Más contenido relacionado



  1. Gravimetric Analysis
  2. 2 Points to be covered • Principle and steps involved in gravimetric analysis. • Purity of the precipitate: co-precipitation and post precipitation, • Estimation of barium sulphate
  3. 3 Introduction • Gravimetric Analysis is a group of analytical methods in which the amount of analyte is determined by the measurement of the mass of a pure substance containing the analyte. Types of Gravimetric Analyses: • There are two main types of gravimetric analyses: A) Precipitation – analyte must first be converted to a solid (precipitate) by precipitation with an appropriate reagent. – The precipitates from solution is filtered, washed, purified (if necessary) and weighed. B) Volatilization – In this method the analyte or its decomposition products are volatilised (dried) and then collected and weighed, or alternatively, the mass of the volatilised product is determined indirectly by the loss of mass of the sample.
  4. 11/1/2018 Deokate U A 4 Criteria For a successful determination • For a successful determination in gravimetric analysis the following criteria should be met 1. The desired substance must be completely precipitated. In most determination the precipitate is of such low solubility that losses from dissolution are negligible. An additional factor is the common ion effect, this further decrease the solubility of the precipitate. – E.g. When Ag+ is precipitated out by addition of Cl- – Ag+ + Cl- = AgCl • The low solubility of AgCl is reduced further by the excess of Cl- which is added force to the reaction to proceed towards right side.
  5. 11/1/2018 Deokate U A 5 Criteria For a successful determination 2. The weighed form of the product should be of known composition. 3. The product should be pure and easily filtered. 4. Easy in handling i.e. ppt filtering, washing drying and weighing. • It is usually difficult to obtain a product which is pure or which is free from impurities. • This could be reduced by careful precipitation and sufficient washing.
  6. 11/1/2018 Deokate U A 6 Advantages of Gravimetric Analysis • Accurate and precise: Gravimetric analysis is potentially more accurate and more precise than volumetric analysis • Possible sources of errors can be checked: Gravimetric analysis avoids problems with temperature fluctuations, calibration errors, and other problems associated with volumetric analysis. • It is an ABSOLUTE method. • Relatively inexpensive
  7. 11/1/2018 Deokate U A 7 Disadvantages • But there are potential problems with gravimetric analysis that must be avoided to get good results. • Proper lab technique is critical • Careful and time consuming. • Scrupulously clean glassware. • Very accurate weighing. • Coprecipitation.
  8. 11/1/2018 Deokate U A 8 Properties of precipitate • The ppt should be so insoluble that no significant loss occurs during filtration and washing • Physical nature of ppt should be such that it cab be easily separated by filtration • The PPT should be stable to atmospheric condn. • The ppt must be convertible to pure compound of definite composition, either by ignition or by simple chemical operations such as evaporations. • Have large crystals (Easier to filter large crystals) • Be free of contaminants
  9. 11/1/2018 Deokate U A 9 Particle Size and Filterability of Precipitates • Precipitates made up of large particles are generally desirable in gravimetric work because large particles are easy to filter and wash free of impurities. In addition, such precipitates are usually purer than are precipitates made up of fine particles. • Three types of ppt are produced – Crystalline, Curdy and gelatinous etc.
  10. 11/1/2018 Deokate U A 10 Process of precipitation • It is a most imp step in gravimetric analysis • Involves both physical and chemical process • The physical process consists of three steps 1) Super saturation: the solution phase contains more dissolved salt than at equilibrium. The driving force will be for the system to approach equilibrium (saturation). 2) Nucleation : initial phase of precipitation. A min number of particle will gather together to form a nucleus of particle or precipitate (solid phase). Higher degree of super saturation, the greater rate of nucleation • nucleation involves the formation of ion pairs and finally a group of ions formed. • it is of two types 1. Spontaneous and 2. Induced 3) Crystal growth : particle enlargement process. Nucleus will grow by deposition of particles precipitate onto the nucleus and forming a crystal of a specific geometric shape. Involving two steps diffusion of ion to surface of nucleus and Deposition on surface.
  11. 11/1/2018 Deokate U A 11 Precipitation process (Von weimarn eq) • Von weimarn discover – the particle size of precipitates is inversely proportional to the relative supersaturation of the sol. during the precipitation process. – The von Weimarn Ratio (The lower the better) – von Weimarn ratio = (Q – S)/S • A measure of relative supersaturation or supersaturation ratio • If high, get excessive nucleation, lots of small crystals, large surface area • If low, get larger, fewer crystals, small surface area • S = solubility of precipitate at equilibrium, ( Keep it high with high temperatures, adjusting pH) • Q = concentration of reagents before precipitation (Keep it low by using dilute solutions, stir mixture well, add reactants slowly) • Can lower S later by cooling mixture after crystals have formed
  12. What Factors Determine Particle Size? The particle size of solids formed by precipitation varies enormously. At one extreme are colloidal suspension, whose tiny particles are invisible to the naked eye (10-7 to 10-4 cm in diameter). Colloidal particles show no tendency to settle from solution, nor are they easily filtered. At the other extreme are particles with dimensions on the order of tenths of millimeter or greater. The temporary dispersion of such particles in the liquid phase is called a crystalline suspension. The particles of a crystalline suspension tend to settle s1 p 1/1 o /20 n 18 taneously and areDr eoe kata e Ud A ily filtered. 15
  13. 11/1/2018 Deokate U A 13 The particle size of a precipitate is influenced by experimental variables as precipitate solubility, temperature, reactant concentrations, and the rate at which reactants are mixed. The particle size is related to a single property of the system called its relative supersaturation, where relative supersaturation = (Q – S) / S In this equation, Q is the concentration of the solute at any instant and S is its equilibrium solubility. When (Q – S)/ S is large, the precipitate tends to be colloidal. when (Q – S) / S is small, a crystalline solid is more likely.
  14. 11/1/2018 Deokate U A 14 • How do Precipitates Form? Precipitates form in two ways, by nucleation and by particle growth. The particle size of a freshly formed precipitate is determined by which way is faster. In nucleation, a few ions, atoms, or molecules (perhaps as few as four or five) come together to form a stable solid. Often, these nuclei form on the surface of suspended solid contaminants, such as dust particles. Further precipitation then involves a competition between additional nucleation and growth on existing nuclei (particle growth). If nucleation predominates, a precipitate containing a large number of small particles results; if growth predominates, a smaller number of larger particles is produced.
  15. 11/1/2018 Deokate U A 15 Controlling Particle Size variables Experimental supersaturation and thus that minimize lead to crystalline precipitates include elevated increase the solubility of the precipitate (S temperatures to in Equation), dilute solutions (to minimize Q), and slow addition of the precipitating agent with good stirring. The last two measures also minimize the concentration of the solute (Q) at any given instant. Larger particles can also be obtained by pH control, provided the solubility of the precipitate depends on pH.
  16. 11/1/2018 Deokate U A 16 Colloidal Precipitates Coagulation hastened by heating, stirring, and adding of Colloids: Coagulation can be an electrolyte to the medium. Colloidal suspensions are stable because all the particles present are either positively or negatively charged. This charge results from cations or anions that are bound to the surface of the particles. The process by which ions are retained on the surface of a solid is known as adsorption. We can readily demonstrate that colloidal particles are charged by observing their migration when placed in an electrical field.
  17. 11/1/2018 Deokate U A 17
  18. Peptization of Colloids Peptization refers to the process by which a coagulated colloid reverts to its original dispersed state. When a coagulated colloid is washed, some of the electrolyte responsible for its coagulation is leached from the internal liquid in contact with the solid particles. Removal of this electrolyte has the effect of increasing the volume of the counter- ion layer. The repulsive forces responsible for the original colloidal state are then reestablished, and particles detach themselves from the coagulated mass. The washings become cloudy as the freshly d 11 i/s 1/p 201e 8 rsed particles pass Deot kah te U rA ough the filter . 21
  19. 11/1/2018 Deokate U A 19 Crystalline Precipitates Crystalline precipitates are generally more easily filtered and purified than coagulated colloids. In addition, the size of individual crystalline particles, and thus their filterability, can be controlled to a degree. The particle size of crystalline solids can often be improved significantly by minimizing Q, maximizing S, or both in Equation. Minimization of Q is generally accomplished by using dilute solution and adding the precipitating from hot solution or by adjusting the pH of the precipitation medium. Digestion of crystalline precipitates (without stirring) for some time after formation frequently yields a purer, more filterable product. The improvement in filterability results from the dissolution and recrystallization.
  20. 11/1/2018 Deokate U A 20 Purity of precipitate • When the ppt is separated out from solution it is always not preferably pure and may be contaminated even after washing • The amount of impurities depends on nature of PPt and condition of pptn • It may be due to – Co-precipitation – Post precipitation, Surface adsorption – Mixed crystal formation – Occlusion and Mechanical Entrapment
  21. 11/1/2018 Deokate U A 21 Coprecipitation Coprecipitation is the phenomenon in which soluble compounds are removed from solution during precipitate formation. There are four types of coprecipitation: i) surface adsorption, ii) mixed-crystal formation, iii) occlusion, iv) mechanical entrapment Surface adsorption and mixed crystal formation are equilibrium processes, whereas occlusion and mechanical entrapment arise from the kinetics of crystal growth.
  22. 11/1/2018 Deokate U A 22 Surface Adsorption Adsorption is a common source of coprecipitation that is likely to cause significant contamination of precipitates with large specific surface areas, that is coagulated colloids. Coagulation of a colloid does not significantly decrease the amount of adsorption because the coagulated solid still contains large internal surface areas that remain exposed to the solvent. The coprecipitated contaminant on the coagulated colloid consists of the lattice ion originally adsorbed on the surface before coagulation and the counter ion of opposite charge held in the film of solution immediately adjacent to the particle. The net effect of surface adsorption is therefore the carrying down of an otherwise soluble compound as a surface contaminant.
  23. 11/1/2018 Deokate U A 23 Minimizing Adsorbed Impurities on Colloids The purity of many coagulated colloids is improved by digestion. During this process, water is expelled from the solid to give a denser mass that has a smaller specific surface area for adsorption. Washing a coagulate colloid with a solution containing a volatile electrolyte may also be helpful because any nonvolatile electrolyte added earlier to cause coagulation is displace by the volatile species. Washing generally does not remove much of the primarily adsorbed ions because the attraction between these ions and the surface of the solid is too strong. Exchange occurs, however between existing counter ions and ions in the wash liquid.
  24. 11/1/2018 Deokate U A 24 Reprecipitation A drastic but effective way to minimize the effects of adsorption is reprecipitation, or double precipitation. Here, the filtered solid is redissolved and reprecipitated. The first precipitate ordinarily carries down only a fraction of the contaminant present in the original solvent. Thus, the solution containing the redissolved precipitate has a significantly lower contaminant concentration than the original, and even less adsorption occurs during the second precipitation. Reprecipitation adds substantially to the time required for an analysis.
  25. 11/1/2018 Deokate U A 25 Mixed-Crystal Formation In mixed-crystal formation, one of the ions in the crystal lattice of a solid is replaced by an ion of another element. For this exchange to occur, it is necessary that the two ions have the same charge and that their sizes differ by no more than about 5%. Furthermore, the two salts must belong to the same crystal class. For example, MgKPO4, in MgNH4PO4, SrSO4 in BaSO4, and MnS in CdS. The extent of mixed-crystal contamination increases as increases. Mixed-crystal formation is the ratio of contaminant to analyte concentration troublesome because little can be done about it. Separation of the interfering ion may have to be carried out before the final precipitation step. Alternatively, a different precipitating reagent may be used.
  26. 11/1/2018 Deokate U A 26 Occlusion and Mechanical Entrapment When a crystal is growing rapidly during precipitate formation, foreign ions in the counter-ion layer may become trapped, or occluded, within the growing crystal. Mechanical entrapment occurs when crystals lie close together during growth. Here, several crystals grow together and in so doing trap a portion of the solution in a tiny pocket. Both occlusion and mechanical entrapment are at a minimum when the rate of precipitate formation is low, that is, under conditions of low supersaturation. Digestion is often remarkably helpful in reducing these types of copreipitation. The rapid solution and reprecipitation that goes on at the elevated temperature of digestion opens up the pockets and allows the impurities to escape into the solution.
  27. 11/1/2018 Deokate U A 27 Precipitation from Homogeneous Solution technique in which a precipitating agent Precipitation from homogeneous solution is a is generated in a solution of the analyte by a slow chemical reaction. Local reagent excesses do not occur because the precipitating agent appears gradually and homogeneously throughout the solution and reacts immediately with the analyte. As a result, the relative supersaturation is kept low homogeneously formed precipitates, during the entire precipitation. In general, both colloidal and crystalline, are better suited for analysis than a solid formed by direct addition of a precipitating reagent.
  28. 11/1/2018 Deokate U A 28 Steps in a gravimetric analysis 1. Preparation of the solution 2. Precipitation 3. Digestion 4. Filtration 5. Washing 6. Drying or ignition 7. Weighing 8. Calculation
  29. 11/1/2018 Deokate U A 29 1.Preparation of analyte solution 1st • Gravimetric analysis usually involves precipitation of analyte from solution. step – Sampling; Representative of bulk 2nd step - prepare the analyte solution (Dissolution) • May need : – preliminary separation to separate potential interferences before precipitating analyte – adjustment of solution condition (pH/temp/vol/conc of test substance) to maintain low solubility of precipitate & max precipitate formation. Eg Calcium oxalate insoluble in basic medium – Most of the substances are readily solute in water and can be used as such. Some required special treatment as treatment with HCl, HNO3, Aquaregia or fusing with basic flux
  30. 11/1/2018 Deokate U A 30 2.Precipitation process : • The precipitating reagent is added at a concentration that favors the formation of a "good" precipitate. • This may require low concentration, extensive heating (often described as "digestion"), or careful control of the pH. • A large excess of pptant should be avoided because this increases chances of adsorption on ppt. • Test for completeness of pptation • No new ppt should be formed after addition of drop of ppting agent.
  31. 11/1/2018 Deokate U A 31 3.Digestion of the Precipitate • Let precipitate stand in contact with mother liquor (the solution from which it was precipitated), usually at high temp • This process is called digestion, or Ostwaldripening. The small particles tend to dissolve and precipitate on the surfaces of the larger crystals • Digestion make larger crystals, reduce surface contamination, reduce crystals imperfection
  32. 11/1/2018 Deokate U A 32 • Ppt is separated from mother liquor • Choice depends on nature of ppt , cost of media and heating temp required for drying. • Filtration medium used are – Filter papers – Filter pulps – Filter mats – Crucible fitted with porous plate (Sintered glass filters) – Crucible to be used at high temperature 4. Filtration
  33. Filtration media • Filter papers: eg ash less filter papers – No 41 (Fast), 40 (Medium), 42(Slow) – 541, 540, 542 Grater mechanical strength • Filter pulps • Filter mats (Gooch crucible) • Sintered glass crucibles – Types G1 (100-120 ), G2 (40-50 ), G3 (20-30 ), G4 (5-10 ) – They are resistant to chemical and easy to clean 11/1/2018 Deokate U A 33
  34. • Sintered glass crucibles are used to filter the precipitates. • The crucibles first cleaned thoroughly and then subjected to the same regimen of heating and cooling as that required for the precipitate. • This process is repeated until constant mass has been achieved, that is, until consecutive weighing differ by 0.3 mg or less. 11/1/2018 Deokate U A 34
  35. 11/1/2018 Deokate U A 35 5. Washing • Co precipitated impurities esp those on surface, removed by washing the precipitate • Wet precipitate with mother liquor and which will also be remove by washing • Need to add electrolyte to the wash liquid bcoz some precipitate cannot be wash with pure water, peptization occur. • Eg HNO3 for AgCl precipitate
  36. 11/1/2018 Deokate U A 36 6) Drying or ignition • To remove solvent and wash electrolytes • Done by heating at 110 to 120°C for 1 to 2 hrs. • Converts hygroscopic compound to non- hygroscopic compound • May used high temp if precipitate must be converted to a more suitable form before weighing • Eg MgNH4PO4 convert to pyrophosphate Mg2P2O7 by heating at 900°C.
  37. 11/1/2018 Deokate U A 37 7) Weighing • After the precipitate is allowed to cool (preferably in a desiccator to keep it from absorbing moisture), it is weighed (in the crucible). • Properly calibrated analytical balance • Good weighing technique
  38. 11/1/2018 Deokate U A 38 Organic Precipitates • Organic precipitating agents have the advantages of giving precipitates with very solubility in water and a favorable gravimetric factor. • Most of them are chelating agents that forms slightly insoluble, uncharged chelates with the metal ions.
  39. 11/1/2018 Deokate U A 39
  40. 11/1/2018 Deokate U A 40 Gravimetric Analysis: Weight Relationship • in gravimetric method – the analyte (solute) is converted to precipitate which is then weight • From the weight of the precipitate formed in a gravimetric analysis, we can calculate the weight of the analyte • Gravimetric factor (GF) = weight of analyte per unit weight of precipitate. • Obtain from ratio of F Wt of the analyte per F Wt precipitate, multiplied by moles of analyte per mole of precipitate obtained from each mole of analyte • GF = f wt analyte (g/mol) x a (mol analyte/mol precipitate) • f wt precipitate (g/mol) b • = g analyte / g precipitate
  41. 11/1/2018 Deokate U A 41 Example 1 of gravimetric factor • If Cl2 in a sample is converted to chloride and precipitated as AgCl, the weight of Cl2 that gives • 1g of AgCl is? • F wt Cl = 35.453, F wt Ag = 107.868 • GF = f wt analyte (g/mol) x a (mol analyte/mol precipitate) • f wt precipitate (g/mol) b • = g analyte / g precipitate • GF = f wt analyte (g/mol) x a (mol analyte/mol precipitate) • f wt precipitate (g/mol) b • = g analyte / g precipitate • g Cl2 = g AgCl x f wt analyte (g/mol) x a • f wt precipitate (g/mol) b • = 1 AgCl x 70.906 g/mol x 1 mol • 143.321 g/mol 2 mol • = 0.2474 g
  42. 11/1/2018 Deokate U A 42 EXAMPLE 2 • A 0.3516g sample of commercial phosphate detergent was ignited at a red heat to destroy the organic matter. The residue was then taken up in hot HCl which converted P to H3PO4. The phosphate was precipitated with Mg2+ followed by aqueous NH3 to form MgNH4PO4.6H2O. After being filtered and washed, the precipitate was converted to Mg2P2O7 (FW=222.57) by ignition at 100ºC. This residue weighed 0.2161g. Calculate the percent P (FW = 30.974) in the sample.
  43. 11/1/2018 Deokate U A 43
  44. 11/1/2018 Deokate U A 44