Ch. 20 Applications of oxidation/reduction titrations 1.Auxiliary oxidizing and reducing reagents Fe(II), Fe(III) - Auxiliary reducing reagents A number of metals : good reduction reagents(Zn, Cd, Pb, Ni, Cu, Ag etc.) * Sticks or coils of the metal - Immersed in the analyte solution Solid remove # Jones reductor : Amalgamated Zn. diameter : 2 c m length : 40 – 50 cm

Auxiliary oxidizing reagents  Sodium Bismutate(powerful oxidizing agent) Half-reaction  Ammonium Peroxysulfate : (NH 4 ) 2 S 2 O 8  Sodium Peroxide and Hydrogen Peroxide After oxidation is complete, excess can be removed by boiling

2. Applying standards reducing agents  Iron(II) solutions : Prepared from Iron(II) ammonium sulfate(Fe(NH 4 ) 2 (SO 4 ) 2 -6 H 2 O(Mohr’s salt) Iron(II) ethylene diamine sulfate(FeC 2 H 4 (NH 3 ) 2 (SO 4 ) 2 -(Oesper’s salt)  Sodium thiosulfate

-Detecting end points in Iodine/Thiosulfate titrations * Color of iodine : 5 * M-I 2 : discernible color (1 drop of a 0.05M-iodine solution in 100 mL) Colorless analyte solution – Iodine color(end point) * Starch Iodine containing solution : deep blue color -Solubility of sodium thiosulfate solution Resistant to air oxidation Decompose to * Variables that influence the rate of this reaction (pH, Microorganisms, Concentration of solution, the presence of Cu(II) ions, exposure to sunlight)

-Standarizing thiosulfate solutions : KI – excellent primary standard for thiosulfate solutions * Other standard : Potassium dichromate Potassium bromate Potassium hydrogen iodate Potassium hexacyanoferrate(III) Metallic copper S 2 O 3 -2 로 적정

Applications of sodium thiosulfate solutions

3. Applying standard oxidizing agents  Choice of oxidizing agents depend on : the strength of analyte as a reduction agent the rate of reaction the stability of the standard oxidant solutions the availability of the oxidant solutions the cost the availability of a satisfactory indicator

(1)α-Napthoflavone (2) 1,10-phenanthroline iron(II) complex(ferroin) (3) diphenylamine sulfonic acid (a)Indefinitely stable (b) moderately stable, requires periodic standardization (c) somewhat unstable, requires frequent standardization

Formal potential of Ce(IV) : 1M-H 2 SO 4 solution * 1M-Perchloric acid : 1.70 V 1M-Nitric acid : 1.61 V - In Perchloric acid and Nitric acid : Ce(IV) not very stable - Application is limited Comparing the two reagents The oxidizing strength : Comparable * Ce(IV) solution in sulfuric acid : stable Mn solution in sulfuric : decompose slowly - require occasional restandarization The strong oxidants – Potassium permangante and Cerium(IV)

-Detecting end point Potassium permanganate : intense purple color 0.02M-KMnO mL – 100 Ml water – color 감지 Very dilute KMnO 4 solution (end point) : Diphenylamine sulfonic acid or 1,10-phenathroline complex of iron(II) * The end point is not permanent(MnO 4- react slowly with Mn 2+ ) : equilibrium constant Ce(IV) : Yellow orange, but the color intensity is low * Indicator : Iron complex of 1,10-phenanthrolin or one of its substituted derivatives.(T19.2)

-Preparation and stability of standard solutions Aqueous solution of KMnO 4 are not entirely stable because of water oxidation : * This reaction can be catalyzed by Light, Heat, Acids, Bases, Mn(II), MnO 2 * Before standardization : Remove the MnO 2 : stability of KMnO 4 solution increase(stand for 24 hr) * Stored in the dark. * In the case of any solid is detected in the solution or on the walls of the storage bottle : Filtration and Restandardization(every 1 or 2 weeks)

Ce(IV) solution : Table 20-4 Cerium ammonium nitrate, Ceric hydroxide Surfuric acid solution of Ce(IV) : remarkably stable, can be stored for months or heated at 100 ℃ prolonged periods without a change in concentration.

Standardizing Permangnate and Ce(IV) solutions  Sodium oxalate : widely used primary standard

Potassium dichromate  Dichromate titration : generally carried out in 1M-HCl or 1M-H 2 SO 4 solution Formal potential for the half reaction : 1.0 to 1.1 V * Indefinitely stable : can be boiled without decomposition do not react with HCl * Primary standard reagent : available commercially modest cost * Disadvantage : Lower electrode potential Slow reaction rate  Preparing Dichromate solutions - Reagent grade : Pure Drying at ℃ before being weighted * Color of Dichromate : weak - Indicator : Diphenylamine sulfonic acid (oxidized form : violet, Reduced form : colorless)  Applying potassium dichromate solutions

Iodine  Iodine : weak oxidizing agent * primary for the determination of strong reductants Properties of iodine solutions : low solubility(0.001M) : KI Air oxidation

Potassium bromate as a source of bromine Primary standard KBrO 3 : stable(identifically) # Directly titration with KBrO 3 are relatively few. Instead, convenient and widely used stable source of bromine. * Add unmeasured excess KBr in acidic solution of the analyte - measured volume of standard KBrO 3 ☞ Standard bromine solution(Stability ↓) * The primary use of standard KBrO 3 : the determination of organic compounds that react with bromine( 반응속도 ↓) - add a measured excess of standard KBrO 3 to the solution (sample + excess KBr) - To determine the excess bromine, introduce an excess of KI - Liberated iodine is then titrated with standard sodium thiosulfate