Pharmaceutical Analysis (BP102T) � UNIT III�Precipitation Titration

Precipitation titrations:
Mohr’s method, Volhard’s, Modified Volhard’s, Fajans method, estimation of sodium chloride.

Complexometric titration:
Classification, metal ion indicators, masking and demasking reagents, estimation of Magnesium sulphate, and calcium gluconate.

Gravimetry:
Principle and steps involved in gravimetric analysis. Purity of the precipitate: co-precipitation and post precipitation, Estimation of barium sulphate.

1. Prof. Amol . S. Dighe Department of pharmaceutical Chemistry Pravara Rural College of Pharmacy, Loni Pharmaceutical Analysis (BP102T) UNIT III Precipitation Titration

2. UNIT-III Precipitation titrations: Mohr’s method, Volhard’s, Modified Volhard’s, Fajans method, estimation of sodium chloride. Complexometric titration: Classification, metal ion indicators, masking and demasking reagents, estimation of Magnesium sulphate, and calcium gluconate. Gravimetry : Principle and steps involved in gravimetric analysis. Purity of the precipitate: co-precipitation and post precipitation, Estimation of barium sulphate.

3. FAQ What is precipitation titration Discuss precipitation titration. Precipitation titration is a type of titration in which analyte and titrant react to form insoluble substance which is also known as precipitate. At the end point of titration suitable indicator react with the excess of titrant and leads to formation of either colored precipitation or colored compound which can be noted as an end point of titration. Many a time precipitation titrations are referred as Argentometric titration. Typically silver nitrate is the most commonly used titrant / reagent used in precipitation titration. These types of titrations are having application for determination of chloride in the sample.

4. Detection of End point FAQ: Explain in detail end point detection methods in precipitation titration. Compare Mohrs method with volhards method Details account on mohrs method with example and give principles. Give advantage of volhards method over mohrs method. Explain in details volhards method Compares volhards method and fajans method Explain principle of volhards method and elaborate its application in determination of chloride. Write a short note k fajan method. Write a short note on adsorption indicator .

5. For end point in case of precipitation titration, following are the methods which are based on formation of colored precipitate, formation of soluble colored compound, principle of adsorption or formation of turbidity. 1) formation of colored precipitate. ( Mohr’s Method) 2) formation of soluble colored compound. ( Volhard’s Method) 3) adsorption indicators. ( Fajan’s Method) 1) Formation of colored precipitate. ( Mohr’s Method) In this method the precipitating reagent reacts with the ion to be analyzed and the precipitate id formed. At the end point , when all the analyte is reacted with the titrant added, the next drop of titrant will then react with the indicator to form colured precipitate. The formation of such colored precipitate will mark the endpoint of titration. Consider the example of titration of 0.1 M silver nitrate solution with 0.1 M sodium chloride solution using potassium chromate solution as an indicator. The solubility product of silver chloride is 1.2 X 10 -10 whereas solubility product of silver chromate is 1.7 X 10 -12.

6. Solubility product of both these salt can be represented as follows. During the titration when the precipitating agents silver nitrate is added to the solution of sodium chloride , precipitate of silver chloride will be formed. When silver chromate is just precipitatated there must be and equilibrium of both salt in solution silver chromate and silver chloride.

7. This Mohr's method can be applied for the determination of bromides in the very similar manner. Mohr’s method should be applied only to neutral or slightly alkaline solution that is within PH 6.5 to 9. If the solution in which mohr’s method is to be performed by titration using potassium chromate as an indicator is acidic then and it lead to formation of dichromate ion and ultimately endpoint cannot be detected. Reaction: In the alkaline solution silver hydroxide starts precipitating out instead of silver chromate. Hence for this method just acidic or neutral condition must be maintained. Simple method of maintaining solution form acidic to neutral is to ad excess of calcium carbonate or sodium bicarbonate to the solution.

8. To make the solution alkaline to the acidic or neutral this can be done by acidic or neutral this can be done by addition of acetic acid and small excess of calcium carbonate. Another one condition for this method is that , this methods must be performed at lower temperature because solubility product of silver chromate increase with increase in temperature. Precipitation titration based on Mohr’s method: Principle: Standardization of silver nitrate is based on precipitation type of titration ( Mohr’s Method) Here sodium chloride is used as primary standard which reacts with silver nitrate to give white precipitate of silver chloride. Here potassium chromate is used as an indicator. After completion of reaction the excess drop o silver nitrate react with potassium chromate to give reddish brown precipitate of silver chromate.

9. Preparation and standardization of 0.1 M silver nitrate: 1) preparation of 0.1 m silver nitrate: Weight accurately about 17 gm of silver nitrate dissolve it in sufficient quantity of distilled water to produce 1000 ml. 2) Standardization of 0.1 M silver nitrate: Prepare 0.1 M solution of dry sodium chloride, pipette out 10 ml of the standard sodium chloride solution into a conical flask, add potassium chromate solution 5% as indicator and titrate with the silver nitrate solution, continues the titration, until a permanent reddish brown precipitate is obtained. Reaction: Factor: Each ml of 0.1 N AgNO3 is equivalent to 0.0585 gm of NaCl

10. 2) Formation of soluble colored compound. ( Volhard’s Method) In this method when the precipitating reagent is added to the analyte at the equivalent point, when all the analyte is react with precipitating agent, the excess amount of titrant solution will react with the indicator to form a soluble colored compound. Example of the method: titration of silver ion in the presence of nitric acid with standard ammonium thiocynate solution using ferric ammonium sulphate solution as an indicator. This method is applied to the determination of halides in acidic solution. Here an excess of known volume of silver nitrate solution is added to the acidic halide solution thereby silver halide gets precipitated. In the next step when all the silver nitrate is consumed by the titrant ammonium thiocyanate, the next drop of ammonium thiocyanate will react with the indicator ferric ammonium sulphate solution to give ferrous thiocyanate complex which has a reddish color. Consider an example of titration of silver chloride solution in the presence of silver nitrate and nitric acid with ammonium thiocynate solution.

11. Preparation and standardization of 0.1 M ammonium thiocynate. Principle: Standardization of ammonium sulphate is based on precipitation type of titration ( Volhard’s method) Here silver nitrate react with ammonium thiocynate to give white precipitate of silver thiocyanate. When all the silver nitrate gets precipitated the next drop o ammonium thiocynate reacts with ferric ammonium sulphate which is used as an indicator. This gives the end point of titration as white to reddish yellow color. Procedure: 1) preparation of 0.1 m ammonium thiocyanate: Dissolve 7.612 gm of ammonium thiocynate in sufficient water to produce 1000 ml. 2) Standardization of 0.1 m ammonium thiocyanate: Pipette out 30 ml of 0.1 M silver nitrate into a glass Stoppard flask, dilute with 50 ml water, and add 2 ml of concentrated nitric acid and 2 ml of ferric ammonium sulphate solution. Titrate with the ammonium thiocyante solution to first appearance of red brown colour Factor: each ml of 0.1 N AgNO3 is equivalent to 0.0585 gm of Nacl

12. Mohr’s method Volhard’s method End point red precipitate of silver chromate End point red soluble complex of ferric thiocyanate Condition for titration: neutral to alkaline (PH- 6.5-9.0) Condition for titration: Acidic solution Titration of iodide and cyanate is not possible Can be used for determination of chloride, bromide and iodide. As solubility of silver chromate increases with rising temperature, titration are carried out at room temperature. As the color of ferric thiocyanate complex fades above 25 0c , the titration are carried below 20 oc Application of precipitation titration: Precipitation titration are used in quantitative determination of halides like chloride, bromide, and iodide. Precipitation reaction find its application in gravimetric analysis of compound. Precipitation titration in use for assay of APIs and formulations. Precipitation reaction are also used to purify organic an inorganic compound .

13. Adsorption Indicator: ( Fajan’s method) In this method at the end point o titration, the indicators get adsorbed on the surface of the precipitate. This adsorption process is accompanied by the color change. Here the indicator gets dissociated and the ions get adsorbed by the precipitate resulting in color change. The indicator used in this method are usually dyes, following are the examples of dyes. Acidic dyes: Fluorescein , eosin. Basic dyes: Rhodamine series For a dye to act as an adsorption indicator it must govern following conditions, The indicator ion must have a change opposite to that of the ion of precipitating agent. Dissolution should be concentrated enough to give sharp color change at the equivalence point. The indicator must be getting secondarily adsorbed only after the equivalence point. Limitation: Reaction of silver halide with the dye indicator is light sensitive reaction hence the titration involving adsorption indicators must be carried out with minimum exposure .

14. Assay of sodium chloride by Fajan’s method: Principle: Assay of sodium chloride is based on precipitation type of titration by fajan’s method Here silver nitrate react with sodium chloride to give white precipitate of silver chloride. On this precipitate of silver chloride the ions present in the solution starts adsorbing. Initially sodium ions and chloride ions starts adsorbing on the precipitate. At the end point when the indicator eosin is present in the solution it get adsorbed, all these reaction are possible only in acidic conditions so to maintain acidic media acetic acid is added. The end point is white to pink color. Procedure: Weight 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 titarte with silver nitrate. End point appearance of pink colour Factor: each ml of 0.1N AgNO3 is equivalent to 0.0585 gm of NaCl

15. Complexometric Titration Complexometric titration are those titration in which sample metal ions is transformed into a complex ion by addition of a reagent which is known as ligand. The complex formed over here is stable and water soluble. M + L ……………. ML Where , M = Metal ions, L= Ligands , ML = Complex Complexometric titration are very similar to acid base titration where in acid base titration acid is that species which donate proton or accepts electron, similarly in case of Complexometric titration metal ions accept electrons and ligand donate the electrons.

16. Types of Complexometric titration FAQ: State and explain various type of EDTA titration. Add a note on types of Complexometric titration . Why EDTA is widely used as complexing agent? Write a short note on types of Complexometric titration . Write a short note on replacement Complexometric titration . Classify Complexometric titration with example. Write details about types of Complexometric titrations. IMP Describe the principle and step involved in Complexometric titration . Give classification of Complexometric titration.

17. Types of Complexometric titration are as follows 1) Direct titration 2) Back titration 3) Replacement or Substitution titration 4) Alkalimetric titration 5) Miscellaneous method 1) Direct titration: The solution containing metal ion to be determined is buffered to the desired PH and titrated directly with the standard EDTA solution. At the equivalence point the concentration of metal ion being determined decrease which is detected as an endpoint with suitable indicator. E.g. titration of magnesium salts with EDTA. 2) Back titration: ( Indirect titration) Many metals cannot be directly titrated as they may get precipitated from the precipitation from the solution in the PH range necessary for titration or they may form inert complexes or a suitable metal indicator is not available to detect the endpoint of titration.

18. In such cases back titration is preferred method. Here an excess of standard EDTA solution is added, the resulting solution is the buffered to the desired PH and excess of EDTA is back titration with a standard metal ion solution. E.g titration of manganese and aluminum salts with EDTA solution using lead nitrate solution as titrant . 3) Replacement or substitution titration: Replacement titration may be used for metal ions that do not react with metal indicator or metal ion which form EDTA complex that are more stable than those of other metals such as magnesium and calcium. The metal cation to be determined may be treated with the magnesium complex of EDTA when the reaction take place as follows. E.g assay of calcium gluconate

19. 4) Alkalimetric method: In every EDTA and metal ion reaction there is release of protons that is acid. This liberated acid is then titrated which standard alkali. To detect the endpoint either visual PH indicator or potential metric method can be adopted. Alternatively iodide- iodate mixture is added to the solution along with EDTA solution and the liberated iodine is titrated against standard solution of sodium thiosulphate using starch solution as an indicator. 5) Miscellaneous method: Exchange reaction between the tetracyanonickelate ( II) ion and the element to be determined is carried out whereby nickel ions are set free. Thus silver and gold which cannot be titrated complex metrically can be determined by this way.

20. Detection of Endpoint FAQ: Discuss on metallochromic indicators Write a short note on metallochromic indicators.\ What is PM indicator Detection of endpoint in Complexometric titration can be performed by two methods, 1) visual indicators A) metallochromic indicators/ PM indicators/ metal indicators B) PH indicators in Complexometric titration 2) Instrumental methods.

21. 1) visual indicators A) metallochromic indicators/ PM indicators/ metal indicators Success of a titration depends upon precise determination of endpoint. The most common procedure uses metal indicators For visual detection of endpoint metal indicator should satisfy the following criteria 1) the color reaction must be such that before the end point , when nearly all the metal ion are complexes with EDTA the solution is strongly color. 2) the color reaction should be specific. 3) the metal indicator complex must posses sufficient stability. 4) the color contrast between the free indicator and the metal indicator complex should be readily observable. 5) the indicator must be very sensitive to two metal ions so that the color changes occurs as near to the equivalence point as possible. Consider the reaction of metal ‘M’ with the indicator ‘In’ the stability constant or formation of M-In complex can be represent as

22. During titration of metal ion M with titrant EDTA , metal EDTA complex formation occurs. Stability constant for formation 0f M-EDTA can be represent as follows . Thus titration proceeds colored metal indicator complex which is initially formed brakes and metal EDTA complex is formed. The color of indicator and those of complexes vary with PH Therefore , PH and stability of metal EDTA complex should be taken into consideration foe selecting an indicator. Commonly used PM indicator: 1) Calcon ( Mordant Black)- calcon mixture which is one part calcon and 99 parts of sodium 2) Catechol Violet – 0.1 % solution is used as an indicator. 3) Mordant black II- 4) Xylenol orange 5) Mordant Black- 3 6) 5) Mordant Red 7

23. B) PH indicators in Complexometric titration PH indicator are also used in complexometric titration typically when the titration is performed by alkalimetric method. In this method the acid liberated during the reaction between metal ion and the complexing agent is titrated with alkali and the end point is detected by using PH indicator . Usually methyl red or methyl orange is used in this type of method.

24. Titration of mixture : FAQ: What is the need for masking and demasking agents. Discuss masking and demasking process with suitable examples. Write a short note on masking and demasking agents. Masking agents: Masking agents is a substance that will form complex more strongly with the metal than that of titration under condition of titration. Masking agents are reagents which prevent interfering ion form reaction without physical separation Masking agents act either by precipitation or by formation of complex more stable than the interfering ion edetate complex. for example

25. Demasking agents: Demasking is a process in which the masked substance regains its ability to enter into the particular reaction. This enable to determine a series of metal ion in one solution containing many cations. The cyanide complex of zinc may be damasked with formaldehyde or with chloral hydrate.

26. FAQ: How will you standardize 0.05 disodium edetate solution. How will you prepare & std 0.1 M EDTA. Explain assay of calcium gluconate. Disodium EDTA is a disodium salt of ethylenediamine tetra acetic acid. It is very versatile complexing agents. It is more preferred complexing agents over the other complxing agents as it possesses following properties. Disodium EDTA is water soluble compound. Complexes formed are also water soluble. Its forms complex with most of the metal ions in 1:1 ration. The complex formation reaction with EDTA are quantitative .

27. Principle: Standardization of disodium ededate is based on complexometric type of titration. EDTA forms complex with Zn in 1:1 ratio. This complex stable at PH 10 , so ammonia buffer PH 10 is used. Zinc dust is dissolved by using hydrochloric acid. Procedure: Preparation of 0.05 M EDTA Weight accurately about 18. g of disodium EDTA dissolve it in sufficient quantity of water and make up the volume to 1000 ml with water Standardization og 0.05 m EDTA: weight accurately about 0.8 gm of zinc dust, dissolve by gently warming in 6 ml of conc. HCl and 0.1 ml of bromine water. Boil to remove excess bromine, cool and add sufficient water to make up the volume to 200 ml. add 50 mg of mordant black –II mixture and titrate with disodium EDTA until the solution turns green. Factor: Each ml of 0.05 M disodium EDTA is equivalent to 0.00327 gm o zn

28. Estimation of magnesium sulphate : Principle: Complexometric titration are particularly useful for the determination of a mixture of different metal ions in solution. Magnesium can be easily determined by EDTA titration in the PH 10 against erichrome black – T , Procedure: Weight 0.3 gm magnesium sulphate and add 50 ml distilled water. Add 10 ml of ammonia, ammonia chloride solution or ammonium buffer solution. Titrate against EDTA using mordant black- II mixture till the color changes from red to blue. Factor: Each ml of 0.05 M EDTA = 0.01232 gm of magnesium sulphate.

29. Estimation of Calcium Gluconate: Principle: Assay of calcium gluconate is based on complexometric replacement type of titration. It involves replacement of Mg 2+ ions from its Mg-indicator complex. Color changes with this indicator is not very distinct. Procedure: Weight accurately about 0.5 gm sample dissolve in 50 ml of warm water. Cool add 5 ml of 0.05 m magnesium sulphate and 10 ml of strong ammonia solution. Titrate with 0.05 M disodium edatate using mordant black – II mixture as an indicator. From volume of 0.05 M disodium edatate required subtract the volume of magnesium sulphate solution added Factor: Each ml of 0.05 M EDTA = 0.02242 gm of calcium gluconate.

30. Application of complexometric titration : Used to determine concentration of metal ion in solution. Can be used to determine how mach of calcium. Magnesium or other minerals is in a food product. Can be used determine the concentration of certain toxic metals. Water hardness can be determined by titrating it with EDTA .

31. Gravimetric analysis FAQ; Write a short note on classification , merit and demerits of gravimetric analysis. Describe the principle involved in gravimetric analysis. Gravimetric analysis is the method used for the quantitative analysis of an analyte based on determination of its weight/mass. The principle of gravimetric analysis is that once the analyte is converted to the compound fixed composition, the weight of residue is measured and the relative quantities of the analyte are determined. Classification of Gravimetric analysis: 1) precipitation method. 2) Volatilization method 3) Electro analytical method 4) Thermo gravimetric method

32. Advantage of gravimetric analysis: It is accurate and precise when using modern analytical balance. Possible sources of error are readily checked. It is absolute method, it involves direct measurement without any form of calibration being measured. Determine the atomic masses of many elements for six figure accuracy. Gravimetric provides very little room for instrumental errors and does not require a series of standard for calculation of an unknown. Disadvantage of gravimetric analysis: The chemist often prefers modern instrumental methods when they can be used. Gravimetric analysis usually only provides for the analysis of a single element or a imitated group of elements ,at a time

33. Step involved in gravimetric analysis ( unit operation) FAQ: Discuss in detail the unit operation in gravimetric analysis. Write about solvent used in washing of precipitate. Write short note on filter media in gravimetric. Write short note on ostwald ripening. Describe steps involved in gravimetric analysis. Steps in gravimetric analysis are 1) Sampling 2) preparation of solution/ dissolution 3) precipitation 4) testing the completeness of precipitate 5) Digestion/ aging of precipitate 6) Filtration 7) Washing of precipitate 8) Drying or ignition of precipitation 9) Weighing 10) Calculation

34. 1) Sampling: For quantitative analysis small sample is taken Sample should be homogenous and in the form of powder. The sample should have all its ideal physical properties. 2) Dissolution /Preparation of solution: Take a clean beaker to dissolve the sample. Transfer the sample to the beaker add sufficient water to get the clear solution adjust the proper solution condition prior to the precipitation process like temperature, PH, concentration of other constituent, volume of solution. If the substance to be analyzed is insoluble in water check its solubility in acidic solution or alkaline solution. If heating is necessary for dissolution one can use heating on water bath or on the flame. 3) Precipitation: Selection of proper precipitant is a critical step in gravimetric analysis the precipitating agent selected should be specific and if not specific it should be at least selective

35. 3) Precipitation: Selection of proper precipitant is a critical step in gravimetric analysis the precipitating agent selected should be specific and if not specific it should be at least selective The precipitant must react with analyte to form precipitate which can be easily filtered washed free for contaminants. Precipitation should be carried out in a chemically inert. 4) testing the completeness of precipitation: To check the completeness of precipitation process to the beaker which contains precipitate with a analyte a little more precipitant is added from the side of beaker and observed for the turbidity. If there is formation of turbidity that means one needs to add more precipitant so that the process of precipitation will complete. 5) Digestion or Ostwald ripening: Digestion is usually carried out at higher temperature to enhance the rate of reaction or speed up the process.

36. During the process of precipitation when the precipitate is in contact with mother liquor solution from which it is precipitated it cause formation of larger crystals this process is referred as digestion or Ostwald ripening. In the digestion process small particles of precipitate that re-dissolves because of increased temperature Due to digestion the precipitate become more regular and dense with crystalline particle and hence this lead to easy filtration of precipitate. 6) Filtration: For separation of precipitate from mother liquor various filter media used. The selection of filter media should be based upon the nature of precipitate and the cost of media. Following are various filter media available Filter paper Filter pulp Filter mats Permanent porous filter disc.

37. 7) Washing of precipitate: Precipitate should not be washed with water but it should be washed with the suitable electrolyte solution. Ideal washing solution should not react with the precipitate but it should dissolve foreign matter easily so as to separate from the precipitate. A) washing with solution of precipitant: In this case the precipitate is washed with a little excess of the dilute precipitating agents itself. E,g precipitate of lead sulphate is washed with sulphuric acid which is a precipitant. B) washing with a solution of electrolyte: In this case the precipitate is washed with an electrolyte solution in which the electrolyte solution has an ion in common with the precipitating agents. e,.g precipitate of silver chloride is washed with solution of dilute nitric acid where solution of silver nitrate is used as a precipitating agents.

38. 8) Drying: After washing , the precipitate is heated to remove water and adsorbed electrolyte. The drying temperature depends upon the nature of precipitate and the filter media used. If the driving temperature is below 250 oc it will be carried out in an oven. If the precipitate need to be dried above 250 0c and below 1200 0c then it must be performed in muffle furnace. 9) weighing: After drying the residue of precipitate is cool to room temperature in a desiccators and then weighed to calculate the content of anlayte. 10) Calculation: To determine % yield following formula is used. % A = g A/ G sample X 100 gA : gram of anlayte G sample : grams of sample taken for the analysis.

39. Purity of precipitate: FAQ: Differentiate between co-precipitation and post-precipitation. What are the factors which affects the purity of precipitate in gravimetric analysis. Write a short note co-precipitation and post-precipitation. Co-precipitation: Co-precipitation can also be stated as precipitation of any extraneous matter which is not generally precipitated under the normal condition of precipitation. E.g precipitation of barium chloride with sulphuric acid as a precipitating agent in the presence of potassium permanganate. When a small amount of potassium permanganate is mixed with solution of barium chloride, due to potassium permanganate the solution will acquire violet color. To this mixture excess of sulphuric acid is added as a precipitating agents followed by addition of reducing agents to reduce potassium permanganate. Types of co-precipitation: 1) surface adsorption 2) Occlusion 3) Mixed crystal formation 4) Mechanical entrapment

40. 1 ) surface adsorption: It is very common source of co-precipitation The precipitate with large surface area is more prone to do. This type of co-precipitation adsorption not only takes place in colloids but it can also so happened in crystalline solids. The ions that are most strongly adsorbed by ionic substance are that ions which from least soluble salts. Factor affecting surface adsorption: A) Concentration: adsorption is directly proportional to the concentration. Greater the concentration greater will be the absorption. B) temperature: Adsorption is an exothermic process and hence it is favored by decrease in temperature. C) Precipitation condition: Adsorption is affected by experimental conditions such as concentration and temp. D) Nature of adsorbed ion: Adsorbent with ionic crystal lattice preferably adsorb ions which forms sparingly soluble or weakly dissociating compound with oppositely charged ions in the lattice.

41. 2) Occlusion: When the crystal is growing rapidly during the precipitate formation foreign ions in the counter ion layer may be trapped or occluded within the growing crystal. Amount of occluded material is greatest in case of crystals that are formed by super saturation. 3) mixed crystal formation: In this case one of the ions in the crystal lattice of a solid is replaced by ion of the other element. This is possible only when the two ions having same charge and their sizes do not differ by not more than 5% The two salts must belong to the same crystal class. 4) Mechanical entrapment: When crystals are close together during crystal growth, mechanical entrapment occurs. Several crystal grow together and in this process it traps a portion of solution in the tiny poclets .

42. Post- precipitation: Sometimes when precipitation is allowed to stand in the presence of mother liquor second substance will also start to form a precipitate with the precipitating reagents this is referred as post precipitation. This generally takes place with sparingly soluble substance which forms supersaturated solution. They usually have an ion in common with the primary precipitate. E.g when calcium oxalate is precipitated in the presence of magnesium oxalate does not precipitate immediately because it tend to form supersaturated solution. Post- precipitation differs from co-precipitation in the following manner. 1) extent of contamination increase with the time of contact with mother liquor in case of post precipitation 2) extent of contamination increase with faster rate of education by mechanical 3) magnitude of contamination by post precipitation is grater than that of magnitude of contamination by co-precipitation

43. Difference between co-precipitation and post- precipitation. Co-precipitation Post- precipitation Definition Co-precipitation is a type of precipitation where soluble compounds in a solution are removed during the course of precipitation. Post precipitation is a type of precipitation where the precipitation of undesirable compound occurs after the formation of the precipitate of the desirable compound. Degree of contamination High low Time of precipitation During the desirable precipitation After the desirable precipitation example Precipitation of silver ions with other ions during the silver chloride precipitation Formation of calcium oxalate after the precipitation of magnesium oxalate.

44. Estimation of barium as barium sulphate: Procedure: 20.8 gm of BaCl2 is weighed and dissolved in 1000 ml of distilled water. Transfer 25 ml of the barium chloride sample is measured and put into 500 ml beaker. Add 0.5 ml o conc. H2SO4 and make up the volume to 100 ml with distilled water. The solution is heated to near boiling point and slowly dilute sulphuric acid is added with efficient stirring till the completion of precipitate. After the solution is allowed to settle for precipitate to form at the basement. A few drop o dilute sulphuric acid is added to the clear solution at the precipitate check all the sulphate in the solution is precipitated out. A filter proper of constant weight and cooled in a dediccator was weighted and it mass noted. The precipitated BaSO4 was filtered using the weighed whatman filter paper no 1 . Wash the precipitate 3-4 times with hot water and then oven for drying 110 0c

45. Application of gravimetric analysis: gravimetric analysis finds its application in quantitative estimation of organic compounds. Gravimetric is useful in determining thermal stability of compound which will serve as the basis for other thermal methods like TGA . For determination of calcium in water. For determination of complex mixture of compounds.

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