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Redox

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  AN OXIDATION-REDUCTIONTITRATION Experiment Done Using Equivalents and Normality T itration is a very general way of using the reaction between two com-pounds to determine the amount of one of them. You have already usedtitrations to determine the amount of acid present in an unknown sample and,f rom this, the equivalent weight of the acid. Now you will use the re a c t i o nbetween an oxidizing agent (KMnO 4 ) and a reducing agent (Na 2 C 2 O 4 ) tod e t e rmine the amount of sodium oxalate, Na 2 C 2 O 4 , in an unknown sample.This titration is particularly convenient since it provides its own endpoint: aslong as reducing agent is present, the purple KMnO 4 will be reduced to near-ly colorless Mn 2 + ; when all of the reducing agent has been used, the next pur-ple drop will remain in solution to signal completion of the titration, that is,the endpoint.To determine the number of equivalents of Na 2 C 2 O 4 p resent in a sample,we need to know the exact normality of the KMnO 4 used in the titration, aswell as the volume of the oxidizing agent used to reach the endpoint. That is,we will need to standardize the KMnO 4 solution first. The KMnO 4 ( o funknown normality) is titrated against a known weight of pure ferrous ammo-nium sulfate [Fe(NH 4 ) 2 ( S O 4 ) 2 ã 6H 2 O], a source of the Fe 2 + ion. Once youhave determined the normality of the KMnO 4 solution you can use it as thes t a n d a rd in your titration of Na 2 C 2 O 4 in your unknown.While the process of titration seems straightforw a rd enough, sources ofe rror are common enough to make it a challenge (as you may have alre a d yfound in the acid-base titration experiment). Your first thoughts about sourc e sof error probably center on the accuracy of the endpoint. In practice, you arenot likely to overshoot the endpoint by more than a few drops, so we mustlook for other sources of error as well. More likely chances for error include:ãIncomplete mixing of the KMnO 4 solution initially. This will mean thateach buret full will have a diff e rent concentration.ãWeighing errors, simple misreading of the balance and/or loss of samplewhen transferring it to the flask.ãB u ret reading errors such as air bubbles in the tip, unre c o rded initial re a d-ings, careless or incorrect readings, and buret leakage during titration.ãLoss of sample in titration through splashing, incomplete dissolution ofsample, or neglecting to wash down the sides of the flask occasionally.Consistency of results, while no guarantee of accuracy, is an encouraging Chemistry 111 Lab: Redox TitrationPage H-3  sign. A consistency check is given in the pro c e d u re to allow you to evaluateyour results (and carry out more titrations if needed). EXPERIMENTAL PROCEDURE A .S t a n d a rdization of a Potassium Permanganate Solution 1 .Clean and rinse your graduated cylinder and Erlenmeyer flasks. Clean ab u ret and rinse with distilled water until the water drains cleanly from thei n v e rted bure t .2 .P re p a re a dilute solution of KMnO 4 by placing about 60 mL of the con-centrated solution in your 500 mL Erlenmeyer flask and dilute with dis-tilled water to a total volume of 300 mL. Stopper the flask and shake thor-oughly to mix the solution. You can now determine the exact concentra-tion of this solution by titrating it against a known quantity of ferrous ion,F e 2 + .3 .In your Pre - L a b o r a t o ry Assignment you calculated the mass ofF e ( N H 4 ) 2 ( S O 4 ) 2 ã 6H 2 O re q u i red in the titration. Weigh out this mass ona piece of weighing paper on the analytical balance; weigh to the neare s t0.001 g and be sure that you re c o rd your balance readings. Place the sam-ple in a 125 or 250 mL flask, dissolve it in roughly 30 mL of distilled water,and add 10 mL of 3 M H 2 S O 4 .4 .Titrate your sample of Fe(NH 4 ) 2 ( S O 4 ) 2 ã 6H 2 O with the KMnO 4 s o l u-tion using the following pro c e d u re: (You may want to do a pre l i m i n a rytitration to see how the end point comes out.) ( a )Place a piece of white paper under the flask so that the color at the end-point can be seen clearly. ( b )Fill the buret and be sure the solution completely fills the tip. Recordthe initial buret reading on the Report Form. ( c )Add KMnO 4 solution from the buret, swirling the flask constantly. ( d )Occasionally rinse down the walls of the flask with distilled waterf rom your wash bottle. Titrate until the last drop of KMnO 4 a d d e dleaves a lasting pink color. The first appearance of a permanent pinkcolor occurs when the KMnO 4 that is added becomes equivalent to theamount of Fe 2 + p resent. ( e )R e c o rd the final buret reading on the re p o rt form .5 .Titrate another sample of Fe(NH 4 ) 2 ( S O 4 ) 2 ã 6H 2 O, adding the first 20mL of KMnO 4 rapidly and then approaching the endpoint with care .R e c o rd your data and then check for consistency (using the same methodoutlined in the Acid-Base Ti t r a t i o n ) . Page H-4Chemistry 111 Lab: Redox TitrationThe dilution procedure canbe carried out fairlyroughly, since you will stan-dardize the solution later.The exact amount of wateris not important. TheH 2 S O 4 is needed for prop-er acidity control.  These values from successive titrations should agree within ±0.5 mL. If not,you will need to carry out more titrations until satisfactory agreement isre a c h e d . B .Analysis of Sodium Oxalate in an Unknown Solid Mixture Obtain an unknown from your instru c t o r. Be sure to re c o rd the unknownnumber on your re p o rt form. Weigh out a sample of approximately 1 gram(but weigh it accurately to the nearest 0.001 g) and transfer to a clean 125 or250 mL Erlenmeyer flask. Dissolve your sample in approximately 50 mL ofdistilled water and 25 mL 3 M H 2 S O 4 . Swirl to dissolve. Heat the solution to between 55 and 60 ˚C. Titrate the sample with stan-d a rdized KMnO 4 in the usual manner, maintaining the temperature in the 55-60 ˚C range. Record your data on the Report Form. Repeat the titration withanother sample of the unknown. Check your results for consistency as out-lined above in the standardization section. Do further titrations if necessary.  Consistency Check = volume KMnO 4  used (mL) mass of sample (g) = volume KMnO 4  per gram Chemistry 111 Lab: Redox TitrationPage H-5Add the acid cautiously; your unknown containsN a 2 C O 3 and foaming mayo c c u r . Your instructor will demon-strate ways to maintain thetemperature. Temperaturecontrol is important here.
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