1. Gravimetric and Combustion Analysis

2. Gravimetric Analysis In gravimetric analysis, the analyte is reacted and the product is collected, massed, and then the mass of product is used to back calculate the initial moles of analyte. There are 2 kinds of gravimetric analysis: precipitation, and volatilization

3. Gravimetric Analysis Precipitation is the common gravimetric analysis that all students conduct. Here a slightly soluble or insoluble product is precipitated out, then dried and massed. The mass of product is then used to calculate the quantity of analyte in the original sample.

4. Gravimetric Analysis Volatilization occurs when the product is a gas, which is typically collected and massed. Carbon dioxide is the common volatilization product in acid/base reactions or in biodegradation reactions. It is easily collected via a second reaction and massed.

5. Gravimetric Analysis Gravimetric analysis is still used to produce standards as well as for some special reactions. Although highly accurate, it can be time and labor consuming.

7. Precipitation Reactions If you conduct a gravimetric analysis, one of the most important things is to pick the right precipitate to form. This precipitate should be fairly insoluble, form nice crystals that are easily filtered with no impurities, have a known composition, and be stable upon drying. That is actually a tall order, so the conditions are carefully controlled to maximize the precipitate yield.

8. Precipitation Crystals One of the most important aspects of a good gravimetric analysis is the particle size of the precipitate. Ideally, it forms nice, large crystals which are easily collected, don’t clog the filter, and don’t collect as many impurities due to their smaller surface area.

9. Precipitation Crystals Large crystals dispersed in a solution are called a crystalline suspension, and the particles will settle easily. Large crystals may have diameters of 0.1 mm or more. Small, fine crystals and colloids are the most difficult to collect.

10. Precipitation Crystals Colloids are particles so small, with diameters of less than 10 -4 cm, that they can’t be seen until you shine a flashlight through them, and so small they go right through most filters. Colloidal particles disperse throughout a solution to form colloidal suspensions. As stated, they are too small to be seen, and they must be treated to force the colloid particles to form filterable crystals.

11. Precipitation Crystals What is interesting in what we see as a mature science, is that the mechanism of precipitate formation and crystal size is not truly understood. However, there are several factors which help determine the particle size of a ppt.

12. Factors in Particle Size solubility of the ppt temperature reactant concentrations electrolyte concentrations how quickly the reactants are mixed together to form the ppt

13. Relative Supersaturation There is an equation which relates the particle size to the relative supersaturation of a solution: where S is the solubility of the ppt and Q is the actual concentration of the solute in sln

14. Relative Supersaturation If Q is higher than S, then the solution is supersaturated. As two reagents are mixed, it is actually typical to have supersaturation, even if it is just localized.

15. Relative Supersaturation But the higher the relative supersaturation, the more likely it is to have colloidal particles, whereas the lower the relative supersaturation, the more likely it is to have solid crystals. So the trick during ppt reactions is to keep the relative supersaturation low to minimize colloids.

16. How does the relative supersaturation affect the particle size of a ppt?

17. PPT Formation There are two ways that ppts form: –nucleation –particle growth

18. PPT Formation In nucleation, a very small number of particles stick together to form a stable solid. It may be as few as 4 particles that form this stable solid. They may form spontaneously or they may form around a small foreign particle such as dust. If the ppt forms mostly by nucleation, either a colloid or very fine crystals will result.

19. PPT Formation In particle growth, more solute particles add to a solid. If enough add to a nucleated solid, then it becomes a crystal. As more particle growth occurs, the larger the crystals become. Obviously, we want to maximize the process of particle growth, as it forms large crystals.

20. PPT Formation and Supersaturation In highly supersaturated solutions, nucleation occurs much faster than particle growth So colloids are quite common as well as very fine crystals in these solutions

21. PPT Formation and Supersaturation To try to minimize the supersaturation: –have dilute solutions (lower Q), –mix reagents together very slowly with vigorous mixing to lower localized supersaturation –mix at higher temperatures where the solubility is higher (higher S)

22. PPT Formation and Supersaturation Depending on the ppt formed, we can also adjust the pH to one where the solute is moderately soluble (higher S) to try to get large crystal growth. The pH is then adjusted to maximize the ppt formation once the large crystals have begun to grow.

23. PPT Formation and Supersaturation However, the solubilities of many compounds, like sulfides and hydroxides, are so low that they usually form colloids. Some compounds, like silver chloride, tend to form colloids or very fine crystals even though it is not that insoluble.

24. Coagulating a Colloid So we have a colloid. What is its structure like, what keeps it from forming crystals, and how can we overcome this?

25. Structure of a Colloid A colloid has two layers around a solid core. At the center is the colloidal solid with its positive cations electrostatically bound to the negative anions. This could be called a crystallite and it is the core, not a layer.

26. Structure of a Colloid Then at the surface of the crystallite, there are positive and negative charges due to the cations and anions of the solute. So excess solute ions adsorb loosely to the surface.

27. Structure of a Colloid This is the primary adsorption layer, and it will have a positive or negative charge, depending on the excess reagent. If the excess reagent is the cation of the solute, then the overall charge will be positive. Example: excess silver nitrate added to sodium chloride. The primary adsorption layer will be predominately silver ions.

28. Structure of a Colloid Because of the overall charge of the adsorption layer, a second layer called the counter-ion layer forms. This is also composed of the excess reagent along with other ions in solution. It will impart the opposite overall charge to the entire colloidal particle. So if the adsorption layer is positive, the counter- ion layer will be negative.

29. Structure of a Colloid Together, the two layers comprise what is called an electrical double layer surrounding a solid core.

31. Structure of a Colloid Why do two colloid particles resist aggregating to form a crystal? If two colloid particles, each with a negative charge, come close to one another, they will repel! So colloid particles are stable and resist crystal formation.

32. Coagulating a Colloid What can be done to overcome this colloid stability and force crystals to form? High heat, stirring, only a slight excess of the excess reagent, and the addition of an electrolyte can force a colloid to coagulate into crystals.

33. Coagulating a Colloid High heat, initially with stirring, is thought to lower the thickness of the double layer, thus making it easier for two colloid particles to collide and coagulate. The higher kinetic energy will also help them gain enough energy to overcome the repulsion.

34. Coagulating a Colloid If too much of the excess reagent is added, then the double layer increases in volume as more of the excess solute ions will be adsorbed to the surface, which in turn requires a larger counter-ion layer.

35. Coagulating a Colloid So it is important to make sure that there is only a slight excess of the excess reagent. Thus the diameter of the double layer will be minimized, enabling neighboring colloids to coagulate.

36. Coagulating a Colloid On the other hand, the addition of a suitable electrolyte like nitric acid or hydrochloric acid may also lower the diameter of the double layer. Now the high concentration of the appropriate ion will make it easier to form the counter-ion layer and its thickness will be reduced. Again, two neighboring colloids can get closer together, making it easier to coagulate.

37. Digesting Once a colloid starts to coagulate, it is best to digest the solution. Digestion is when the heated solution with the coagulating crystals sits undisturbed for an hour or more.

38. Digesting Typically, the colloidal suspension is stirred with heating until crystals start to coagulate. Then stirring is stopped, and the solution is heated to almost boiling for at least 10 minutes. Finally, the solution is allowed to cool slowly and sit undisturbed for several hours. Digestion results in larger, purer crystals which are easier to filter.

39. Filtration Once the crystals have formed and digested, they need to be filtered. The washing step can be a problem, as peptization of the coagulated colloid may occur. This means that the coagulated colloid reverts to a smaller colloidal particle.

40. Filtration Washing with pure water often causes this problem as this lowers the concentration of counter-ions, which then causes the double layer to increase in volume, and the coagulated solid may break back into smaller colloids. These colloids will then go right through the filter, and the filtrate may look cloudy.

41. Filtration Typically, the wash solvent is a dilute solution of the electrolyte. This keeps the double layer intact, minimizing peptization. This electrolyte will then volatilize during the drying step. The filtered and washed crystals are then dried to constant mass.

42. Coprecipitation of Impurities During the precipitation process, other soluble compounds may also be removed from the solution phase. These other compounds are carried out of solution by the desired crystals. They are impurities and they are said to have coprecipitated. These are NOT other insoluble compounds, but by several mechanisms, have been carried out of solution.

43. Coprecipitation of Impurities Coprecipitation occurs in several ways: –adsorption onto the surface of the crystals, –inclusions (absorption into crystal) –occlusions (absorption) Inclusions occur when ions of the impurity occupy lattice sites in the crystal, while occlusions are just particles which are physically trapped inside the crystal

44. Reprecipitation If coprecipitation occurs or is known to be a common occurrence with this solute, then reprecipitation of the solute should be conducted. In reprecipitation, the filtered precipitate containing impurities is redissolved and then the crystals are reprecipitated.

45. Reprecipitation This technique effectively lowers the concentration of impurities, so the second precipitation will contain fewer impurities. This is a common technique for iron and aluminum hydroxides which coprecipitate other more soluble hydroxides.

46. Gathering Agents Occasionally, reprecipitation is intentionally used to gather a trace component that coprecipitates. When the precipitate is redissolved in a very small amount of solvent, the trace component has been effectively concentrated. In this case, the precipitate used to gather the trace component is called the gathering agent.

47. Masking Agents Masking agents can also be used to prevent coprecipitation. The masking agents react with the impurities to from highly soluble complexes to keep them in solution.

48. Homogeneous Precipitation In homogeneous precipitation, the precipitate is formed through a second chemical reaction. First, a reagent is treated in a manner so that it forms what is called a precipitating agent or reagent. The precipitating reagent then reacts with the solute ion to form the desired solid precipitate.

49. Homogeneous Precipitation As the precipitating reagent is generated in the solution gradually, this limits the relative supersaturation of the precipitate. So crystals are more likely to form, be larger, and be more pure. This is relatively common for the precipitation of hydroxide salts where urea is used to generate the precipitating agent hydroxide.

51. Drying a Precipitate Drying a precipitate seems easy. Many compounds can be easily dried at around 110°C to remove any water which is adsorbed. Other compounds need much higher heat to remove water. The temperature must be carefully decided as many compounds will decompose if the heat is too high.

52. Igniting a Precipitate Yet other precipitates have a variable composition and must be further treated to form a compound of uniform composition. One common way to treat variable composition compounds is through ignition: high heating. This is common with iron analysis. Variable composition ferric bicarbonate hydrates are ignited at around 850°C to produce anhydrous ferric oxide.

54. Gravimetric Calculations

55. Combustion Analysis Combustion analysis is still used to determine the amount of C, H, N, O, S, and halogens in an unknown sample. In the classic freshman combustion problem, a hydrocarbon is combusted in excess oxygen gas to produce water vapor and carbon dioxide gas. The water and carbon dioxide are trapped and the mass of these products is obtained. Then calculations begin.

57. Combustion Analysis Today, elemental combustion analyzers measure C, N, H, and S at the same time. Oxygen analysis is done through pyrolysis with no oxygen gas and halogen analysis occurs through an automated titration.