Iodometry Experiment: Determining Copper Content using Iodine and Thiosulfate, Lecture notes of Stoichiometry

Download Iodometry Experiment: Determining Copper Content using Iodine and Thiosulfate and more Lecture notes Stoichiometry in PDF only on Docsity! 7/20/2010 1 Iodometric Determination of Copper I US O C t C i M d B f d Aft 1982n ne en o ns a e e ore an er 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. 7/20/2010 2 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 7/20/2010 5 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 M d f di l i di thi lf t• a e rom sso v ng so um osu a e pentahydrate salt • Solution is standardized against a primary standard • Iodine oxidizes thiosulfate to the tetrathionate ion: I2 + 2 S2O3 2- 2 I- + S4O6 2- 7/20/2010 6 Primary Standard I di i l d i t d d• o ne s rare y use as a pr mary s an ar 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 Cu2+ + 5 I- 2 CuI(s) + I3- I3- + 2 S2O3 2- 3 I- + S4O6 2- • 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 7/20/2010 7 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 point 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) → Cu(H2O)6 2+(aq) + 2 NO2(g) + 2 NO3-(aq) • Once the pennies have been dissolved, the copper is free to react with iodide ions 7/20/2010 10 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 7/20/2010 11 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 7/20/2010 12 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 7/20/2010 15 Confidence Interval t-test • The 95% confidence interval range with two degrees of freedom for pre-1982 pennies was found to be – 96.95 to 93.20 %wt copper – The true value given at the Department of Treasury website is • (1962–1982) 95% copper, 5% zinc (brass) – which falls within the CI range • The 95% confidence interval range with two degrees of freedom for post-1982 pennies was found to be – 2 863 to 2 165 %wt copper. . – The true value given at the Department of Treasury website is • (1982– present) 97.5% zinc core, 2.5% copper plating – which falls within the CI range Sources of Error • Copper(I) iodide forms a weak complex with molecular iodine which slows down its reaction with thiosulfate. – As a consequence, once the starch indicator has turned from blue to colorless, the blue color returns after a few seconds as I2 is slowly released into the solution by the CuI[I2] complex – This "after-bluing" can be avoided by adding potassium thiocyanate just before the end point is reached – The thiocyanate ion, SCN-, replaces the complexed I2 from CuI[I2], releasing the I2 to solution where its reaction with thiosulfate is rapid. • Iodine may be lost by evaporation from the solution – This is minimized by having a large excess of iodide in order to keep the iodine in the tri-iodide ion state – Titrations involving iodine should be made in cold solutions in order to minimize loss through evaporation. 7/20/2010 16 Sources of Error • In acidic solutions, the iodide ion can oxidize from the atmosphere – Therefore, prompt titration of the liberated iodine is necessary in order to prevent oxidation. • If the starch solution is not made fresh daily or improperly prepared, the indicator will not behave properly at the endpoint resulting in a sluggish or improper endpoint . Sources of Error • The addition of concentrated H2SO4 and continued heating until white sulfur trioxide fumes appear will ensure that all the HNO3 has been removed – NO3 - will oxidize I- and would will seriously interfere with the procedure. • The presence of the concentrated sulfuric acid aids in the exclusion of the remaining nitrogen oxides from the hot solution. • The red oxides first boil off, then fumes of sulfur trioxide beginning to escape. 7/20/2010 17 Sources of Error • If excess ammonia is added, copper(II) hydroxide redissolves forming the deep blue , copper(II) ammonia complex, Cu(NH3)4 • If the excess of ammonia is large, some ammonium acetate will be produced later which may keep the reaction between copper(II) and iodide ions from being complete • The excess ammonia can be removed by boiling, and the precipitate will re-form Sources of Error • Concentrated sulfuric acid has a very strong affinity for water • With nitric acid itself, sulfuric acid acts as both an acid and a dehydrating agent • Combined with the heating and evaporation of water in the sample, this resulted in a supersaturated aqueous ionic solution which forced some precipitates out at certain stages in the experiment • Before the sulfuric acid was added to the pre-1982 penny solutions, the concentration caused copper nitrate to precipitate out • After the sulfuric acid was added, it caused copper sulfate to precipitate out in well formed crystals • These two precipitates were easily redissolved by adding more de- ionized water and slightly mixing Copper Sulfate Crystal