Color EDTA titration

Finding the End Point with a Visual Indicator Most indicators for complexation titrations are organic dyes that form stable complexes with metal ions. These dyes are known as metallochromic indicators. To function as an indicator for an EDTA titration, the metal-indicator complex must possess a color different from that of the uncomplexed indicator. Furthermore, the formation constant for the metal-indicator complex must be less favorable than that for the metal-EDTA complex. 

Figure 12-14 Guide to EDTA titrations of common metals. Light color shows pH region in which reaction with EDTA is quantitative. Dark color shows pH region in which auxiliary complexing agent is required to prevent metal from precipitating. Calmagite is more stable than Eriochrome black T (EB) and can be substituted for EB. [Adapted from K. Ueno, Guide tor Selecting Conditions of EDTA Titrations." J. Chem. Ed. 1965,42,432.]...

An important factor in the application of EDTA titration methods has been the development of suitable metal ion indicators, which permit visual titrations to be carried out in dilute solutions. A metal ion indicator is usually a dyestuff that forms metal ion complexes of a color different from that of the uncomplexed indicator. The complex forms over some characteristic range of values of pM, exactly as an add-base indicator forms a hydrogen ion complex over a characteristic range of pH... 

The scope of the present treatment does not include details of the various instrumental methods for the detection of EDTA titration end points. Nevertheless, we may mention spectrophotometric detection methods, which are of two types. The first is based on instrumental observation of the color changes of metal ion indicators. The second is based on the absorption of radiation in the visible or ultraviolet regions of the spectrum by the metal-EDTA complex. For example, MgY shows appreciable absorbance at a wavelength of 222 nm, whereas the reagent HjY ... 

The second method is by ethylene diamine tetra acetic acid (EDTA) titration, in which an indicator, Eriochrome Black T is added to the water sample, which develops a red color due to the presence of ( k. When EDTA titer is added, Ca is complexed and yields an end point at which the color changes to blue. To ensure a sharp end point, a small amount of magnesium salt of EDTA is added. The titration is performed at room temperature and at a pH of 10. 

The use of Eriochrome Black T as an indicator in the Zn -EDTA titration illustrates a case in which the indicator metal-complex is so stable that the color change occurs after the equivalence point, (log P nin equivalence point). Since log znin changes more rapidly with pH than does pZn (Why ), it is possible to adjust the pH to reduce the difference between them to a reasonably small value. (Table 9.2). 

Note the analogy of this result with that outlined for the effect of solubility product in precipitation titrations discussed above. A great many compounds have been proposed as indicators for metal ions in EDTA titrations. These species are generally organic compounds that form colored chelates with metal ions in a range of pM that is characteristic of the cation and dye. One example is Eriochrome black T, which is blue at pH 7 and red when complexed with a variety of metal ions. 

In a back titration, a slight excess of the metal salt solution must sometimes be added to yield the color of the metal-indicator complex. Where metal ions are easily hydrolyzed, the complexing agent is best added at a suitable, low pH and only when the metal is fully complexed is the pH adjusted upward to the value required for the back titration. In back titrations, solutions of the following metal ions are commonly employed Cu(II), Mg, Mn(II), Pb(II), Th(IV), and Zn. These solutions are usually prepared in the approximate strength desired from their nitrate salts (or the solution of the metal or its oxide or carbonate in nitric acid), and a minimum amount of acid is added to repress hydrolysis of the metal ion. The solutions are then standardized against an EDTA solution (or other chelon solution) of known strength. 

Titration curves for 10 M Mg + with 10 M EDTA using calmagite as an indicator at (a) pH = 9, (b) pH = 10, and (c) pH = 11. The range of pMg and volume of titrant over which the indicator is expected to change color is shown for each titration curve. 

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. 

Into a conical flask, pipette a 50.0 or 100.0 mL aliquot of the solution and adjust the pH to 1-2 with aqueous ammonia solution (use pH test-paper). Add five drops of xylenol orange indicator and titrate with additional 0.05 M EDTA until the colour changes sharply from red to yellow. This gives the bismuth content. Record the total (combined) volume of EDTA solution used. Now add small amounts of hexamine (ca 5g) until an intense red-violet coloration persists, and titrate with the standard EDTA to a yellow end point the further consumption of EDTA corresponds to the lead-plus-cadmium content. 

Give your EDTA solution one final shake to ensure its homogeneity. Then, clean a 50-mL buret, rinse it several times with your EDTA solution (the titrant), and fill, as usual, with your titrant. Be sure to eliminate the air bubbles from the stopcock and tip. Titrate the solutions in your flask, one at a time, until the last trace of red disappears in each with a fraction of a drop. This final color change will be from a violet color to a deep sky blue, but should be a sharp change. All three titrations should agree to within 0.05 mL of each other. If they do not, repeat until you have three that do. Record all readings in your notebook. 

The indicator must give up its metal ion to the EDTA. If a metal does not freely dissociate from an indicator, the metal is said to block the indicator. Eriochrome black T is blocked by Cu2+, Ni2+, Co2+, Cr3+, Fe3+, and Al3+. It cannot be used for the direct titration of any of these metals. It can be used for a back titration, however. For example, excess standard Question What will the color change be EDTA can be added to Cu2+. Then indicator is added and the excess EDTA is back-titrated... 

In a direct titration, analyte is titrated with standard EDTA. The analyte is buffered to a pH at which the conditional formation constant for the metal-EDTA complex is large and the color of the free indicator is distinctly different from that of the metal-indicator complex. 

Calcium ion was titrated with EDTA at pH 11. using Cal-magite as indicator (Table 12-3). Which is the principal species of Calmagite at pH 11 What color was observed before the equivalence point After the equivalence point ... 

To a solution containing 25.00 mL of 0.043 32 M Cu(C104)2 plus 15 mL of 1 M acetate buffer (pH 4.5) were added 25.00 mL of unknown sulfide solution with vigorous stirring. The CuS precipitate was filtered and washed with hot water. Then ammonia was added to the filtrate (which contained excess Cu2+) until the blue color of Cu(NH3) + was observed. Titration with 0.039 27 M EDTA required 12.11 mL to reach the murexide end point. Calculate the molarity of sulfide in the unknown. 

The excess yellow Bil4 is then titrated with EDTA. The end point occurs when the yellow color disappears. (Sodium thiosulfate is used in the reaction to prevent the liberated I from being oxidized to yellow aqueous I2 by 02 from the air.) The precipitation is fairly selective for Cs+. The ions Li+. Na+, K+. and low concentrations of Rb+ do not interfere, although Tl+ does. Suppose that 25.00 mL of unknown containing Cs were treated with 25.00 mL of0.086 40 M NaBiI4 and the unreacted Bil4 required 14.24 mL of 0.043 7 M EDTA for complete titration. Find the concentration of Cs+ in the unknown. 

Color Plate 6 Titration of Cu(II) with EDTA, Using Auxiliary Complexing Agent (Section 12-5)... 

M C11SO4 before titration (left). Color of Cu(II)-ammonia complex after adding ammonia buffer, pH 10 teenier). End-point color when all ammonia ligands have been displaced by EDTA (right). 

Color Plate 7 Titration of Mg2+ by EDTA, Using Eriochrome Black T Indicator (Demonstration 12-1)... 

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 ... 

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. 

To determine the amount of calcium in a water sample, e.g. the titration with EDTA (ethylenediaminetetraacetate, C2H4N2(CH2COOH)4) can be used. First of all, NaOH is added to the sample to obtain a pH value of at least 12. Then, a color indicator is admixed and titration with EDTA performed until the color changes. In doing so, all Ca is converted to a Ca-EDTA complex and detected in this form. 

The amount of EDTA that has to be added until the color changes is unknown. Therefore, EDTA is added step by step and the titration is continued beyond the point of color change. The point of color change will be determined afterwards using the obtained graph. 

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. 

Assay Accurately weigh about 500 mg of the residue obtained in the test for Loss on Ignition (below), dissolve it in a 1 50 mixture of hydrochloric acid water, dilute with water to 100.0 mL, and mix. Transfer 50.0 mL of this solution into a 250-mL Erlenmeyer flask, add 10 mL of ammonia-ammonium chloride buffer TS and 12 drops of eriochrome black TS, and titrate with 0.1 M disodium EDTA until the wine red color changes to pure blue. Each milliliter of 0.1 M disodium EDTA is equivalent to 12.04 mg of MgS04. 

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