitatively displaced from the EDTA and are complexed by the triethanolamine small quantities of cobalt are converted into a triethanolamine complex during the titration. Relatively high concentrations of copper can be masked in the alkaline medium by the addition of thioglycollic acid until colourless. Manganese, if present in quantities of more than 1 mg, may be oxidised by air and forms a manganese(III)-triethanolamine complex, which is intensely green in colour this does not occur if a little hydroxylammonium chloride solution is added. 

Procedure. Charge the titration cell (Fig. 17.24) with 10.00 mL of the copper ion solution, 20 mL of the acetate buffer (pH = 2.2), and about 120mL of water. Position the cell in the spectrophotometer and set the wavelength scale at 745 nm. Adjust the slit width so that the reading on the absorbance scale is zero. Stir the solution and titrate with the standard EDTA record the absorbance every 0.50 mL until the value is about 0.20 and subsequently every 0.20 mL. Continue the titration until about 1.0 mL after the end point the latter occurs when the absorbance readings become fairly constant. Plot absorbance against mL of titrant added the intersection of the two straight lines (see Fig. 17.23 C) is the end point. 

UNDERWOOD, A. L. Simultaneous titration of iron and copper with EDTA. 

Chlorpromazine formed an insoluble 1 1 complex with lead picrate, and 5 3 complexes with the picrates of cadmium, copper, and zinc [70]. The sample (0.1 g) was dissolved in 15 mL of 95% ethanol, and the solution adjusted to pH 9 with 0.1 N NaOH. After adding 25 mL of a 0.02 M picrate reagent (30 mL of Pb), the solution was set aside for 2 hours. The precipitate was collected on a sintered glass fuimel, and the unconsumed metal in the filtrate was titrated directly with 0.02M EDTA at pH 10.4 (after adding 0.5 g of potassium sodium tartrate for Pb). Eriochrome black T was used as the indicator. 

Figure 1 Titration curves for H,edta. Curve 1 H4edta curve 2 H4edta + lithium curve 3 H4edta + magnesium curve 4 H4edta + copper. The quantity a is in moles of strong base per mole H4edta. Total ligand and metal concentration ...

The equilibrium between Mg2+ and the edta anion L4 can be compared to that between NH3 and H+, because the corresponding equilibrium constants are very dose in magnitude. In the latter case, it is possible to titrate NH3 with a solution of a strong acid in order to determine quantitatively its total concentration. It is therefore quite evident that, based on the values of the equilibrium constants of Scheme 3, the quantitative determination of the Mg2+ using edta should be possible. Because the other cations form more stable complexes than Mg2+, the complexometric titration should be of wide application. Some caution is necessary concerning the pH value at which the determination is done, because the ligand can be protonated, with consequent decrease of its chelating power. However, in the case of copper(II), its edta complex is already completely formed at pH 3 and therefore a titration is possible under these conditions. 

Singhal et al. [306] have determined oxamyl residues (methyl-N. N -dimclh-yl-N- [methylcarbamoyl] oxy-thioaminimidate) by a method based on reaction with carbon disulfide and copper in which excess copper is added to an extract of the soil and the excess copper is determined by titration with 0.001 M EDTA to the l-(2-pyridylazo)-2-naphthol endpoint. 

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]... 

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. 

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. 

A 0.3284-g sample of brass (containing lead, zinc, copper, and tin) was dissolved in nitric acid. The sparingly soluble SnOi -4H20 was removed by filtration, and the combined fdtrate and washings were then diluted to 500.0 mL. A 10.00-mL aliquot was suitably buffered titration of the lead, zinc, and copper in this aliquot required 37.56 mL of 0.002500 M EDTA. The copper in a 25.00-mL aliquot was ma.sked with thiosulfate the lead and zinc were then titrated with 27.67 mL of the EDTA solution. Cyanide ion was used to mask the copper and zinc in a 100-niL aliquot 10.80 mL of the EDTA solution was needed to titrate the lead ion. Determine the composition of the brass sample evaluate the percentage of tin by difference. 

Some wet chemical sample preparation such as pH or oxidation state adjustment is normally required for most metal ion determinations. Then complexometric titration using EDTA, as already mentioned, or diphenylthio-carbazone ( dithazone Eq. 4.27) may be used for cadmium, copper, lead, mercury, or zinc determinations. 

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. 

The shift in electrode potential caused by the complexing agent is contained in the lecond term of Equation 2.18. In this case, it amounts to a shift of —0.526 V. The important practical consequences of chelation and complexation will be discussed in more detail later. Fdr example, one can determine copper ion by direct potentiometry using a copper-ion-selective electrode, or via a potentiometric titration with EDTA using the electrode as an endpoint detector. 

Quantitatively transfer the solution from the previous ammonia analysis in part C to a 250-mL or larger beaker. Add 0.5 M aqueous ammonia, NH3, slowly from your buret until the pH of the solution is 9-10. Add 20. mL of the pH 10. buffer. If the resulting solution is cloudy, slowly with stirring add more buffer until the solution clears or until the total volume of added buffer is 30. mL. Quantitatively transfer the solution to a 500-mL volumetric flask, and dilute the solution with distilled water to the "mark." Mix the solution thoroughly. Rinse and fill your buret with this solution. Measure precisely 50.00 mL of the solution into a clean 250-mL Erlenmeyer flask. Add an additional 125 mL of distilled water to the flask, and heat the solution to 50-60°C (Laboratory Methods D). Add about 0.1 g of Murexide Tablet indicator (0.1 g is approximately the amount if you have 0.5 cm on the end of your spatula) to the flask. Be careful you should not add excess indicator. Clean the buret, and fill it with your standardized EDTA solution from part B. Titrate the warm metal ion solution with EDTA to a blue endpoint which persists for at least 30. seconds. Record the initial and final volumes of EDTA solution in TABLE 17.ID. Repeat the analysis with a second 50.00-mL portion of the solution from sample 1 in part C. Then repeat the copper analysis in duplicate again if a second sample is available from part c. 

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