Principles precipitation titration-.pptx

Principles and types of Diazotization titration. INdicators for titration

1. Precipitation titration By PRAMOD KUMAR

2. Definition A special type of titrimetric procedure involves the formation of precipitates during the course of titration. The titrant react with the analyte forming an insoluble material and the titration continues till the very last amount of analyte is consumed. The first drop of titrant in excess will react with an indicator resulting in a color change and announcing the termination of the titration . Quantitative precipitation can be used for volumetric determination. The titration involves precipitation are called precipitation titrations Two type Direct Indirect

3. Reaction must satisfy this condition Precipitate must be practically insoluble. Precipitation must be rapid. Possible to detect equivalent point. Co-precipitation should be minimal Method based on precipitation of insoluble silver is known as Argentiometry . Halogens can be determined by precipitation as sparingly soluble mercurous salts HgCl2 and HgI2 is called as Mercurometry .

4. Solubility product ( Ksp ) Solubility product is the product of the concentration of ions in the saturated solution of a sparingly soluble salt as AgCl is constant at a given temperature.

5. When the ionic product exceeds the solubility products the solution is super saturated and precipitation will occur. When the ionic product is less than the solubility product the solution is unsaturated.

6. In quantitative analysis excess precipitating agent is always employed to ensure complete precipitation. If little excess of H2SO4 is employed, the ionic product far exceeds the solubility product and there is complete precipitation. Oxalic acid cause complete precipitation of calcium oxalate from solution of calcium acetate but not from calcium chloride and calcium nitrate.

7. Acetic acid is a weak acid than oxalic acid thus it does not suppress the dissociated oxalic acid. The concentration of oxalate ion is sufficient to keep ionic product greater than solubility product of calcium oxalate. In case of calcium chloride HCl is formed which is strong acid and highly dissociated. It suppresses the dissociation of oxalic acid by common ion effect . The oxalate ion concentration falls below the value required to exceed the solubility product of calcium oxalate. The precipitation is therefore incomplete. This explains why calcium oxalate dissolves in HCl but not in oxalic acid.

8. Effect of acid upon the solubility of a precipitate Sparingly soluble salt of a strong acid, the effect of the addition of acid will be similar to that of any other indifferent electrolyte but if the sparingly soluble salt of weak acid will have solvent effect upon it.

9. Effect of temperature upon solubility of a precipitate The solubility of the precipitate encountered in the quantitative analysis increases with rise of temperature is small but with other it is quite appreciable. The solubility of silver chloride at 10 C and 100 C is 1.72 mg and 21.1 mg respectively. In case of barium sulphate at these two temperature is 2.2 and 3.9 mg respectively. Where ever possible it is advantageous to filter while the solution is hot; the rate of filtration is increased. The solubility of foreign substances, thus rendering their removal from the precipitate more complete

10. Effect of solvent upon the solubility of the precipitate The solubility of most inorganic compound is reduced by the addition of organic solvent such as methane, ethanol, propanol and acetone Addition of 20 % ethanol renders solubility of lead sulphate negligible thus permitting quantitative separation. Similarly, calcium sulphate a separates quantitatively from 50 % ethanol.

11. Mohr’ method Volhard’s method

12. In 1856 Mohr introduced it. Determination of halide – chloride with silver nitrate using potassium chromate solution as indicator. It is especially useful for the determination of chloride. Precipitated silver chromate, through sparingly soluble in water is more soluble than silver chloride and the red color due to silver chromate does not appear until all the chloride has been precipitate as silver chloride

13. Precaution In this method, neutral medium should be used since, in alkaline solutions, silver will react with the hydroxide ions forming AgOH .( Ksp 2.3x10 -8 ) In acidic solutions, chromate will be converted to dichromate. Therefore, the pH of solution should be kept at about 7. There is always some error in this method because a dilute chromate solution is used due to the intense color of the indicator. This will require additional amount of Ag + for the Ag 2 CrO 4 to form.

14. Limitation Allowable PH range is 6.5 to 10 . Below PH 6.5 there is increased in solubility of silver chromate . above PH 10 the end point comes too late and Silver hydroxide is also precipitated.(co precipitation will occur) If the solution is alkaline make it acidic with nitric acid, then neutralize it by adding sodium bi carbonate or borax

15. Preparation of 0.1 M silver nitrate – weigh 17 g of silver nitrate dissolved it in 1000 ml of distilled water Weigh accurately 0.1 g of sodium chloride dissolve in 5 ml of water, 5 ml of acetic acid , 50 ml of methanol, 0.15 ml of eosin stir preferably with magnetic stirrer and titrate with silver nitrate. End point appearance of pink colour [ Rose milk color ] 0.01699g of AgNO3=0.005845g of NaCl=1ml of of 0.1M AgNO3

16. In 1874 Volhard designed the method of estimation of silver ions [ AgNO3 ] in dilute acid solutions by titrating against a standard thiocyanate solution in the presence of ferric salt [ Ferric ammonium sulphate ] as indicator. It has been extended to estimate chloride, bromide and other several analysis. Indirect Titration Volhard Method :

17. Ammonium or potassium thiocyanate solution is used in conjunction with 0.1 M AgNO3 in the assay of substances which react with nitrate but which cannot be determined by direct titrations with silver nitrate solution. In this method to the halide solution, a known excess of silver nitrate is titrated with potassium thiocyanate solution is called Volhard’s method.(Indirect Titration) In this method the precipitate of Silver chloride is filtered off The filtrate is titrated with standard thiocyanate solution using ferric ammonium sulphate solution as indicator. The excess Ag + is then titrated with standard SCN - solution until a red color is obtained which results from the reaction At the endpoint a permanent red colour is produced due to the formation of ferric thiocyanate . Fe 3+ + SCN -  Fe(SCN) 2+

18. Modified volhard’s method Cold Well’s method Especially NaCl or KCl are determined. In case of chloride it is usual to filter of the silver chloride or coagulate the precipitate by means of either dibutyl phthalate preferred or nitro benzene. The excess of silver nitrate is back titrated with potassium or ammonium thiocyanate using ferric alum as indicator. The nitro benzene or dibutyl phthalate is added to coagulate the silver chloride precipitate so that it will not interfere with the titration of excess of silver nitrate by forming a layer over silver chloride and this avoids the need for filtration.

19. The indicator system is very sensitive and usually good results are obtained. The medium should be acidic to avoid the formation of Fe(OH) 3 However, the use of acidic medium together with added SCN - titrant increase the solubility of the precipitate leading to significant errors. The reason for removing the precipitate of silver chloride as it react with thiocyanate SCN and change in Titre value . The suspension is boiled for few minutes to coagulate AgCl ppt. This removes most of the absorbed Ag+ from its surface. Potassium Nitrate is added as a coagulate to Coagulate the ppt ofAgCl3. by doing so Ag+ readsortion of Ag+ is prevented by prescnce of KNO3 In determination of iodide and bromide is not needed because the reaction is negligible.

20. Preparation of 0.1 M Ammonium thiocyanate Dissolve 7.612g of Ammonium thiocyanate in 1000ml of distilled water. Procedure: Pipette 30 ml of silver nitrate into a flask dilute with 50 ml of water, add 2 ml of nitric acid, 2 ml of ferric ammonium sulphate solution and titrate with ammonium thiocyanate solution to the first appearance of reddish-brown color.

21. In 1923-24 Fajan introduced the method Adsorption indicator is used The action of these indicators are based on the simple fact that the endpoint the indicators get adsorbed by the precipitate[AgCl] and during the process of adsorption, a change in colour of the indicator will takes place which may result in a substance of different colour Fluorescein and its derivatives are adsorbed to the surface of colloidal AgCl. After all chloride is used, the first drop of Ag + will react with fluorescein (FI - ) forming a reddish color.   Ag + + FI -  AgF Among these methods, the Volhard Method is widely used because we can detect the end point of precepitation titration very well. Fajans Method( Adsorption IndicatorMethod )

22. Adsorption indicator At the equivalence point the indicator is adsorbed by the precipitate and during the process of adsorption a change occurs in the indicator which leads to a substance of different colour . Thus, they termed as adsorption indicator. Eg Fluorescein , Eosin, Tatrazine , Rhodamine

23. Limitations of Precipitation Titration A few number of ions such as halide ions (Cl - , Br - , l - ) can be titrated by precipitation method. Co-precipitation may be occurred. It is very difficult to detect the end point.

24. Titration Curves The titration curve for a precipitation titration follows the change in either the analyte’s or titrant’s concentration as a function of the volume of titrant.  For example, in an analysis for Cl –  using Ag+ as a titrant Ag+( aq )+ Cl – ( aq ) < = = = = > AgCl( s ) the titration curve may be a plot of  pAg  or  pCl  as a function of the titrant’s volume. As we have done with previous titrations, we first show how to calculate the  titration curve and then demonstrate how to quickly sketch the titration curve. Calculating the Titration Curve  As an example, let’s calculate the titration curve for the titration of 50.0 mL of 0.0500 M Cl – with 0.100 M Ag+.  The reaction in this case is Ag+( aq )+ Cl – ( aq ) < = = = = > AgCl( s )

25. The first task is to calculate the volume of Ag+ needed to reach the equivalence point. The stoichiometry of the reaction requires that shows that we need 25.0 mL of Ag+ to reach the equivalence point. Before the equivalence point Cl– is in excess. The concentration of unreacted Cl– after adding 10.0 mL of Ag+, for example, is If the titration curve follows the change in concentration for Cl–, then we calculate pCl  as pCl = –log[Cl–] = –log(2.50 x 10–2) = 1.60

26. However, if we wish to follow the change in concentration for Ag+ then we must first calculate its concentration. To do so we use the  K sp  expression for AgCl gives a pAg of 8.14 . At the equivalence point, we know that the concentrations of Ag+ and Cl– are equal. Using the solubility product expression At the equivalence point, therefore, pAg and pCl are both 4.89 . After the equivalence point, the titration mixture contains excess Ag+. The concentration of Ag+ after adding 35.0 mL of titrant is K sp  = [Ag+][Cl–] = 1.8 x 10–10

27. or a  pCl  of 7.82.