By…… Ganesh Bharskar Precipitation Titration

Precipitation Titration A special type of titremetric 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. Example : AgNO 3 + NaCl  AgCl + NaNO 3

Characteristics of Precipitation Titration  They are fast and the stoichiometry is known and reproducibile, (no secondary reactions of interference.)  They are complete or can be quantified depending on the amount of solubility product (in general a precipitation titration is considered complete when Kps < )  An indicator can be used to find the equivalence point or titration end point which, for this type of titration, corresponds to when precipitation of the analyte under analysis is complete.

PRINCIPLE OF PRECIPITATION TITRATION “Themainprincipleoftheprecipitation titrationsisthatthequantityoftheadded reagentorprecipitantis tothesubstancebeing precipitating equivalent precipitate”

6 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. Factors affecting the solubility of precipitate

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.

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.

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.

8/26/ 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.  Precipitate should not interfere in the sharp detection of the end point  Titration results should not differ apprecially due to adorptions on the precipitate

Methods of Precipitation Titration According to end point detection method, three main procedures are widely used depending on the type of application. These are : a) ) Mohr Method Volhard Method Fajans Method

 Mohr Method : This method utilizes chromate as an indicator. Chromate forms a precipilate with Ag + but this precipitate has a greater solubility than that of AgCl, for example. Therefore, AgCl is formed first and after all Cl - is consumed, the first drop of Ag + in excess will react with the chromate indicator giving a reddish precipitate. 2 Ag + +CrO 4  Ag 2 CrO 4 2- Karl Friedrich Mohr ( )

 In this method, neutral medium should be used since, in alkaline solutions, silver will react with the hydroxide ions forming AgOH. 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 dilule 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.

CrO Ag + Ag2CrO4 Yellow red Red Ag2CrO4 should start precipitating at the equivalence point

Thisisanindirectchloridemethodfor excessamountof determinationwherean standardAg + isaddedtothechloridesolution containing Fe 3+ as an indicator. The excess Ag + is then titrated with standard SCN - solution untill a red color is obtained which results from the reaction: Fe 3+ + SCN -  Fe(SCN) 2+  Volhard Method : Jacob Volhard ( )

 Theindicatorsystemisvery sensitiveand usuallygoodresultsareobtained.Themedium should be acidic to avoid the formation ofFe(OH) 3 However,the useofacidicmediumtogether withaddedSCN - titrantincreasethesolubilityof theprecipitateleadingtosignificanterrors. problemhadbeenovercomebytwo This main procedures.

MODIFIED VOLHARD’S METHOD volhard’s method need of filtration before back titration for removal of PPT silver chloride. volhards method fading end point. silver chloride is more soluble than silver thiocynate. Caldwell.J.R And Moyer V.H found that Nitrobenzene form insoluble layer over the PPT as the result of this silver chloride does not interfere in thiocynate titration. Addition nitrobenene improves the endpoint.

STEP INVOLVED IN MODIFIED VOLHARD’S METHOD:- 1.add silver nitrate in analyte 2. after add nitrobenzene in above solution 3.add ferric ion indicator 4. titrated with thiocynate titrant 5. at endpoint ppt red colur is produce.

 Fajans Method : Kazimierz Fajans ( ) 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.

The precipitation titration in which silver ions is titratedwith 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 Fajan’s method (indicator adsorption method)

The method is generally used for the quantitative analysis of halide ions or thiocyanate ions. For eg :titration of Cl – ions with AgN0 3 in presence of adsorption indicator.here AgN0 3 is kept in burette and the Cl – ion solution with indicator is taken in titration flask 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

27 a)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 N0 3 –.

28 b) At the end point,anion of indicator in (weak organic acid or base )replace the negatively charged ion N0 3 – from the second layer and give the intense color. -This intense color gives the end point of titration

Precautions: 30  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 the presence of dextrin the color change is reversible and if the end point is over run,one can back titrate `with a standard chloride solution.

Adsorption Indicator

MOHR’S METHOD It is Direct Type of titration Potassium chromate & potassium dichromate indicator are user in this method Coloured precipitation obtain in this titration It is performed at pH 6-9 because in acidic condition H + react with chromate ion and to form chromic acid in alkaline condition Ag(OH2) precipitated out It is applicable for chloride and bromide not for iodide because AgI adsorbed chromate ions VOLHARD’S METHOD It is indirect or Back Type of titration Ferric nitrate and ferric ammonium sulphate indicator are used in this titration Coloured solution obtain in this titration It is performed at pH 6-9 in acidic condition to prevents precipitation of fe +++ ion Ag(OH2) It is mostly applicable for iodide and bromide not for chloride because AgI has less solubility than AgBr nand AgCl

Comparison of silver titration methods

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.

How to overcome the problems of precipitation Titration?  In the assay of substances which react with nitrate but which can’t be determined by direct titration with silver nitrate solution.  Excess standard silver nitrate solution is added together with concentrated nitric acid and the excess silver nitrate titrated with 0.1N ammonium thiocynate solution. (This is called Volhard Method)

 In the case of chlorides it is usually filtering off the AgCl or to coagulate the precipitate by means of nitrobenzene, which is non-toxic, because AgCl reacts slowly with ammonium thiocyanate. Thismakestheendpointsratherthatsince production complexwith of rediron thethiocyanate involvesthe (III).Thiocyanate ions.

What types of drug are analysed by this method  Carbromal.  KClInfusion.  NaClInfusion  Thiamine Hydrochloride

Indicator of Precipitation Titration  Potassium Chromate (K 2 CrO 4 )  Silver Chromate (Ag 2 CrO 4 )