In this problem you will identify a metal using complexometric titrations. EDTA (the disodium salt o — Organic Chemistry Chemistry Question
Identification of a metal
In this problem you will identify a metal using complexometric titrations. EDTA (the disodium salt of ethylene-diamine-tetraacetic acid) forms stable complexes with most di- and trivalent metal ions.
M2+ + H2Y2– = MY2– + 2 H+
M3+ + H2Y2– = MY– + 2 H+
where M is the metal and Y4– is the anion formed from EDTA.
While there is an excess of uncomplexed metal ions present, they bond to the indicator molecules. At the end of the reaction all ions form an EDTA complex and the indicator molecules will be liberated bringing about a change in color. Thus the end point of the reaction is when addition of EDTA does not change the hue of the solution. One has to titrate until a steady color is reached. A previously titrated sample can be used for comparison.
Chemicals and reagents
• EDTA, disodium salt, c = 0.05 mol dm–3
• Nitric acid, 65 %
• Nitric acid, 10 %
• Ammonia, 5 %
• Methylthymol blue
Procedure
First a metal sample is dissolved in nitric acid. After setting the pH of the resulting solution to approximately 2, it is titrated with EDTA. In another measurement the oxide of the metal is dissolved and the solution is titrated again with EDTA. (The oxide was prepared previously by evaporating the nitric acid solution of the metal and calcining the residue.)
Many solutions used are very acidic, treat them with due caution.
Titration of the solution prepared from metal
Weigh accurately about 150 mg of the unknown metal into a titration flask. Carefully add 3 cm3 conc. nitric acid using working under the hood. Complete dissolution may take 10 minutes. Dilute the solutions to 50 cm3 and then return to your desk. First add 8 cm3 of 5 % ammonia solution, then continue adding the ammonia solution dropwise until the contents of the flask start to opalize from the hydroxide precipitate. Add immediately 5 cm3 of 10 % nitric acid. Add two pinches of solid methylthymol blue indicator. Titrate with a EDTA solution (c = 0.0500 mol dm–3) until a steady yellow color is attained. Repeat as necessary.
Titration of the solution prepared from metal oxide
Weigh accurately about 1.000 g of the oxide, dissolve it in 5 cm3 conc. nitric acid and then dilute to 100 cm3 in a volumetric flask. Transfer 20.00 cm3 portions into a titration flask and dilute to 50 cm3. Start adding 5 % ammonia dropwise until the appearance of a precipitate. Immediately add 5 cm3 of 10 % nitric acid solution. Add two pinches of solid methylthymol blue indicator. Titrate with a EDTA solution (c = 0.0500 mol dm–3) until a steady yellow color is attained. Repeat as necessary.
Identify the metal based on calculations.
Model Answer
If m1 is the mass of the metal and the volume of EDTA solution consumed is V1, the following equation can be written for the molar mass of the metal Me: M(Me) = m1 / (c_EDTA * V1). Let n denote the oxidation number of the metal in the oxide. From here it follows that the formula of the oxide is M2On. If m2 is the mass of the oxide weighed and the volume of EDTA solution consumed is V2, the following equation can be written: 20.00 / 100.00 * m2 / (2 * M(Me) + n * M(O)) = c_EDTA * V2, which can be simplified to: 0.4 * m2 / (c_EDTA * V2) - 2 * m1 / (c_EDTA * V1) = n * M(O). Bismuth is best used as the unknown. In this case the volumes consumed should be V1 = 14.36 cm3 and V2 = 17.17 cm3 to give M(Me) = 208.9 g mol–1 and n = 3. Actual volumes measured are reproducible and lower, but the identification of the metal is unambiguous.
Give the formula of the oxide.
Model Answer
The formula of the oxide is Bi2O3.