Three Simple Titrations S. Hessam Moosavi Mehr July 4, 2008
Titration of Iron using EDTA1 Iron (III) forms stable complexes with EDTA as shown in the scheme below, H2 Y2− + Fe3+ −→ FeY− + 2H+ where Y denotes EDTA. A solution of salicylic acid is used to indicate the endpoint. Salicylic acid forms a less stable, purple-colored complex with iron. EDTA forms more stable complexes, and eventually displaces salicylate out of the small amount of salicylate-iron complex present. The endpoint is thus characterized by a change of color from purple to yellow.
Reagents 1. Sodium dihydrogen EDTA (NaH2 Y) 2. 1% salicylic acid in alcohol solution 3. Acetic acid/sodium acetate buffer
Apparatus 1. 250 ml volumetric flask 2. 50 ml volumetric pipette 3. 500 ml Erlenmeyer flask
Procedure Dilute the solution provided in the 250 ml volumetric flask to mark using distilled water. Use the volumetric pipette to transfer 50 ml of the resulting solution to the Erlenmeyer flask. Use the appropriate amount of buffer solution to stabilize the pH about 2–3. Add 7 drops of indicator and perform the titration with EDTA until the solution changes from purple to yellow.
Report Format • Report the number of milligrams of iron (III) nitrate present in the original sample given to you. 1 Ethylenediamine
tetra-acetic acid
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• Report the volume of EDTA you used in each titration.
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Note to Lab Instructor Each student’s sample should contain on the order of 0.1 gr of iron. Use iron (III) chloride to prepare the solutions— this means each student should receive about 0.3 gr FeCl3 in their sample. Use 20% sodium acetate solution and acetic acid to prepare the buffer. Also, the appropriate amount of buffer for the experiment should be determined in advance. You should be able tell your students how much to use.
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Titration of Ascorbic Acid using Potassium Iodate2 Ascorbic acid, commonly known as Vitamin C, can be easily oxidized to yield two electrons as shown below. Ascorbic acid −→ Dehydroascorbic acid + 2H+ + 2e− When coupled with the reduction of the iodate ion, IO3− + 6e− + 6H+ −→ I− + 3H2 O it forms the basis titration of ascorbic acid using iodate. The titration is carried out in 1 M HCl with the endpoint detected using starch as an indicator. When all of the ascorbic acid present has been oxidized by iodate, the next drop of will turn a minute amount of iodide anions to iodine IO3− + 5I− + 6H+ −→ 3I2 + 3H2 O resulting in the tell-tale blue color of the starch-iodine complex.
Reagents 1. Potassium iodate titrant 2. Solution of 2 M HCl 3. Starch indicator
Apparatus 1. 50 ml burette 2. 250 ml volumetric flask 3. 250 ml Erlenmeyer flask 4. 25 ml volumetric pipette
Procedure Dilute the solution provided in the 250 ml volumetric flask to mark using distilled water. Use the volumetric pipette to transfer 25 ml of the resulting solution to the Erlenmeyer flask. Use a graduated cylinder to transfer 25 ml of 2 M HCl into the same flask. Add 40 drops of starch solution and titrate with potassium iodate up to a permanent blue colour. 2 From
the practical examination in IChO 2003
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Report Format • Report the volumes of titrant used for each titration. • Report the milligrams of ascorbic acid present in the original sample given to you.
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Note to Lab Instructor The iodate titrant should be prepared in advanced and its molarity/normality announced before the experiment begins.
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Silver Titration of Chloride using Fluorecein Indicator The most commonly used method for determination of chloride is the Mohr titration using silver nitrate as titrant and chromate as an indicator.
Reagents 1. Fluorescein, eosin, or phonosafranine 2. Silver nitrate titrant
0.1
Procedure
Stub
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Volhard Titration of Chloride An excess of silver nitrate is added to a chloride solution. The excess silver is then back-titrated using thiocyanate, SCN− . The endpoint is detected through the formation of the crimson [Fe(SCN)]2+ complex.
Reagents 1. Fe (III) indicator 2. 0.1 N AgCl solution 3. 0.1 N KSCN solution 4. 6 N HNO3 5. Nitrobenzene
Apparatus 1. 100 ml volumetric flask 2. 25 ml volumetric pipette 3. 250 ml Erlenmeyer flask
Procedure Dilute the given sample to the mark, then use the volumetric pipette to transfer a 25 ml sample to the Erlenmeyer flask. Add 10 ml 6 N HNO3 , 25 ml 0.1 N AgNO3 , 3 ml nitrobenzene, and 1 ml Fe (III) indicator. Shake vigorously for one minute and leave the flask until the solution becomes clear. Perform the titration with SCN− until a permanent pink color appears.
Report Format • Report the volumes of titrant you used in each titration. • Report the number of milligrams of NaCl in the original sample
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Note to Lab Instructor The indicator solution should contain 2% Fe (III) along with a small amount of nitric acid to prevent the iron from hydrolysis.
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