Objectives

1. Objectives • Mention the main steps of gravimetric analysis. • Define supersaturation. • Identify types of impurities in precipitates. • Define peptization. • Define gravimetric factor. • Apply gravimetric analysis to different samples.

2. Gravimetric analysis • Gravimetric analysis is one of the most accurate and precise methods of macro-quantitative analysis. • In this process, the analyte is selectively 1. converted to an insoluble form. The separated precipitate is 2.dried or ignited,possibly to another form and is 3. accurately weighed. • From the weight of the precipitate and knowledge of its chemical composition, we can calculate the weight of the analyte in the desired form.

3. Steps of Gravimetric analysis • 1. Precipitation. • 2. Digestion. • 3. Filtration. • 4. Washing. • 5. Drying or ignition. • 6. Weighing. • 7. Calculations

4. 1. Mechanism of precipitation • When a solution of precipitating agent is added to a test solution to form a precipitate, such as in the addition of AgNO3 to a chloride solution to precipitate AgCl, • The actual precipitation occurs in a series of steps: • 1.Super saturation, (The ionic product of concentrations should be  the solubility product). • 2. Nucleation, where a minimum number of particles come together to produce microscopic nuclei of the solid phase. • 3. Particle growth, where nuclei join together to form a crystal of a certain geometric shape.

5. Mechanism of precipitation (cont.)

6. Mechanism of precipitation (cont.)

7. Notice that: • Although nucleation should occur theoretically spontaneously, but induced nucleation can occur on dust particles, scratches on the vessel surface. • The higher the degree of supersaturation, the greater the rate of nucleation. • This means the formation of great number of nuclei per unit time, increasing also the rate of particle growth. • This increases the total surface area of the precipitate thus more possibility of entrapment of impurities.

8. Degree of supersaturation • Von Weimarn discovered that the particles size of precipitates is inversely proportional to the relative supersaturation of the solution during the precipitation process: • Relative supersaturation (RSS) = • Where Q is the concentration of the mixed reagents before precipitation occurs and is referred to as the degree of supersaturation and S is the solubility of the precipitate at equilibrium. • This ratio is called the Von Weimarn Ratio.

9. Degree of supersaturation (cont.) • When a solution is supersaturated, it is in a state of metastable equilibrium and this favours rapid nucleation to form a large number of small particles, that is • High RSS Many small crystals (high Surface area) • Low RSS Fewer, larger crystals (low surface area) • Thus, we want Q low and S high during precipitation.

10. Favourable conditions for precipitation • 1. Precipitate from dilute solutions. (This keeps Q low) • 2. Add dilute precipitating agents slowly with constant stirring. (This Keeps Q low) • 3. Precipitate from hot solution. (This increases solubility) • 4. Precipitate at as low pH as possible. (This increases the solubility of the precipitate)

11. The best conditions can be obtained by using concentrations at the point of interaction of the above two curves (*). *

12. 2. Digestion • We have two types of precipitates, according to the size of the formed particles, colloidal (1- 100 m) or amorphous, crystalline (> 100 m) precipitates. • Digestion of 1.Crystalline precipitates can be obtained by allowing the precipitate to stand with the mother liquor for a long time. • 2. Colloidal precipitate is digested by standing with the mother liquor (the solution from which it was precipitated) at high temperature.

13. Digestion (cont.) • During digestion, • (i)the small particles tend to dissolve and re-precipitate on the surfaces of large crystal. • In addition, (ii)individual particles agglomerate to together around a counter ion until they finally cement together by forming connecting bridges. • Also, (iii) imperfections of the crystals tend to disappear and adsorbed or trapped impurities tend to go into solution.

14. Notice that: • For colloidal precipitates, e.g. AgCl the ions are arranged in a fixed pattern. • The surface of the precipitate tend to absorb the ion of the precipitated particle that is in excess in the solution, (Ag+ or Cl-) • This primary adsorbed layer attracts oppositely charged ion in a secondary layer, so the particle will have an overall neutral charge. • These secondary layers come closer to form coagulated precipitate.

15. Impurities in precipitates • 1. Occlusion • This occurs when materials that are not part of the crystal structure are trapped within the crystal. • For example, water or any counter ion can be occluded in any precipitate (AgCl). This causes deformation in the crystal. • This type is hard to be removed, digestion can decrease it to a certain extent.

16. Impurities in precipitates (cont.) • 2. Inclusion (isomorphous replacement) • This occurs when a compound that is isomorphous to the precipitate is entrapped within the crystal. • Isomorphous means they have the same type of formula and crystals in similar geometric form. • Example, K+ has nearly the same size of NH4+ so it can replace it in Magnesium ammonium phosphate. • Digestion cannot handle this type and mixed crystals will be formed.

17. Impurities in precipitates (cont.) • 3. Surface adsorption • Surface adsorption is very common especially in colloidal precipitates. • Example, AgCl, BaSO4, where each of them will have a primary adsorption layer of the lattice ion present in excess followed by a secondary layer of the counter ion of opposite charge so as to have a neutral net charge. • These adsorbed layers can often be removed by washing where they can be replaced by ions than can be easily volatilized.

18. Impurities in precipitates (cont.) • 4. Post precipitation • When the precipitate is allowed to stand in contact with the mother liquor, a second substance will slowly form a precipitate on the surface of the original one. • Examples, When calcium oxalate is precipitated in the presence of magnesium ions, magnesium oxalate may be if the solution is left without filtration for a long time. • Digestion will increase the extent of such type, dissolution and reprecipitation will decrease the extent of post precipitation.

19. Washing and filtering the precipitates • Coprecipitated impurities especially occluded or surface adsorbed ions can be removed by washing of the precipitate after filtration. • The precipitate will also be wet with the mother liquor which is also removed by washing. • Colloidal precipitates can not be washed with pure water, because peptization occurs. This is the reverse of coagulation.

20. Peptization • As we said before that coagulated particles have neutral layer of adsorbed primary and counter ions. • The presence of another electrolyte will cause the counter ions to be forced into closer contact with primary layer, thus promoting coagulation. • Washing with water will dilute and remove foreign ions and the counter ion will occupy a larger volume, with more solvent molecules between it and the primary layer.

21. Peptization (cont.) • The results is that the repulsive forces increases and the particles reverts to the colloidal state pass through the filter (peptization occurs). • This can be prevented by adding an electrolyte to the washing liquid. e.g. HNO3 is used to wash AgCl. • N.B.This electrolyte should be volatile at the temperature of drying or ignition and must not dissolve the precipitate.

23. Drying or ignition • If the collected precipitate is in a form suitable for weighing, it must be heated to remove water and to remove adsorbed electrolyte from the wash liquid. • This drying can be done by heating at 110 to 120°C for 1-2H. • Ignition at much higher temperature is usually required if a precipitate must be converted to a more suitable form for weighing. • In this case, the weighed form of the precipitate might be different from the precipitated form. Examples are given in the table

24. TABLE FOR COMMON GRAVIMETRIC ANALYSIS

25. Drying or ignition ( cont.)

26. Calculations in Gravimetric analysis • If we start with 10 ml of the analyte A which is precipitated in the form of P by the following equation • nA + B mP + S nMwt mMwt W2 W1 •  W2 = W1 (nMwtanalyte/mMwtppt) • Where W2 is the weight of the analyte ion (only) present in 10 ml of solution and W1 is the weight of precipitate (ppt). • (nMwtanalyte/mMwtppt) is called the gravimetric or conversion factor. • It can be calculated from the ratio of the molecular weight of the substance sought (analyzed) to that of substance weighed.

27. Example 1 • Calculate the gravimetric factor for each of the following: P Ag3PO4, K2HPO4 AgPO4, Bi2S3 BaSO4 As2O3Ag3AsO4, K2OKB(C6H5)4 • P/Ag3PO4 = atwt P/ Mwt Ag3PO4 • K2HPO4/Ag3PO4= Mwt K2HPO4/MwtAg3PO4 • Bi2S3/BaSO4 = MWt Bi2S3/3MWt BaSO4 • As2O3/Ag3AsO4 = Mwt As2O3/2MWt Ag3AsO4 • K2O/KB(C6H5)4 = Mwt K2O/ 2MWt KB(C6H5)4

28. Example 2 • Orthophosphate (PO43-) is determined by weighing as ammonium phosphomolybdate, (NH4)PO4.12MoO3. Calculate the percent P in the sample and the percent P2O5 if 1.1682g precipitate were obtained from a 0.2711 g sample. • Remember: Wt of analyte = Wt of ppt x Gravimetric Factor • Wt of P = 1.1682 X % P = (0.0193/0.2711) x 100 = 7.11%

29. Example 2 (cont.) In general,

30. Example 3 • An ore is analyzed for the manganese content by converting the manganese to Mn3O4 and weighing it. If a 1.52 g sample yields Mn3O4 weighing 0.126g, what would be the percent Mn and Mn2O3 in the sample?

31. Example 3 (cont.)