Gravimetric Methods of Analysis methods based on measurement of weight of an analyte or a compound containing the analyte Precipitation methods based on isolation of an insoluble precipitate of known composition Volatilization methods analyte is volatized, weighed and the loss is determined

Properties of precipitates To obtain good results, you must be able to produce a “pure” precipitate that can be covered with high efficiency. We want the precipitate to: have low solubility, so that no significant loss of the analyte occurs during filtration and washing easily filtered and washed free of contaminants unreactive with the air, water, … very pure know composition after after it is dried or, if necessary, ignited.

Particle size and filterability of precipitates precipitates Colloidal suspensions size range : – mm no tendency to settle difficult or impossible to filter Colloidal suspensions size range : – 10 mm will settle spontaneously readily filtered typically of higher purity than colloids

Particle size and filterability of precipitates precipitates We have some control on particle size based on how we add our reagents ( precipitating agent or precipitant). Relative supersaturation (RSS) where Q = the concentration of the solute at any time S = equilibrium solubility of solute RSS can be used to estimate/control the type of precipitate that is formed.

Particle size and filterability of precipitates precipitates If RSS is large, the precipitate tends to be colloidal. If RSS is small, the precipitate tends to be crystalline. We can keep RSS small by: using dilute solution and reagent slowly adding precipitant stirring the solution warming the solution

Mechanism of precipitation formation Precipitates form in two way: Nucleation Particle Growth a process in which a minimum number of atoms, ions, or molecules join together to give a stable solid (nuclei). spontaneous induced the three dimensional growth of a particle nucleus into a lager crystal.

Mechanism of precipitation formation nucleation spontaneous induced Spontaneous nucleation will occur on its own. Induced nucleation required a ‘seed’ particle to get things started (dust, another crystal, glass fragment, …)

Mechanism of precipitation formation Particle Growth Once nucleation site has formed, other ions are attracted to the site. This will result in the information of large, filterable particles. If done properly, it also reduces contaminates since they don’t ‘fit’ in the crystal structure.

Mechanism of precipitation formation Nucleation and particle growth are competing processes although at least some nucleation must initially occur. solution nucleation particle growth We want the particle growth to be the predominate factor in precipitate formation.

Mechanism of precipitation formation Rate of nucleation is e  RSS Rate of particle growth is  RSS If RSS is large, nucleation is favored and colloidal suspensions tend to be formed. If RSS is small, particle growth will predominate resulting in crystalline in precipitates.

Mechanism of precipitation formation Rate of nucleation Rate of particle growthcrystalformation colloidalformation RSS K sp

Mechanism of precipitation formation The goal is to form crystalline precipitates so RSS must be minimised. This can be done by: Increasing S Decrease Q Increase temperature Control pH Use dilute solution Slowly add reagents Stirring the solution

Mechanism of precipitation formation Example: we will use the following precipitate formation through out: AgNO 3 (aq) + NaCl(aq) AgCl(s) + NaNO 3 (aq) precipitant colloidal precipitate

Colloidal precipitates  gCl) n Ag +              silver chloride particle primary absorbed layer counter ion layer water Start of precipitation

Colloidal precipitates initially, there is little free chloride in the solution due to the excess of Ag + the outer layer of the precipitate contains both Ag + and Cl - which tend to attract additional Ag + to the primary absorbed layer surface – primary absorbed layer this primary layer is simply an extension of the precipitate but we have run out of Cl - the counter ion in excess (nitrate) is attracted to this layer by electrostatic forces – maintains electrical neutrality

Colloidal precipitates since the overall structure tends to appear negative to the solution, it attracts water molecules that move with the particle. High [Ag + ]Low [Ag + ] at high silver concentrations, we have a higher charge on the precipitate. This results in the larger particle. the problem is even worse when the counter ion is large.

Colloidal precipitates Net result at high silver concentrations and with large counter ions, we tend to form stable colloids. the charged structures have difficultly approaching each other since there are similarly charged. Note The same type of problem will occur when chloride is in excess - towards end of precipitation

Colloidal precipitates It would seem that the solution is to keep the [Ag + ] low at all times. this is impossible to do - as Ag + is added, we create locally high concentrations even when using dilute solutions. since we can’t eliminate the problem, we need ways to minimise its effect

Obtaining a good precipitates Several steps can be taken to help precipitate the colloid and reduce impurities. two competing process. Any steps taken must address two competing process. Coagulation Peptization a process by which a coagulated colloid is reconverted to a colloidal suspension. a process where colloidal particles ‘lump’ together into larger particles.

Two common approaches for causing a colloid to coagulate: heating heating to the solution adding adding an electrolyte to the solution. Heating the colloid while stirring significantly decrease the number of absorbed ions per particle reduce the size of the counter ion layer  make it easier for the particles to approach each other

Coagulation of colloids Increasing the electrolyte concentration of the solution (HNO 3 for AgCl) (HNO 3 for AgCl) reduce the volume of the ionic atmosphere and allow particles to come closer together before electrostatic repulsion becomes significant.

Addressing peptization Washing a colloid to remove excess counter ion or trapped impurities can result in peptization. There are several common approaches that can be used to address this problem.

Treatment of colloidal precipitates Use a volatile electrolyte a relative volatile salt can be used to wash the precipitate this replaces the less volatile, excess counter ion heating precipitate (during drying) will remove the volatile electrolyte

Treatment of colloidal precipitates Digestion and aging Digestion heating the solution for about an hour after precipitate formation  help to remove weakly bound water Aging storing the solution, unheated, overnight.  allow trapped contaminates time to ‘work their way out’ a denser precipitate Both can result in a denser precipitate that is easier to filter.

Crystalline precipitates This type of precipitates is much easier to work with. easy to filter and purify In general, the best approach is : slowly form precipitate using warm and dilute solution. Then, digest without stirring.

Coprecipitation ‘a phenomenon in which otherwise soluble compounds are removed from solution during precipitate formation. Sources of coprecipitation surface absorption occlusion mixed crystal formation mechanical entrapment

Coprecipitation Surface adsorption the surface of precipitate will contain a primary absorbed ions.  as a result, counter ions will be present. example AgCl, the precipitate will commonly contain nitrate. Occurs with colloidal and crystalline ppts.

Coprecipitation AgCl precipitate if nitrate is co-precipitated, our result will be too high  nitrate weights more than chloride the results will be variable since the surface area may differ sample to sample. a lower weight counter ion will result in our results being too low.

Coprecipitation Dealing with surface adsorption washing washing with volatile electrolyte help but not too much  the attraction of counter ion is usually too strong better approach  the excess can be removed during drying ex: For AgCl, washing with HCl is a reasonable approach.

Coprecipitation Dealing with surface adsorption reprecipitation dissolve filtered precipitate and precipitate a second time only a small portion of the contaminate makes it into the second solution the second precipitate will be purer

Mix-Crystal Formation ‘a type of co-precipitation in which a contaminate ion (similar ion) replaces the analyte ion in the lattice of a crystalline precipitate’ example Determination of SO 4 2- as BaSO 4 The presence of Pb will cause a mixed crystal containing PbSO 4.

Mix-Crystal Formation this is a problem with both colloidal and crystalline ppts there is no easy solution to minimise the problem when encountered, your only choices are to remove the interferences prior to precipitation or to select a different reagent

Occlusion and Entrapment Occlusion If the crystal growth is too rapid, some counter ions don’t have time to escape from the surface. counter ions rapid precipitate formation

Occlusion and Entrapment Mechanical Entrapment occur when crystals lie close together during growth several crystal grow together and in so doing trap a portion of the solution in a tiny pocket rapid growth and merging particles

Occlusion and Entrapment The best way to deal with these problem is to slow things down using dilute, warm solution during precipitate give counter ions time to leave and help break up pocket digestion and aging precipitate provides additional time for this to occur as well

Precipitation from Homogeneous Solution Basis reagent is added to in an unreactive form to sample solution this permits of complete mixing of all materials some properties of solution is changed to slowly convert to the reagent to a reactive form the solution is slowly and uniformly changes – no locally high concentration

Precipitation from Homogeneous Solution example Dimethyl sulphate – (CH 3 O) 2 SO 4 is used to for the homogeneous generation of sulphate ion. (CH3O) 2 SO 4 + 4H 2 O  SO CH 3 OH + 2H 3 O + Eliments precipitated: Ba, Ca, Sr and Pb and sulphate.

Precipitate Drying after filtration, the precipitate must be dried to constant weight - remove excess solvent - drive off any volatile species in some cases, the precipitate is heated to a point where it decomposes to a stable form for weighing thermobalances can be used to determine optimum drying time and temperature

> 1000 o C > o C

Summary of method a relative slow method of analysis - most time is spent waiting minimal requirements - major equipment is a good balance and an oven no calibration is required - results are based on formula weight accuracy ppt

Summary of method sensitivity - analyte concentration should be over 1%. Below that, you may encounter problem to solubility. selectivity - not very specific but can be make reasonably selective.