Titrasi PresipitasiOutline 1. Essence and classification of methods precipitation titration 2. Titration Curves 3. Argentometry 3. Argentometry 4. Thiocyanatometry 5. Mercurometry 6. Sulphatometry 7. Hexacianoferratometry Precipitation Titratons Precipitation titrimetry, which is based on reactions that yield ionic compounds of limited solubility, is one of the oldest analytical techniques. The slow rate of formation of most precipitates, however, limits the number of precipitating agents that can be used in titrations precipitating agents that can be used in titrations to a handful. The most widely used and important precipitating reagent, silver nitrate, which is used for the determination of the halogens, the halogen-like anions. Titrations with silver nitrate are sometimes called argentometric titrations. Requirements • The analyte should be dissolved in water and give an ion which would be active in precipitation reaction. • The precipitate should be practically insoluble (K <10 -8 to 10 -10 , S<10 -5 mol/L). (K sp <10 -8 to 10 -10 , S<10 -5 mol/L). • There is no coprecipitation. • Precipitate should form quickly enough. • There should be a way for determining the equivalence point. Classification of precipitation titration methods (based on titrant): 1. Argentometry 2. Thiocyanatometry 3. Mercurometry 3. Mercurometry 4. Sulphatometry 5. Hexacianoferratometry 2. Titration Curves Curve plotting of titration is based on a rule of solubility product. And accordingly nB mA B A n m + = n m B A Ksp ] [ ] [ · = The Shapes of Titration Curves Titration curves for precipitation reactions are derived in a completely analogous way to the methods described for titrations involving strong acids and strong bases. P-functions are derived for the preequivalence-point region, the post- equivalence point region, and the equivalence equivalence point region, and the equivalence point for a typical precipitation titraton. Most indicators for argentometric titrations respond to changes in the concentration of silver ions. As a consequence, titraton curves for precipitation reactions usually consist of a plot of pAg versus volume of AgNO 3 . Precipitation titration curve for 50.0 mL of 0.0500 M Cl – with 0.100 MAg + . (a) pCl versus volume of titrant; (b) pAg versus volume of titrant. The factors which define value of inflection points of titration on curves of precipitation titration • Concentration of titrant solutions and the analyte (the higher the concentration, the sharper the slope of the inflection point.) sharper the slope of the inflection point.) • Solubility of a precipitate (The smaller the solubility, the sharper the slope of the inflection point) Influence of precipitate solubility on titration inflection point Influence of other factors on inflection point of the precipitation titration • Temperature (the higher the temperature, the higher the solubility of a precipitate and the less sharp slope inflection point) and the less sharp slope inflection point) • Ionic strength of the solution (the higher the ionic strength of a solution, the higher the solubility of a precipitate and the less sharp slope inflection point) Titration of 50 mL of 0.05 M NaCl by 0.1000M AgNO 3 continued pAg = –log 10 [Ag+]. Precipitation AgCl Volume AgNO3 Sharp increase in Free Ag+ Successful titrations of ions by Siver Nitrate pAg = –log 10 [Ag+]. Precipitation AgI Precipitation AgBr Volume AgNO3 A Sharp increase in Free Ag+ signals the endpoint in each case Precipitation AgCl Successful titrations of Mixtures by Siver Nitrate pAg = –log 10 [Ag+]. Precipitation AgI first Volume AgNO3 A Sharp increase in Free Ag+ signals the endpoint in each case Precipitation AgCl second Sigmoidal titration curve Linear segment titration curve. Spectrophotometric titration curve of transferrin with ferric nitriloacetate. Titration curve for the titration of 50.00 ml of 0.1000M AgNO 3 with 0.1000MKSCN. Factors influencing the sharpness of end points 1) Reagent concentration [T], [A] | ¬ e T + 2) Completeness of reaction Solubility+ ¬ e.p. change jump| I – Ksp=8.3 ×10 –17 I Ksp=8.3 ×10 Br – Ksp=5.0 ×10 –13 Cl – Ksp=1.8 ×10 –10 IO 3 – Ksp=3.0×10 –8 BrO 3 – Ksp=5.7×10 –5 Titration of a mixture by potentiometric detection Two precipitable ions (ex. Cl – , I – ) Titrant (ex. Ag + ) Ex. 0.0502M KI + 0.0500MKCl Ex. 0.0502M KI + 0.0500MKCl 0.0845 M AgNO 3 Ksp AgI << Ksp AgCl 8.3×10 –17 1.8 ×10 –10 Coprecipitation [Ag + ][Cl – ] / [Ag + ][I – ] = [Cl – ] / [I – ] = 8.3×10 –10 /1.8×10 –17 = 1/ 4.6 ×10 –7 Titration curves for 50.0 ml of a solution 0.0800 M in Cl ÷ and 0.0500 M in I ÷ or Br ÷ . End Point for Argentometric Titrations Three types of end points are: (1) chemical, (2) potentiometric, (3) amperometric. Potentiometric end points are obtained by measuring the potential. To obtain an amperometric end point, the current generated between a pair of silver microelectrodes is measured and plotted as a function of reagent measured and plotted as a function of reagent volume. The chemical end point consists of a color change or the appearance or disappearance of turbidity. The requirements are (1) the color change should occur over a limited range in the p-function, and (2) the color change should take place within the steep portion of the titration curve. 3. Argentometry This is a precipitation titration in which Ag + is the titrant. X - + Ag + = AgX+ where: X - = Cl - , Br - , I - , CN - , SCN - , etc.. • Titrant: AgNO 3 – secondary standard solution • Titrant: AgNO 3 – secondary standard solution • Standardization оn primary standard solution of Sodium chloride AgNO AgNO 33 ++ NaCl NaCl == AgCl++ + NaNO + NaNO 33 • I ndicator for standardization- 5 % Potassium chromate K 2 CrO 4 (to appearance reddish-brown precipitate of Silver chromate): 2AgNO 2AgNO 33 ++ K 2 CrO 4 == Ag 2 CrO 4 +++ 2KNO + 2KNO 33 .. Argentometry: • without indicator: Gay-Lussac method (method of even turbidity) method to point enlightenment • with indicator: Mohr method Fajans – Fisher – Khodacov method Volhard method Gay-Lussac method (method of even turbidity) If solution NaBr titrate by solution AgNO 3 (or on the contrary) there is a reaction: Br - + Ag + = AgBr↓ • For determination of equivalence point it is • For determination of equivalence point it is necessary to select two identical portions of a solution before the titration end. To one of them add a drop of AgNO 3 solution, on another - a drop of NaBr solution at the same concentration. Titration will finish when there will be identical intensity of turbidity in both portions of solution. Method to enlightenment point The method of titration to an enlightenment point can be applicable when insoluble compounds is in colloidal state. For example, determination of І - ions by silver nitrate, AgІ forms, it is adsorbing І - and receive negative charges (colloidal solution of AgІ forms). As more and more І - ions react with Ag + ions, particles As more and more І - ions react with Ag + ions, particles AgІ gradually lose adsorbed by them І - ions, and their charge decreases. In the end of titration occur coagulation of particles and their sedimentation. The solution thus is absolutely clarified. Formation of a Colored Precipitate The Mohr Method Sodium chromate can serve as an indicator for the argentometric determination of chloride, bromide, and cyanide ions by reacting with silver ion to form a brick-red silver chromate (Ag 2 CrO 4 ) precipitate in the equivalence-point region. The reactions involved in the determination of chloride and bromide (X - ) are titration reaction: Ag + + X - AgX (s) [white] indicator reaction: 2Ag + + CrO 4 2- Ag 2 CrO 4(s) [red] The solubility of silver chromate is several times grater than that of silver chloride or silver bromide. Mohr method • Titrant: AgNO 3 – secondary standard solution • Stanardization on primary standard solution of sodium chloride NaCl (by a measured volume of primary standard solution): AgNO AgNO 33 ++ NaCl NaCl == AgCl++ + NaNO + NaNO 33 • Indicator - 5 % potassium chromate K 2 CrO 4 (to formation precipitate of reddish-brown formation precipitate of reddish-brown Ag 2 CrO 4 ): 2AgNO 3 + K 2 CrO 4 = Ag 2 CrO 4 ++ 2KNO 3 • Determinate substance: chloride Cl - , bromide Br - . • Medium: рН~ 6,5-10,3. • Usage: quantitative definition of sodium chloride, potassium chloride, sodium bromide, potassium bromide, etc. Restrictions of usage of Mohr method: • It is impossible to use titation in acidic solutions: 2CrO 4 2- + 2H + = Cr 2 O 7 2- + H 2 O • It is impossible to use titration in the presence of ammonia, etc. ions, molecules which can be ligands on relation to Silver ions • It is impossible to use titration in the presence of many cations (Ba 2+ , Pb 2+ , etc.) which form the many cations (Ba , Pb , etc.) which form the painted precipitates with chromate ions CrO 4 2- • It is impossible to use titration in the presence of reducers which reduce chromate ions CrO 4 2- to Cr 3+ ions • It is impossible to use titration in the presence of many anions (PO 4 3- , AsO 4 3- , AsO 3 3- , S 2- etc.) which with Silver ions give the painted precipitates Adsorption Indicators: The Fajans Method An adsorption indicator is an organic compound that tends to be adsorbed onto the surface of the solid in a precipitation titration. Ideally, the adsorption occurs near the equivalence point and results not only in a color change but also in a transfer of color from the solution to the solid (or the reverse). Fluorescein is a typical adsorption indicator useful for Fluorescein is a typical adsorption indicator useful for the titration of chloride ion with silver nitrate. In aqueous solution, fluorescein partially dissociates into hydronium ions and negatively charged fluoresceinate ion that are yellow-green. The fluoresceinate ion forms an intensely red silver salt. Titrations involving adsorption indicators are rapid, accurate, and reliable. Fajans – Fisher – Khodacov method • Titrant: AgNO 3 – secondary standard solution • Standardization on primary standard solution of sodium chloride NaCl (by a measured volume of primary standard solution): solution): • Medium: рН~ 6,5-10,3 (for determination of clorides) and рН~ 2,0-10,3 (for determination of bromides and iodides). • Indicators of method: – dichlorofluoroscein (for determination of clorides) – eosine (for determination bromides and iodides) The mechanism of indicators action AgNO 3 + NaCl = AgCl+ + NaNO 3 HInd · H + + Ind - Adsorption indicator whose color when adsorbed to the precipitate is different from that when it is in solution Colour Indicator Indicator Solution Solution Surface of Surface of precipitate precipitate Dichloro- fluoroscein greenish yellow pink eosine eosine yellowish yellowish--red red redish redish -- violet violet • A number adsorption abilities of anion at рН~7 on a precipitate surface of Silver chloride • I - > CN - > SCN - > Br - > eosine > Сl - > dichlorofluoroscein > NO 3 - > ClO 4 - eosine eosine yellowish yellowish--red red redish redish -- violet violet Conditions of titration: • Acidity of solutions • Concentration of reacting solutions • The account adsorption abilities of indicators and ions which present in a solution • Titration near equivalence point is necessary to spend slowly to spend slowly • Titration with adsorption indicators spend in a diffused light Use: • Quantitative definition of chlorides, bromides, iodides, thiocyanides, cyanides. The Volhard Method (Colored Complex) In the Volhard method, silver ions are titrated with a standard solution of thiocyanate ion: Ag + + SCN - AgSCN(s) Iron (III) serves as the indicator. The solution turns red with the first slight excess of thiocyanate ion: Fe 3+ + SCN - Fe(SCN) 2+ red The titration must be carried out in acidic solution to prevent precipitation of iron(III) as the hydrated oxide. | | | || | K Fe SCN Fe SCN f = = × ( ) . 105 10 3 - 2+ 3+ …continued… The most important application of the Volhard method is for the indirect determination of halide ions. A measured excess of standard silver nitrate solution is excess of standard silver nitrate solution is added to the sample, and the excess silver ion is determined by back-titration with a standard thiocyanate solution. Volhard method • Titrant: AgNO 3 , ammonium or potassium thiocyanide NH 4 SCN, KSCN - secondary standard solution • Stardadization AgNO 3 on primary standard solution NaCl, NH 4 SCN, KSCN on standardization solution AgNO 3 : AgNO 3 + NH 4 SCN = AgSCN+ + NH 4 NO 3 AgNO 3 + NH 4 SCN = AgSCN+ + NH 4 NO 3 • Indicator by standardization of ammonium or potassium thiocyanide with iron (ІІІ) salts (NH 4 Fe(SO 4 ) 2 ·12H 2 O in presence of nitric acid) to the formation of the reddish colored Fe(SCN) 2+ complex: Fe 3+ + SCN - = [Fe(SCN)] 2+ • Medium: in presence of nitric acid • Indicators of method: iron (ІІІ) salts NH 4 Fe(SO 4 ) 2 ·12H 2 O in presence of nitric acid • Determinate substance: halogenides, thiocyanides, cyanides, sulphides, carbonates, chromates, oxalates, arsenates etc. Until equivalence point (e.p.) Hal - + Ag + (excess) = AgHal+ Ag + (rest) + SCN - = AgSCN+ After e. p. Fe 3+ + SCN- = [Fe(SCN)] 2+ (reddish) !!! At determination of iodides the indicator is added in the end of titration to avoid parallel: end of titration to avoid parallel: 2Fe 3+ + 2I - = 2Fe 2+ + I 2 Advantages of Volhard method Titration possibility: • In very acidic solutions • In the presence of many cations which interfere by definition in Mohr method End point detection in argentometric titration Detection techniques in precipitation titrations : ! Indicator ~ Potentiometry # Light scattering / turbidimetry of nephelometry Titration Mohr Volhard Fajans Titration Ag + + Cl – ÷ AgCl+ Ag + + Cl – ÷ AgCl+ Ag + + Cl – ÷ AgCl+ reaction white Back titration : Ag + + SCN – ÷ AgSCN+ white ÷ + ÷ white End point 2Ag + + CrO 4 2– ÷ Ag 2 CrO 4 + SCN – + Fe 3+ ÷ FeSCN 2+ Electric double layer reaction red soluble red with adsorption Ind. pH 7~10.5 K f = 1.05×10 3 Dichlorofluorescein Use Cl – , Br – , CN – Cl – , Br – , I – Cl – , Br – , I – , SCN – No use I – , SCN – Table 13-3, p.362 4. Thiocyanatometry • This is a precipitation titration in which SCN - is the titrant. • Titrant: ammonium or potassium thiocyanide NH 4 SCN, KSCN - secondary standard solution • Stardadization: on primary standard solution of AgNO 3 : AgNO + NH SCN = AgSCN+ + NH NO AgNO 3 + NH 4 SCN = AgSCN+ + NH 4 NO 3 • Indicator by standardization of ammonium or potassium thiocyanide with iron (ІІІ) salts: Fe 3+ + SCN - = [Fe(SCN)] 2+ • Medium: in presence of nitric acid • Indicator: iron (ІІІ) salts NH 4 Fe(SO 4 ) 2 ·12H 2 O in presence of nitric acid Determine substance: drugs, which contain Silver (Albumosesilber, colloid silver - Kollargol, silver nitrate). !!! At the analysis of drugs which contain nonionic silver, preliminary it is heated with sulphuric and nitric acids (receive ionic compound). compound). !!! At determination of iodides the indicator is added in the end of titration to avoid parallel: 2Fe 3+ + 2I - = 2Fe 2+ + I 2 Advantages of Thiocyanatometry Titration possibility: • In very acidic solutions • In the presence of many cations which interfere by determination in Mohr method method 5. Mercurometry • Mercurometry – this is a precipitation titration in which Hg 2 2+ is the titrant 2Cl - + Hg 2 2+ = Hg 2 Cl 2 + K sp = 1,3·10 -18 2I - + Hg 2 2+ = Hg 2 I 2 + K sp = 4,5 ·10 -29 • Titrant: Hg 2 (NO 3 ) 2 - secondary standard solution • Stardadization: on primary standard solution of NaCl: Hg 2 (NO 3 ) 2 + 2NaCl = Hg 2 Cl 2 + + 2NaNO 3 • Indicators: 1) solution of iron (ІІІ) thiocyanide (from reddish to colourless) 2Fe(SCN) 2+ + Hg 2 2+ = Hg 2 (SCN) 2 + + 2Fe 3+ !!! For the account of titrant volume which is used on indicator titration, do “blind test” 2) 1-2 % spirit diphenylcarbazone (to blue 2) 1-2 % spirit diphenylcarbazone (to blue color) !!! Indicator is added before end of titration • Determinate substance: chlorides and iodides. • Medium: very acidic (may be to 5 mol/L H + ions). Advantages of mercurometry: • Possibility usage titration in very acidic solutions • Titrant is cheaper • Mercury (I) Salts are less soluble, therefore accuracy and sensitivity of titration more, and inflection point on a curve is much more inflection point on a curve is much more • Possibility of titration with the diphenylcarbazone indicator in the muddy and painted solutions • Probably reversive titration of Mercury (І) salts Lack: • Mercury (І) salts are very toxic. 6. Sulphatometry • Sulphatometry – this is a precipitation titration in which SO 4 2- is the titrant Sometimes bariummetry – this is a precipitation titration in which Ba 2+ is the titrant Reaction of method: Ba 2+ + SO 4 2- = BaSO 4 ↓ determinate titrant determinate titrant substance • Titrant: secondary standard solution of H 2 SO 4 , Ba(NO 3 ) 2 , BaCl 2 . • Stardadization: solution of H 2 SO 4 on primary standard solution of Na 2 B 4 O 7 or Na 2 CO 3 (crystal.) with methyl orange; Ba(NO 3 ) 2 or BaCl 2 on secondary standard solution of H 2 SO 4 with nitrchromazo or orthanilic А. • Indicators: use metallochrome indicator - nitrchromazo or (orthanilic С), orthanilic А. BaInd + H 2 SO 4 = BaSO 4 + + H 2 Ind violet white roze precipitate solution Determinate substance: • Direct titration: - by sulphatic acid – content of Barium - by barium chloride or barium nitrate – content of sulphates - by barium chloride or barium nitrate – content of sulphates !!! For completeness of sedimentation of barium ions use as solvent of 50 % ethanol !!! Sometimes, as the indicator use sodium rodizonate Sulphatometry Ex. Ba 2+ + SO 4 2– ÷ BaSO 4 (white)+ T V(titrant, ml) End point Turbidimetry : The intensity of light scattered by particles of precipitate is measured measured Detection of end point: 1) Turbidimetry : stabilizer(glycerol-alcohol mixture) 2) Indicator : rhodizonate + Ba 2+ ÷ Red ppt+ 7. Hexacyanoferratometry Hexacyanoferratometry - this is a precipitation titration in which K 4 [Fe(CN) 6 ] is the titrant • Titrant: secondary standard solution K 4 [Fe(CN) 6 ] • Stardadization: on standard solution of • Stardadization: on standard solution of KMnO 4 in sulphatic acid 5K 4 [Fe(CN) 6 ] + KMnO 4 + H 2 SO 4 = 5K 3 [Fe(CN) 6 ] + MnSO 4 + 3K 2 SO 4 + 4H 2 O • Indicator by standardization - Methyl violet (from yellow-dreen to red-brown). Indicators: diphenylamine at the presence of a small amount of potassium hexacyanoferrate (ІІІ) for definition Zn 2+ , Mn 2+ 3,3-dimetilnaftizin for definition Cu 2+ , Zn 2+ , Cd 2+ , Pb 2+ , Fe 2+ , Ni 2+ etc. alizarin red S for definition Zn 2+ , Pb 2+ , alizarin red S for definition Zn 2+ , Pb 2+ , Th(IV) diphenylcarbazone for definition Cd 2+ Determinate substance: metal cations (direct titration), displacement titration – phosphate ions. sample KCl + K2SO4 mass= 2,44g vol= 250mL Aliquot 1 vol= 50mL Mohr titration 41,4mL 0,05818M AgNO3 Aliquot 2 vol= 50mL add NaB(Ph)4 Vol= 40mL 0,1082M ppt redissolve --> Mohr titration 50mL ppt redissolve --> Mohr titration 50mL Aliquot 1 --> mmol Cl=(V*M)AgNO3 titration 1= 2,40632mmol (1) Aliquot 2 --> mmol K+ = (mmol Cl + 2*mmol SO4) = (V*M)AgNO3 titr2 = 2,90784mmol (2) mmol SO4= 0,5*(mmol(2)-mmol(1))= 0,25076mmol %KCl= 36,71% =D17/D18*F24*(39+35,5)/1000*1/D16*100 %K2SO4= 8,94% 13-28 sample= 1,998g Cl- + CLO4- vol= 250mL Aliquot 1 50mL titration AgNO3 13,97mL 0,08551M Aliquot 2 50mL reduction of Cl)4---> Cl- titration AgNO3= 40,12mL from Aliquot 1 mol Cl-=mol AgNO3= (V*M)AgNO3 titration 1= 1,194575mmol (1) from Aliquot 1 mol Cl-=mol AgNO3= (V*M)AgNO3 titration 1= 1,194575mmol (1) from aliquot 2= mol Cl- + mol ClO4-= (V*M)AgNO3 titration 2= 3,430661mmol (2) mmol ClO4-= (2) -(1) 2,236087mmol %Cl= 10,61% =B19/B20*G26*35,5/1000*1/B18*100 %ClO4= 55,68% =B19/B22*G28*(35,5+4*16)/1000*1/B18*100