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Precipitation titration

This document discusses various precipitation titration methods involving silver ions (Ag+). It describes three main methods: 1) Mohr's method uses silver ions as the titrant and chromate ions as the indicator for titrating halide ions like chloride. Silver halide precipitates first, followed by silver chromate at the endpoint. 2) Volhard's method titrates silver ions with thiocyanate ions in acidic medium using ferric ions as the reddish-brown thiocyanate complex indicator. 3) Fajan's method, or indicator adsorption method, involves adsorption of anionic dye indicators onto the precipitated silver halide particles. The intense color change at the

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Precipitation titration Nirmal raj marasine pharmacist cmc 8/26/2015 1
Precipitation titration The titration in which precipitation reactions takes place is called precipitation titration .for eg :titration of AgN03 with halide ions such as Cl or I O Br  Solubility product the product of molar concentration of ions raised to the power equal to its stoichiometric coefficient presents in the ionic equation In saturated solution at a fixed tempreature is called solubility product. 8/26/2015 2
• It is donated by Ksp • SOLUBILITY : The molar concentration of solute dissolved in solution at saturated condition at any temperature is called solubility .for eg : out of 5 moles of solute ,suppose 2moles of solute is ionized in a one liter solution at any fixed temperature .therefore ,here solubility of solute is equal to 2moleslit. 8/26/2015 3
Principle of precipitation • The product of concentrations of ions presents in a solution at any fixed temperature is called ionic product of the salt • It is denoted by Q • If the solution is saturated solution then the ionic product is called solubility product of the salt • “if the ionic product of the salt is greater than that its solubility product value then the preciptation of the salt takes place otherwise not,this is called the principle of precipitation “ 8/26/2015 4
Depending upon the values of ionic product, the solutions can be classified into three different categories as follows: i. Q=Ksp ,the solution is just saturated and no precipitation takes place . ii. If Q>Ksp ,the solution is supersaturated and precipitation takes place iii. If Q<Ksp ,the solution is unsaturated and more of the solute can dissolve .so no precipitation takes place at this condition 8/26/2015 5
Factors affecting the solubility of precipitate a) Effect of tempreature : with increased temperature solubility of precipitate increases b) Effect of solvent : solubility of inorganic salt is reduced by addition of organic solvent such as ethanol ,methanol ,propanol ,and acetone . • but in presence of only water ,hydration of ions of salt increases due to the high dipole moment of water molecule . 8/26/2015 6
• this hydration produces energy called hydration energy which is sufficient to overcome the attractive force between ion of solid lattice . • the ions in crystals do not have so large an attraction for oraganic solvents ,and hence the solubilities are usually less than in water 8/26/2015 7
c) Effect of acid : The solubilityof the salt of weak acid is affected by the addition of acid . -hydrogen ion of added acid combines with the anions of the salt and forms weak acid thereby increasing the solubility of the salt . d) Formation of complex ions : The increase in solubility of a precipitate upon adding excess precipitating agent is frequently due to the formation of complex ion 8/26/2015 8
Argentometric titration (argentometry): titration involving precipitation with a standard solution of sliver nitrate is called argentometric titration or argentometry conditions required for argentometry :  precipitates should be practically insoulble Precipitation reactions should be rapid and quantitative 8/26/2015 9
Precipitate should not interfere in the sharp detection of the end point Titration results should not differ apprecially due to adorptions on the precipitate Different methods involved in argentometry: a) Mohr’s method b) volhard’s method c) Fajan’s method 8/26/2015 10
Mohr’s method • A precipitaion titration in which sliver ion is used as titrant and chromate ion as indicator is called mohr method . • This method is appplicable for the quantitative analysis of halide ions (cl br and i) THEORY: Solubility product of ppt of sliver halide is greater than that of sliver chromate so that solubility of sliver halide becomes less than that of sliver chromate .there fore ,ppt of sliver halide is formed first than that of silver chromate . 2 Ag+(aq) + CrO4 2–(aq) → Ag2CrO4(s) 8/26/2015 11
• For example ,in titration of AgN03 with Nacl in presence of indicaor The chromate ion, CrO4 2– Here initially AgN03 reacts with Nacl and produce white ppt of Agcl NaCl + AgNO3  AgCl + NaNO3 white ppt 8/26/2015 12
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• Therefore ,concentration of Ag2CrO4 should be 0.014 M at end point of titrations but this gives the intense yellow colour which interfere the colour of end point .there fore ,generally 0003M-0.005M concentration of Ag2CrO4 is maintained at end point . 8/26/2015 14
precaution : 1) The titration must be carried out in ph range of 6.5-9 (i.e neutral or slightly in alkaline) 2) In more alkaline medium sliver hydroxide may be precipitated out 3) in acidic solution ,conccentration of chromate ion decreases greatly as : 8/26/2015 15
In acidic medium due to the decrease in concentration of brick red color can not seen at the end point of titration 4) In actual practice ,the concentration of chromate produces an intense yellow color to such and extent that the end point is masked . -therefore ,normally 0.003 -0.005M concentration should be employed in analytical procedures. 8/26/2015 16
Volhard’s method • A precipitation titration in which Ag+ ion is precipitated by SCN- (thiocyanate ions ) in presence of Fe(lll) ions indicator in acidic medium is called volhard method . • i.e initially Ag+(aq) + SCN–(aq) → AgSCN(s) 8/26/2015 17
• At end point Fe3+(aq) + SCN–(aq) → [FeSCN]2+ reddish brown • Here ,initially thiocyanate react with silver ions and forms precipitate .at end point ,excess of thiocyanate (SCN-) react with Fe(lll) and forms reddish brown complex which indicate the end point of reaction . 8/26/2015 18
• This volhard method is used to determine the concentration of Ag+ ions or concentration of halide ions (i.e. CL-, Br-, I) indirectly i.e. by back titration. precautions: During the titration in resulting solution ,concentration of nitric acid should be 0.5M- 1M.Here ,strong nitric acid is not used because it prevents the formation of thiocyanate iron(lll) complex .Here ,acidic medium prevents the hydrolysis of Fe(lll) as Fe(0H)3 8/26/2015 19
The solution must be free from nitrous acid otherwise it gives the instant red colouration forming the thiocynic acid Temperature of the solution must be maintained below 250C since at an elevated temperature the red colour of the ferric thiocyanate complex fades away rapidly 8/26/2015 20
• When the excess of silver has reacted ,the thiocyanate may react with the silver chloride ,since thiocyanate is less soluble than silver chloride . i.e AgCl + SCN–→ AgSCN + Cl– this will take place before reaction occurs with the iron (lll) ions in the solution ,so there will be considerable titration error . 8/26/2015 21
Fajan’s method (indicator adsorption method) • The precipitation titration in which silver ions is titrated with halide or thiocyanate ions in presence of adsorption indicator is called fajan’s method • Since the adsorption of indicator takes place at end point the method is also called indicator adsorption method • The indicator, which is a dye, exists in solution as the ionized form, usually an anion 8/26/2015 22
• The method is generally used for the quantitative analysis of halide ions or thiocyanate ions . • For eg :titration of Cl– ions with AgN03 in presence of adsorption indicator .here AgN03 is kept in burette and the Cl– ion solution with indicator is taken in titration flask 8/26/2015 23
a) Before the equivalence point :before the eqv.point ,colloidal particles of AgCL are negatively charged due to the adsorption of Cl– from the solution . The adsorbed Cl– from the primary layer which attract the positively charged Na+ ions from the solution to form a more loosely held secondary layer as shown in figure 8/26/2015 24
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b) After the eqv.point ,excess of silver ions Ag+ displace the Cl– ions and form the primary layer which attract the negatively charged nitrate ions N03 – . c) At the end point ,anion of indicator in (weak organic acid or base )replace the negatively charged ion N03 – from the second layer and give the intense color . - This intense color gives the end point of titration 8/26/2015 26
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Precautions: The AgCL should not be allowed to coagulate into large particles at the eqv.point since this will greatly decrease the surface available for adsoption of the indicator . -A protective colloid such as dextrin should be added to keep the precipitate highly dispersed .in thepresence of dextrin the color change is reversible and if the end point is overrun ,one can back titrate `with a standard chloride solution . 8/26/2015 28
The adsorption of indicator should start just before the eqv.point and increase rapidly at the eqv.point .some unsuitable indicators are so strongly adsorbed that they actually displace the primary adsorbed ion well before the eqv.point is reached The pH of the titration medium must be controlled to ensure a sufficient concentration of the ion of the weak acid or base indicator .. 8/26/2015 29
• fluorescein for eg ,has a (Ka =10-7) and in solutions more than acidic than pH 7 the concentration of Fluorescein ions is so small that no color change is observed . - fluoresceion can be used only in the Ph range of about 7 to 10 • It is preferable that the indicator ion be opposite in charge to the ion added as the titrant adsorption of the indicator will then not occur untill excess titrant is present . 8/26/2015 30
Adsorption indicator 8/26/2015 31
Comparison of silver titration methods 8/26/2015 32