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Sodium Thiosulfate • This Method Is Called "Iodometry"

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Sodium Thiosulfate • This Method Is Called 7/20/2010 Iodometric Determination of Copper In US One Cen t Co ins Ma de Be fore an d After 1982 XX YY Pennies are an interesting commodity. They are everywhere , an always present means of currency. The penny is ingrained in our society for many different social and economic reasons. 1 7/20/2010 Penny Composition • Pennies made from 1962 to 1982 consist of 95% copper and 5% zinc. These copper rich pennies are currently worth more as a metal source than their face value. • Pennies minted after 1982 to the present day are only made up of 2.5% copper. This copper is plated onto a zinc core that makes up 97.5% of the penny. • The determination of copper in both pennies made before and after 1982 will be determined using the same method. Introduction to Iodometric Determination 2 7/20/2010 Wet Chemical Analysis • MtdithittlMore accurate and precise than instrumental methods. • Systematic errors have been detected in the past and can be accounted. • More involved process • Help to learn new techniques • Reinforce the techniques learned so far Process selection - Iodine • UdidltUsed widely to quan tittiltitatively measure chemicals reactions • Many analytical procedures are based on the release or uptake • An analyte that is an oxidizing agent is added to excess iodide to produce iodine • The iodine produced is determined by titration with sodium thiosulfate • This method is called "iodometry" 3 7/20/2010 Iodometry • Chosen f or it s st rai ghtf orwar d proce dure • The reaction is rapid and quantitative • The starch used is a clear and determinant indicator • The reagents involved are all easily obtained and in good supply. • Volumetric determination with a back titration is a well documented method of determining the amount of an ion in a solution. Volumetric Determination • A primary standard – selected as appropriate to the ion being measured – dissolved into an aqueous solution – used in a titration to standardize a secondary standard solution of sodium thiosulfate • The analyte is reacted with iodide to oxidize ion to iodine • The iodine is then titrated with thiosulfate with a starch indicator to determine the endpoint • With the known amount of thiosulfate used to titrate the freed iodine, we can determine how much iodide reacted with our analyte and then determine how much analyte is present 4 7/20/2010 Iodometry • The iodine (triiodide) – iodide redox system, - - - I3 + 2 e ֖ 3 I • The iodide ion is a strong reducing agent that many oxidizing agents can react with completely • The amount of iodine liberated in the reaction between iodide ion and an oxidizing agent is a measure of the quantity of oxidizing agent originally present in the solution • The amount of standard sodium thiosulfate solution required to titrate the liberated iodine is then equivalent to the amount of oxidizing agent • Iodometric methods can be used for the quantitative determination of strong oxidizing agents such as potassium dichromate, permanganate, hydrogen peroxide, oxygen, and in my case, the cupric ion. Sodium Thiosulfate • MdMade from disso lilving so dium thiosu lftlfate pentahydrate salt • Solution is standardized against a primary standard • Iodine oxidizes thiosulfate to the tetrathionate ion: 2- - 2- I2 + 2 S2O3 ֖ 2 I + S4O6 5 7/20/2010 Primary Standard • IdiIodine is rare ly use d as a pr imary s tan dar d for thiosulfate because it presents a problem in weighing and maintaining its solution concentration • Pure copper in the form of copper wire is used as the primary standard for sodium thiosulfate when it is to be used for the determination of copper as is the case with this experiment Secondary Standard • Upon addition of excess iodide to a solution of copper(II), a precipitate of CuI is formed along with I2. The liberated iodine is then titrated with standard sodium thiosulfate 2+ - - Cu + 5 I ֖ 2 CuI(s) + I3 2 - 2- - 2- I3 + 2 S2O3 ֖ 3 I + S4O6 • Formation of the precipitate and the addition of excess iodide force the equilibrium to the right in a rapid quantitative reaction • The net stoichiometry of the reaction is 1:1 since 2 moles of copper indirectly requires 2 moles of thiosulfate 6 7/20/2010 Indicator • The endpoint in a titration of iodine with thiosulfate is signaled by the color change of the starch indicator. • The active ingredient in starch is a helical polymer of α-D-gluocse called β-amylose, which forms a deep blue-black complex with molecular iodine • For use in this indirect method, the indicator is added at a ppyoint when virtually all of the iodine has been reduced to iodide ion, this helps the the disappearance of color to be rapid and sudden • The starch indicator solution must be freshly prepared since it will decompose and its sensitivity is decreased Preparation of Copper • Complex ions are ions formed by the bonding of a metal atom or ion to two or more ligands by coordinate covalent bonds • A ligand is a negative ion or neutral molecule attached to the central metal ion in a complex ion • In this experiment, copper and copper-clad pennies are dissolved in a concentrated aqueous solution of nitric acid, HNO3 • In aqueous solution, most of the first-row transition metals form octahedral complex ions with water as their ligands: Cu(s) + 4 HNO3(aq) + 4 H2O(l) → 2+ 3- Cu(H2O)6 (aq) + 2 NO2(g) + 2 NO (aq) • Once the pennies have been dissolved, the copper is free to react with iodide ions 7 7/20/2010 Experimental Procedure Standardizing Thiosulfate • Measure out ~1 L of de-ionized water • Heat to boiling, boil for at least five minutes • Add 25 g of Na2S2O3·5H2O to 1-L volumetric flask, and mix with hot boiled water • Add ~0.1 g of Na2CO3 as a preservative • Fill to the mark and mix • Transfer the solution to a 1-L plastic bottle • Store in darkness when not in use to help prevent decomposition of the thiosulfate, which is catalyzed by light. 8 7/20/2010 Standardizing Thiosulfate • Three pieces of clean copper wire were weighed using about 0.20 to 0.25 g each and placed in seperate 250-mL Erlenmeyer flasks • About 5 mL of 6 M nitric acid is added to each flask • Dissolution is promoted by warming the solution on a hot plate • De-ionized water is added to keep the volume constant. • After the copper has dissolved, ~25 mL of water was added and the solution boiled for a few more minutes to remove more nitrogen oxides • Add 5 mL of urea solution (1 g in 20 mL of water) with continued boiling for another minute. • Boilinggg removes oxides of nitrogen which result from the following reactions: Cu(s) + 8 HNO3 ֖ 3 Cu(NO3)2 + 2 NO + 4 H2O 3 NO + O2 ֖ 2 NO2 2 • and the oxides are decomposed by urea: HNO2 + (NH2)2CO ֖ CO2 + 2 N2 + 3 H2O 2 Standardizing Thiosulfate • The solution is removed from the hot plate and cooled • The excess acid is neutralized with 4 M aqueous ammonia added dropwise in the hood until a blue-green precipitate of copper(II) hydroxide barely form • To dissolve the precipitate and bring the solu tion to a p H bene fic ia l to the titra tion reactions, 5 mL of glacial acetic acid was added to each flask. 9 7/20/2010 Analysis of Pennies • For the pennies, six pennies were selected, three from pre- 1982 minting and three from post-1982 • They were cleaned with soap and water, dried, and then weighed on an analytical balance • Each of the pennies are placed into a 250-mL Erlenmeyer flask • To each flask, ~10 mL of 6 M nitric acid is added and the flasks were placed on a hot plate in the hood to dissolve the pennies • After three hours of heating and additional 6 M nitric acid and 15.9 M nitric acid added in small amounts and de-ionized water to keep the solution level constant, the pennies were dissolved • The flasks are removed from the hot plate and allowed to cool in an ice bath • Then ~10 mL of 9 M sulfuric acid is slowly added to each flask by pouring down the side of the flask • The solution is reheated until white fumes of sulfur trioxide appeared • Throughout the heating process, red fumes of nitrogen oxides could been seen over each of the solutions Analysis of Pennies • The flasks are then removed again and cooled in an ice bath • 4 M aqueous ammonia is added dropwise right until the point where the copper hydroxide precipitate has formed • To dissolve the precipitate and bring the pH to a level appropriate for the titration system, 5 mL of glacial acetic acid is added • The pre-1982 penny solutions has to be diluted to get a proper amount of copper concentration that will be appropriate for the framework of the experiment • The copper solution is quantitatively transferred to a 250 mL volumetric flask and diluted to the mark • After dilution, 25 mL of this solution is transferred with a transfer pipet quantitatively to another 250 mL Erlenmeyer flask which is then ready for titration preparation 10 7/20/2010 Iodometric Titration • At this point, the samples were prepared and titrated one at a time. • Potassium iodide is weighed and then added • The flask is covered with a watch glass and allowed to stand for ~2 minutes • The solution is then titrated with thiosulfate solution until the brownish color of iodine is almost gone indicated by a light tan or creamed coffee color Iodometric Titration • Enough iodine has then been reacted to add the 5mL of starch solution • And 2 g of sodium thiocyanate is added to displace any adsorbed iodine from the precipitate 11 7/20/2010 Iodometric Titration • The flask is swirled for about 15 seconds • Then the titration is completed by adding thiosulfate dropwise • At the end point, the bluish-gray color of the solution disappears and the precipitate appears whitish, or slightly gray • The second and third samples were treated in the same manner and titrated with thiosulfate Data and Analysis 12 7/20/2010 Standardization of Thiosulfate Solution Sodium thiosulfate pentahydrate 248.18 g/mol FM Copper molar mass 63.546 g/mol Mass of thiosulfate used (g) 25.1074 Volume of solution (L) 1.00 Calculated Molarity of thiosulfate 0.101 M solution Standardization of Thiosulfate Solution Trial 1 Trial 2 Trial 3 MfMass of copper w i()ire (g) 0.
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