Precipitation Titration Methods PDF

Precipitation titration  Definition 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 ana...

Precipitation titration  Definition 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.  Quantitative precipitation can be used for volumetric determination. The titration involves precipitation are called precipitation titrations Two type Direct Indirect Reaction must satisfy this condition Precipitate must be practically insoluble. Precipitation must be rapid. Possible to detect equivalent point. 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  Solubility product 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.  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. 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 calciumoxalate from solution of calcium acetate but not from calcium chloride and calcium nitrate.  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. 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  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  Types Mohr’ method Volhard’s method  Mohr’s method Karl Friedrich Mohr (1806-1879)  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  The of depends sensitivity concentration, indicator temperature concentration, on concentration and manner of observing of the electrolyte H+ red ion coloration  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+ + CrO42-  Ag2CrO4  Precaution 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 dilute chromate solution is used due to the intense color of the indicator. This will require additional amount of Ag+ for the Ag2CrO4 to form.  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. If the solution is alkaline make it acidic with nitric acid then neutralise it by adding sodium bi carbonate or borax  Limitation If appreciable amount of Ammonium salts are present the PH should not exceed 7.2. In reverse titration iodides and bromides cannot be titrated.  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 stirr preferably with magnetic stirrer and titrate with silver nitrate. End point appearance of pink colour [ Rose milk colour ]  Volhard Method : Jacob Volhard (1834-1910)  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.  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 0.1M ammonia or potassium thiocyanate solution is called Volhard’s method. In this method the precipitate of Silver chloride is filtered off and the filtrate is titrated with standard thiocyanate solution using ferric ammonium sulphate solution as indicator. At the endpoint a permanent red colour is produced due to the formation of ferric thio cyanate.  This is an indirect method for chloride determination where an excess amount of standard Ag+ is added to the chloride solution containing Fe3+ as an indicator. The excess Ag+ is then titrated with standard SCN- solution untill a red color is obtained which results from the reaction: Fe3+ + SCN-  Fe(SCN)2+ 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 the of precipitate leading to significant errors. This problem had been overcome by two procedures. main Removal of precipitate of silver chloride The reason for removing the precipitate of silver chloride react with thiocyanate SCN to form the change in Titre value. In determination of iodide and bromide is  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  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. Preparation of 0.1 M Ammonium thio cyanate Dissolve 7.612g of Ammonium thio cyanate 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 thio cyanate solution to the first  Fajan’s method Kazimierz Fajans (1887-1975)  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 these methods, the Among used because we can widely Volhard Method detect the end point of precepitation is titration very well. 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 diffi cult to detect the end point.  Determination of endpoint Formation of coloured precipitate In mohr’s method a small quantity of potassium chromate is added as indicator. At the end point the chromate ion combines with silver ion to form the sparingly soluble red silver chromate. Silver chromate sparingly soluble in water but more soluble in silver chloride.  The red colour does not appear until all the chlorides are precipitated. In determination of neutral halide 0.5 ml of 5 % w/v of potassium dichromate is used. In acid solution potassium di chromate cannot be used as indicator because of solubility of potassium dichromate in acid. Acid solution neutralised with chloride free calcium carbonate  The solubility of silver chromate is increased with temperature then the titration must be performed at room temperature. Formation of soluble coloured compound It is exemplified by Volhard for titration of silver in the presence of free nitric acid with standard potassium or ammonium thiocyanate solution. The indicator is ferric nitrate  Addition of thiocyanate produces first a precipitate of silver thiocyanate When this reaction is complete, the slight excess of thiocyanate produces reddish brown colour due to formation of complex. It is applied to the determination of chlorides, bromides and iodides in When excess silver has of reacted, react thiocyanate chloride, since with silver may thiocyanate silversoluble. It is less will takes place before the reaction occurs with Iron III ions which cause titration error  It is necessary to prevent reaction between the thiocyanate and silver chloride. Silver chloride is filtered off before back titrating since at this stage the precipitate will be contaminated with adsorbed silver ions. The suspension should be boiled for a few minutes to coagulate the silver and then remove the most adsorbed silver ions from its surface before filtration. The cold filtrate is titrated.  After the addition of silver nitrate , potassium nitrate is added as coagulant the suspension is boiled for about 3 minutes, cooled and then titrated immediately. Desorption of silver ions occurs and on cooling re adsorption is largely prevented by presence of potassium nitrate. An immiscible liquid is added to coat the silver chloride particles and thereby protect them from interaction  The most sucessful liquid is Nitro benzene – 1 ml for each 50 mg of chloride Or Dibutyl phthalate The suspension is well shaken to coagulate the precipitate before back titration.  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 , AgCl precipitated in the AgCl precipitated in the presence presence of excess of Chloride of excess of silver ions ions  If fluorescein will strongly adsorbed than the nitrate ion will reveal its presence on precipitate not by its own colour, which is that of the solution. Another view is adsorption of fluorescein ion a rearrangement of the structure of the ion occurs with formation of coloured substance.  HO O OH Fluorescein O O Important dye with phthalein group Condensing phthalic anhydride with resorcinol 0.2% solution of sodium salt of fluorescein in water/ alcohol Fajan and wolff recommended 1-2 drops of indicator for 10 ml of neutral 0.1 N halide It is rapidly decomposed by light. It can be used for chlorides bromides iodides and thiocyanate.  Dichloro fluorescein It is weakly acidic solution. Condensing phthalic anhydride and mono chloro resorcinol. 0.1 % solution in 70%  Eosin It is orange colour powder dye sparingly soluble in alcohol. Bromide, iodide and thiocyanate even in dilute solutions may be titrated accurately. B r O H Br O Br O O H Br O  Phenosafranine It is good adsorption indicator for titration of chloride and bromide solution. The dye is green crystalline product which dissolves in water to give a bright red solution. During the titration of chloride or bromide with silver nitrate most of the indicator is adsorbed on the precipitate in its red form, at the equivalence point the colour  Phenosafranine It is not useful in the pressence of sulphuric acid because of bleaching effect on the colour. Good results obtained only when halogen is present in the ionic form. Undissociated compounds such as chromic chloride and mercuric chloride cannot be analysed by this procedure. The same is of course true of any adsorption  Diphenyl carbazone 0.2% alcoholic solution of diphenyl carbazone is used as adsorption indicator. In titration of chloride a colour change from bright red to violet is obtained. With bromide and iodide the change is from yellow to green and with thiocyanate from pink to blue. Since no color change is obtained in the presence of acid. An acid solution must be first neutralised. What types of drug are analysed by this method Carbromal. NaCl Infusion KCl Thiamine Hydrochloride Infusion. Indicator of Precipitation Titration  Potassium Chromate (K2CrO4)  Silver Chromate (Ag2CrO4) Thank you