Alloys containing aluminum and zinc as primary alloying elements have been developed in Japan for us — Organic Chemistry Chemistry Question
Analysis of zinc-aluminum alloy by EDTA titration
Alloys containing aluminum and zinc as primary alloying elements have been developed in Japan for use in industry. The most famous example, extra super duralumin “7075”, is the strongest aluminum alloy used in aircraft manufacturing. Recently, a novel zinc-aluminum alloy that exhibits an interesting mechanical property has also been developed. The alloy exists as a solid at room temperature but is easily spread like a starch syrup under appropriate mechanical tension. This property is known as “super-plasticity,” which facilitates industrial uses of the alloy, including use as a high-performance and semi-permanent seismic damper for protecting buildings from earthquakes. This unique property arises from the alloy’s fine-grained microstructure containing 7 % – 50 % aluminum by mass.
Composition is a fundamental parameter for developing such advanced alloys. In this experiment, assuming the composition assay of this type of alloy, you will be given a test solution which simulates a digested sample of the alloy; 50 cm3 of the solution contains 30 – 35 mg of zinc and 10 – 15 mg of aluminum, and is acidified to pH 1 using hydrochloric acid. You will be required to determine the concentrations of Zn2+ and Al3+ in the sample solution by titration utilizing ethylenediaminetetraacetic acid (EDTA) as a chelating agent. Masking and back-titration techniques should also be employed.
Chemicals and reagents
* test solution (prepare as described above)
* acetic acid solution, c = 0.1 mol dm-3
* ammonium fluoride
* ethylenediamine-N,N,N',N'-tetraacetic acid, c = 0.01 mol dm-3, disodium dihydrate standard solution (accurately prepared)
* hexamethylenetetramine (hexamine) solution, 10 % (w/v)
* methyl orange (MO) solution, 0.1 %
* xylenol orange (XO) ethanol/water (20/80) solution, 0.1 %
* zinc(II) standard solution, c = 0.01 mol dm-3, (accurately prepared, using ZnSO4⋅7 H2O)
Apparatus and glassware
* burette, 25 cm3, 1 rack
* 5 Erlenmeyer flasks, 200 cm3
* hot-plate
* magnetic stirrer
* 4 Pasteur pipettes (droppers)
* pipette, 10 cm3
* safety bulb
* 4 stirring bars (coated with Teflon)
Procedures
1. Pipette 10 cm3 of the sample solution into a 200 cm3 Erlenmeyer flask and place a stirring bar in the flask. Start stirring the solution on a magnetic stirrer and add a few drops of MO indicator. Add 30 cm3 of a EDTA standard solution (0.01 mol dm-3). To adjust the pH of the mixture to ca. 3.5, introduce (dropwise) a 10% hexamine solution into the flask until the MO indicator shows a slight color change, from red to orange. Place the flask on a hot-plate and boil the mixture for a few minutes; then place the flask in an ice bath to cool the mixture. After cooling, place the flask on the magnetic stirrer and add a few drops of the XO indicator solution to the mixture.
2. Adjust the pH to ca. 5.5 as follows: Stir the mixture gently, deliver the 10% hexamine solution dropwise into the flask until the XO indicator changes color from yellow to slightly purplish, and then, add a 0.1 mol dm-3 acetic acid solution in drops until a clear yellow color reappears. Next, titrate the mixture using the standard Zn2+ solution with c = 0.01 mol dm-3 until the color turns to purple. The volume of titrant used in step 2 is defined as “A” cm3. (Caution: Do not discard the titrated mixture; you will need to titrate it continuously in step 3.)
Note: Determining the end point is somewhat difficult, since the color changes gradually from yellow to purple as the end point is approached. When the color is close to purple, read the burette and then add another drop of titrant; if there is a perceptible color change, read the burette again and add another drop. Repeat this process until a drop of titrant causes no color change, and then record the preceding burette reading. If EDTA is still present, the yellow color will return; add more titrant until the color remains purple for at least one minute.
3. Add ca. 1.0 g of NH4F to the titrated mixture in step 1 and heat it on the hot-plate until the mixture boils; note that heating results in the mixture’s color returning to yellow. Remove the flask to an ice bath, and after the mixture cools, place the flask on the magnetic stirrer. If the clear yellow color has disappeared after cooling, add a 0.1 mol dm-3 acetic acid solution dropwise until the color reappears. Next, titrate the mixture again using a 0.01 mol dm-3 Zn2+ standard solution. The volume of the titrant used in step 3 is defined as “B” cm3.
In steps 1 and 2, why is the pH adjusted to ca. 3.5 and ca. 5.5, respectively, in a stepwise manner? Explain the reason considering the difference in the stability of each metal-EDTA and –hydroxyl complex.
Model Answer
It makes it possible to avoid the formation of Al(III)-hydroxyl complex ions upon a reaction between EDTA and Al3+.
In this procedure, all Al3+ and Zn2+ in the sample solution should be reacted with EDTA to obtain the total metal content using the back titration technique in step 1-2.
To this end, adjusting the pH of the solution to around 5.5 is required to allow a quantitative reaction between Zn2+ and EDTA. At around pH 5.5, on the other hand, formation of an Al(III-EDTA chelate complex is considerably suppressed by the formation of various Al(III)-hydroxyl complex ions, and the formation of the Al(III- EDTA chelate complex would be incomplete (Al3+ forms various highly stable hydroxyl complex ions, even in weakly acidic media). Multi-nuclear complexes such as [Al2(OH)2]4+ or [Al13(OH)32]7+, especially, which are kinetically highly inert, are formed at pH 4 or above. In fact, only a limited amount of the Al(III-EDTA chelate complex could be expected to be formed in this pH region in a reaction at room temperature. Al3+ must therefore be reacted with EDTA under boiling conditions after the pH is carefully adjusted to less than 4.
Hence, a lower aluminum content would be estimated due to incomplete formation of Al(III-EDTA chelate if the pH of the sample solution was adjusted to pH 5.5 in one step.
What is the role of ammonium fluoride added to the mixture in step 3?
Model Answer
Ammonium fluoride is used for the quantitative recover of Al3+ from the Al(III-EDTA chelate complex. Fluoride ions react readily with aluminum ions to create a highly stable complex, [AlF6]3-. Adding a large amount of ammonium fluoride to the titration mixture and heating result in a quantitative exchange of EDTA ligands in the Al(III-EDTA chelate with fluoride ions, as follows:
[Al(III)(edta)]– + 6 F– → [AlF6]3– + edta4–
(The change in color of the XO indicator upon heating indicates that the free EDTA was released from the Al(III-EDTA chelate complex by the reaction.)
Hence, the contents of Al3+ in the mixture can be determined by titrating the released free EDTA ligands with the Zn2+ standard solution.
Show the formula for calculating the concentration of the Al3+ and Zn2+ ions in the sample solution based on the results of each titration (A and B).
Model Answer
The total amount of substance of Al3+ and Zn2+ (in mol) in a 10 cm3 sample solution is given by equation 33-1:
n(Al3+ + Zn2+ ) = 0.01 × fedta fZn × (6.00-A) × 0.001 (eq. 33.1)
where fedta and fZn are the factors for the standard solutions of 0.01 mol dm–3 EDTA and 0.01 mol dm–3 Zn2+ used, respectively.
The concentration of Al3+ in the sample solution is given by equation 33-2:
Conc. of Al3+ = 0.01 × fZn × B × 0.001 / 0.010 (eq. 33-2)
The concentration of Zn2+ in the sample solution is given by equation 33-3:
conc. of Zn2+ = 0.001×0.01 × fZn{fedta (6.00 – A) – 0.01 × B } / 0.010 (eq. 33-3)
Calculate the concentrations (in mol cm-3) of Al3+ and Zn2+ ions in the sample solution.
Assuming that the alloy contains only Al and Zn, calculate the composition of the alloy in percent by weight.