Analytical Chemistry (Volummetric analysis) Lecture 8 PDF

Analytical Chemistry (Volummetric analysis) Lecture 8 Prepared by Dr.Ahmed Bahgat  Visual detection of end point ◼ Obviously, the end point can be determined by measuring potential with an indicator electrode relative to a reference and plotting this aga...

Analytical Chemistry (Volummetric analysis) Lecture 8 Prepared by Dr.Ahmed Bahgat  Visual detection of end point ◼ Obviously, the end point can be determined by measuring potential with an indicator electrode relative to a reference and plotting this against the volume of titrant. ◼ But in other titrations, it is usually more convenient to use a visual indicator. ◼ There are three methods for visual detection: 1. Self indication. 2. Starch detection. 3. Redox indicators. 2  Visual detection of end point (cont.) ◼ 1. Self indication ◼ If the reducing or oxidizing agent is coloured, the end point can be detected by the appearance or disappearance of the colour of the reagent. ◼ e.g. KMnO4 where the end point can be detected by the pink colour of permanganate. ◼ 2. Starch Indicator ◼ This indicator is used for titrations involving iodine. Starch forms a dark blue colour with iodine (I2). 3  Starch-Iodine Complex ◼ Starch is the indicator of choice for those procedures involving iodine because it forms an intense blue complex with iodine. ◼ Starch is not a redox indicator; it responds specifically to the presence of I2, not to a change in redox potential. ◼ The active fraction of starch is amylose, a polymer of the sugar α-d-glucose. ◼ In the presence of starch, iodine forms I6 chains inside the amylose helix and the color turns dark blue  Visual detection of end point (cont.) ◼ 3. Redox indicators ◼ These are highly coloured dyes that are weak reducing or oxidizing agents that can be oxidized or reduced and the colour of both forms are different. ◼ There are not many good redox indicators. ◼ Ferroin [tris(1,10-phenanthroline)iron(II) sulphate] is one of the best indicators that can be used in cerium(IV) titrations. It is oxidized from the red to pale blue at the end point. ◼ Diphenylaminesulphonic acid, is used as an indicator for titrations with dichromate in acid solution. The colour at the end point is purple. 5  Titrations with oxidizing agents I- Potassium permanganate ◼ Potassium permenganate is a widely used oxidizing agent (E°= 1.51V) that can act as a self indicator ◼ in acidic medium KMnO4 + 8H+ + 5e- Mn2+ + 4H2O ◼ While in basic medium, it will be precipitated as brown MnO2. ◼ Potassium permanganate is standardized by titration with primary standard sodium oxalate 5H2C2O4 + 2MnO4- + 6H+ 10CO2 + 2Mn2+ + 8H2O ◼ The solution must be heated for rapid reaction. The reaction is catalyzed by the Mn2+ product and it goes very slowly until Mn2+ is formed. ◼ Permanganate titration are not possible in the presence of chloride because it will be oxidized to chlorine. 6 7 8 9 10  Potassium permanganate, KMnO4 Properties It is a strong oxidant with an intense violet color It is not a primary standard Standard solution and stability Aqueous solutions of KMnO4− are not entirely stable because the ions tend to oxidize water: 4MnO4− + 2H2O 4MnO2 (s) + 3O2 (g) + 4OH− This decomposition reaction is catalyzed by light, heat, acids and manganese dioxide. Potassium permanganate should be stored in dark glass bottles & kept away high temp Standardization It is not a primary standard. It is standardized ≠ st. oxalic acid 2MnO4− + 5H2C2O4 + 16H+ 2Mn2+ + 10CO2 + 8H2O Conditions 1. titration is carried out in acidic solution (1 M H2SO4) not HCl - Permanganate titration are not possible in the presence of chloride because it will be oxidized to chlorine. -2. titration is carried out at 70oC, To expel CO2 and to catalyze the reaction, to avoid the precipitation of MnO2 3. The reaction occurs slowly even at elevated temp, once Mn is formed, it act as catalyst for the reaction End point detection Self indicator The intense purple color of MnO4 − serves as indicator  Oxidation with potassium permanganate ◼ KMnO4 is a strong oxidant with an intense violet color. In strongly acidic solutions (pH < 1), it is reduced to Mn2+. ◼ In neutral or alkaline solution, it is reduced to brown solid MnO2. ◼ In strongly alkaline solution ( 2 M NaOH), green manganate ion (MnO42-) is produced. Reduction half-reactions Strongly acidic solutions (pH ≤ 1) MnO4− + 8H+ + 5e Mn2+ + 4H2O E0 = +1.51 V Purple manganous(colorless) Change in oxidation state of Mn: +7 to +2 Eq wt = Mw / 5 5 equivalents ≡ 1 mole 5 milli-equivalents ≡ 1 milli-mole 1mL 1M ≡ 1 milli-mole 1mL 1N ≡ 1 milli-equivalents 1mL 1M ≡ 1mL 5 N Neutral or weakly alkaline solutions MnO4− + 4H+ + 3e MnO2 (s) + 2H2O E0 = 1.69 V Manganese dioxide (brown solid) Change in oxidation state of Mn: +7 to +4 Eq wt = Mw / 3 3 equivalents ≡ 1 mole 3 milli-equivalents ≡ 1 milli-mole Strongly alkaline solutions (2 M NaOH) MnO4− + e MnO4− − E0 = + 0.56 V manganate (green) Change in oxidation state of Mn: +7 to + 6 Eq wt = Mw / 1 1 equivalent ≡ 1 mole 1 milli-equivalent ≡ 1 milli-mole Applications Strongly acidic solutions, 0.1M to 1M H2SO4 By Direct Titrations Determination of reducing species The acid used for acidification is H2SO4 and not HCl as MnO4− oxidizes Cl– in acid medium For MnO4− / Mn2+ E0 = +1.51 V For Cl2 / Cl− E0 = +1.36 V 2 MnO4− + 10Cl− + 16H+ 2 Mn2+ + 5Cl2 + 8H2O Determination of Fe2+ salts e.g. FeSO4 MnO4− + 5Fe2+ + 8H+ Mn2+ + 5Fe3+ + 4H2O  II- potassium dichromate ◼ It is slightly weaker oxidizing agent than potassium permanganate. ◼ The main advantage is its availability as a primary standard material. ◼ It does not react with it HCl so the titrations can be performed in HCl medium. ◼ Cr2O72- + 14H+ + 6e 2Cr3+ + 7H2O ◼ The orange colour of dichromate is not intense to be used to determine the end point, so that is why external indicators should be used e.g diphenylamine sulphonic acid. 16  Cerium (IV) ◼ Is a powerful oxidizing agent. Its potential depends on the acid in which the reaction takes place. It is 1.44 V on using H2SO4 and 1.70 V in perchloric acid. ◼ It can be used in the same titrations as permanganate but the oxidation of chloride is slow. ◼ The salt of cerium, ammonium hexanitrocerate is a primary standard material. ◼ The main disadvantage is the increased cost compared to permanagante. ◼ Ferroin is a suitable indicator for such titrations. 17  Iodimetry ◼ Iodine is a moderately strong oxidizing agent that can be used to titrate reducing agents. Titrations with iodine are called iodimetric titrations. ◼ These reactions are performed in neutral or mildly alkaline pH8 to weakly acid solutions. (GIVE REASON) ◼ If the pH is too alkaline, I2 will disproportionate (undergo oxidation and reduction reaction at the same time) to hypoiodate and iodide ◼ I2 + 2OH- IO- + I- + H2O ◼ Also in acidic medium, Starch which is used to detect the endpoint of such titration is hydrolyzed, and oxidizing power of iodine will also decrease. 18  Iodimetry (cont.) ◼ Because I2 is not a strong oxidizing agent this limits the number of reducing agents that can be titrated against iodine and this increases it selectivity. ◼ Although pure iodine can be obtained by sublimation but its solution should be standardized using As2O3. ◼ Iodine has a low solubility in water but the complex I3- is very soluble. So iodine solutions are prepared by dissolving iodine in concentrated solutions of potassium iodide ◼ I2+I- I3- ◼ Therefore, I3- is the actual species used in titration. 19  Iodometry ◼ Iodide ion is a weak reducing agent and will reduce strong oxidizing agents. It is not used, however, as a titrant mainly because of the lack of convenient visual indicator system, as well as the low speed of the reaction. ◼ When an excess of iodide (I-) is added to a solution of an oxidizing agent, I2 is produced in an amount equivalent to the oxidizing agent present. This I2 can be titrated with reducing agents and the result will be the same as if the oxidizing agent was titrated directly. ◼ The titrating agent is sodium thiosulphate. ◼ This method is called Iodometric method. 20  Iodometry (cont.) ◼ Consider, for example, the determination of dichromate: Cr2O72- + 6I- + 14H+ 2Cr3+ + 3I2 + 7H2O I2 + 2S2O32- 2I- + S4O62- ◼ Each Cr2O72- produces 3I2 which in turn react with 6S2O32-. ◼ Thus ◼ 6 millimoles of S2O32- are equivalent to Cr2O72-. 21  Notice that ◼ We can’t titrate strong oxidizing agents directly with thiosulphate cause it might be oxidized to higher states such as sulphate (SO42-) rather than tetrathionate(S4O62-). ◼ Starch, used to detect the end point, should not be added from the beginning of the titration but near the end of titration where the colour of iodine is pale yellow for two main reasons: ◼ 1. Iodine starch complex is only slowly dissociated and diffuse end point would result in a large amount of iodine is adsorbed on starch. ◼ 2. Iodometric titration usually occur in acid medium which leads to the hydrolysis of starch. 22  Remember ◼ No of moles can be obtained by MV ◼ Or Weight/Mwt(molecules) ◼ Or Weight/At.wt(atoms) ◼ On solving the redox problems, 1. Try to find the relation between the No. of moles of your reactants, 2. Calculate the no. of moles by applying any of the above relations. 23  Example 1 A 0.2 g sample containing copper is analyzed iodometrically. Copper (II) is reduced to copper(I) by iodide: 2Cu2+ + 4I- 2CuI + I2 What is the percent of copper in the sample if 20 ml of 0.10 M Na2S2O3 is required for the titration of the liberated iodine? I2 + 2S2O32- 2I- + S4O62- ◼ One half of I2 is liberated per 2 mole of Cu2+ and since each I2 reacts with 2S2O32-, thus each mole of Cu2+ is equivalent to one mole S2O32-. ◼ (MV)Cu2+ = (MV)S2O32- ( M V ) C u 2 + A t.w t C u 2 + ( 2 0 x 0.1) x 6 3.5 w tcu 2+ = = = 0.1 2 7 g 1000 1000 ◼ % of Cu2+ = (0.127/0.2) x 100 = 63.5 % 24  Example 2 ◼ A solution of Na2S2O3 is standardized iodometrically against 0.1262 g of high purity KBrO3, requiring 44.97 ml Na2S2O3. What is the molarity of the Na2S2O3? KBrO3 + 6I- + 6H+ Br- + 3I2 + 3H2O 3I2 + 6S2O32- 6I- + 3S4O62- ◼ 6 Each mmol S2O32- = mmol BrO3- ◼ 6 (MS O 2- x 44.97) = (126.2 mg/167.01) 2 3 ◼ MS O 2- = 0.1008 M 2 3 25  Example 3 ◼ A 0.1809 sample of pure iron wire was dissolved in acid, reduced to the +2 state, and titrated with 31.33 ml of cerium (IV). Calculate the molar concentration of the Ce4+. ◼ Fe2+ Fe3+ + e ◼ Ce4+ + e Ce3+ ◼ 1 mmole of Fe2+ is equivalent to 1 mmole Ce4+ ◼ wtFemg/Atwt = (MCe4+ x VCe4+) ◼ 180.9 mg/55.84 = MCe4+ x 31.33 ◼ MCe4+ = 0.1034 M 26  Example 4 ◼ Titration of 0.1467 g of primary standard Na2C2O4 required 28.85 ml of potassium permanganate. Calculate the molar concentration of KMnO4 in this solution. 5H2C2O4 + 2MnO4- + 6H+ 10 CO2 + 2 Mn2+ + 8 H2O From the equation, each 2 mmole of permanganate is equivalent to 5 mmole oxalate, thus 1 mmole of permanganate is equivalent to 5/2 mmole oxalate, ◼ (MV)KMnO4 = (wt/Mwt)oxalate X 5/2 ◼ (M x 28.85)KMnO4 = (146.7mg/133.99)oxalate X 5/2 ◼ MKmnO4 = 0.0151 M 27 1-What do you understand by the term “Accuracy”? a. Involves determining the individual constituents of a given sample. b. The degree of agreement between replicate measurements of the same quality. c. The degree of agreement between the measured value and true value. d. Involves determining the level of purity of an analyte. 2) Calculate the equivalent weight of KCl? 3) If the concentration of OH- ions in an aqueous solution is 2.5 x 10-4 then, a. its pH is lower than 7 b. the solution is acidic c. its pOH is lower than 7 d. All 4) The pH of ammonium chloride (0.05 M) is equal to …….. (Kb=1.774X10-5) 5) The number of moles for 200 mL of 2M H2SO4 is ……. 6) For some titrations, methyl orange is a suitable indicator while phenolphthalein is unsuitable. For which one of the following titrations is this case? 7) Calculate the percentage by volume for a solution prepared by adding 80 ml methanol to 300 ml of water 8) Which volume of 0.10 M NaOHaq exactly neutralizes 20.0 mL of 0.05 N H2SO4? 9) Which word equation represents a neutralization reaction? a. base + acid => salt + water b. base +salt => water + acid c. salt + acid =>base + water d. salt + water=> acid + base 10) The number of moles of NaOH needed to neutralize 2 moles of H2SO4 is …… 11) 25.00 mL of 0.100mol L-1 hydrochloric acid solution is added to 12.00 mL of 0.100molL-1 sodium hydroxide solution. Calculate the pH of the resulting Solution. 12) Which of the following practices will lead to an error when titrating a solution of sodium hydroxide with dilute hydrochloric acid? a. Washing the burette with distilled water, then with a little amount of the acid and then filling with the acid. b. Reading to the bottom of the meniscus in the burette. c. Washing the pipette with distilled water then using it to dispense the sodium hydroxide solution. d. Using only a small amount of indicator. 19) A substance that can be considered as electron-pair donor called……. 20) The unit of molality is……. 21) The pH of a sample of human blood was measured to be 7.41 at 25°C. The concentration of hydroxide ions is ……. 22) Which of the following apparatus is less accurate? a. Beaker b. Measuring flask c. Burrette d. Pipette 23) The pH of 70 mL acetic acid (0.05N) after addition of 70 mL of NaOH (0.03 N) is ………. (Ka=1.8X10-5) 24) The pH of acetic acid solution is 6.20. Calculate the Ka for this acid. The initial acid concentration is 0.010 M 25) The normality of a 3.60 M sulphuric acid solution is……… 26) At pH 13, the most predominate form of carbonic acid is …….  ◼Experiment 6 ◼ Redox Titration Standardization of potassium permanganate (KMnO4) Preparation of Oxalic acid (0.1 N) Procedure: 1- Weigh exactly 1.575 g AR oxalic acid in clear dry beaker. 2- Dissolve the solid acid in about 50 ml distilled water. 3- Transfer the soluble acid into 250 ml measuring flask using glass funnel. 4- Complete to the mark using distilled water. 5- Shake the solution three times for complete homogenity of the solution. Standardization of KMnO4 Procedure: 1- 1) Transfer 10 ml oxalic acid solution into a conical flask and add 20 ml, 2N H2SO4. 2) Heat the solution to 60-70 C and titrate slowly with potassium permanganate to the faint pink colour end-point. 3) Repeat the titration another two times. Standardization of KMnO4 (NV)oxalic = (NV)KMnO4