ERT207 Analytical Chemistry Gravimetric Analysis and Precipitation Equilibria Pn Syazni Zainul Kamal

CO2 : Ability to classify and use separation techniques and gravimetric methods for mass determination

Introduction The term gravimetric pertains to a Weight Measurement. Gravimetric method is one in which the analysis is completed by a weighing operation. 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. Gravimetric Methods can also be defined as quantitative methods based on the determining the mass of a pure compound to which the analyte is chemically related. Analyte = constituents to be determined

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.

Example for Precipitation:- Calcium can be determined gravimetrically by precipitation of calcium oxalate and ignition of the oxalate ion to calcium oxide. Ca2+ + C2O42- →CaC2O4 CaC2O4 → CaO + CO2 + CO The precipitate thus obtained are weighed and the mass of calcium oxide is determined.

Example for Volatilisation:- The analyte or its decomposition products are volatilised at a suitable temperature. The volatile product is then collected and weighed, i.e. the mass of the product is indirectly determined from the loss in mass of the sample. Example Water can be separated from most inorganic compounds by ignition, the evolved water can then be absorbed on any one of several solid desiccants. The weight of water evolved may be calculated from the gain in weight of the absorbent.

Other Desirable Properties Not all insoluble precipitates are well suited for gravimetric analysis. It is important to consider what properties are required in order that a precipitate be applicable for a quantitative precipitation method:- Solubility Filterability Chemical Composition Other Desirable Properties

Other Desirable Properties Solubility The product must be sufficiently insoluble to prevent the loss of weight. Filterability Precipitate formed should be adoptable to simple and rapid filtration methods. Chemical Composition The product must be of known chemical composition. Other Desirable Properties Other factors effecting the stability and purity of the precipitate.

For a successful determination in gravimetric analysis the following criteria should be met :- 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.

(2)The weighed form of the product should be of known composition. For a successful determination in gravimetric analysis the following criteria should be met :- (2)The weighed form of the product should be of known composition. (3)The product should be pure and easily filtered. 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.

Gravimetric Analysis Gravimetric analysis is potentially more accurate and more precise than volumetric analysis. Gravimetric analysis avoids problems with temperature fluctuations, calibration errors, and other problems associated with volumetric analysis. But there are potential problems with gravimetric analysis that must be avoided to get good results. Proper lab technique is critical

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

1. Preparation of analyte solution Gravimetric analysis usually involves precipitation of analyte from solution. 1st step – prepare the analyte solution May need preliminary separation to separate potential interferences before precipitating analyte May need adjustment of solution condition to maintain low solubility of precipitate May require pH/temp/volume (of sol.) and/or other adjustments to maximize precipitate formation. Eg Calcium oxalate insoluble in basic medium

2. Precipitation 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. The precipitate should Be sufficiently insoluble Have large crystals (Easier to filter large crystals) Be free of contaminants

Precipitation process : When solution of precipitating agent (AgNO3) added into testing solution (chloride solution) to form AgCl precipitate, Supersaturation : the solution phase contains more dissolved salt than at equilibrium. The driving force will be for the system to approach equilibrium (saturation). - Nucleation : initial phase of precipitation. A min number of particle precipitate will gather together to form a nucleus of particle precipitate (solid phase). Higher degree of supersaturation, the greater rate of nucleation Ag+Cl- Ag+Cl-Ag+Cl- Ag+Cl-Ag+Cl-Ag+Cl-

nucleation involves the formation of ion pairs and finally a group of ions formed. - Particle growth : particle enlargement process. Nucleus will grow and forming a crystal of a specific geometric shape. 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 von Weimarn ratio = (Q – S) S A measure of relative supersaturation or supersaturation ratio The lower the better 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 mixed reactants 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

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 Ostwald ripening. The small particles tend to dissolve and precipitate on the surfaces of the larger crystals Large crystal (small surface area) have lower free energy than small crystal (large surface area) Digestion make larger crystals, reduce surface contamination, reduce crystals imperfection

4. Filtration 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 weighings differ by 0.3 mg or less. (you may see the proper technique of filtration in chapter 2)

5. Washing Coprecipitated impurities esp those on surface can be 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

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.

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

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.

Organic Precipitates

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 the 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

Example 1 If Cl2 in a sample is converted to chloride and precipitated as AgCl, the weight of Cl2 that gives 1 g of AgCl is??? F wt Cl = 35.453 F wt AgCl = 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/moprecipitate) 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 x 1 143.321 2 = 0.2474 g

percent composition by weight of the analyte in the sample : % A = gA x 100% gsample gA – grams of analyte (the desired test substance) gsample – grams of sample taken for analysis

EXAMPLE 2 A 0.3516 gm 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 1000ºC. This residue weighed 0.2161 gm. Calculate the percent P (FW = 30.974) in the sample.

g P = 0.2161 g x 30.974 x 2 222.57 1 = 0.0601g % A = gA x 100% gsample GF = f wt analyte (g/mol) x a (mol analyte/mol precipitate) f wt precipitate (g/mol) b = g analyte / g precipitate g P = 0.2161 g x 30.974 x 2 222.57 1 = 0.0601g % A = gA x 100% gsample = 0.0601 g x 100% 0.3516 g = 0.1709 %

Orthophosphate (PO43-) is determined by weighing as ammonium phosphomolybdate (NH4)PO4.12MoO3. Calculate the percent P and the percent P2O5 if 2.1771g precipitate (ppt) were obtained from a 0.3588g sample. [F wt P = 30.97], [F wt P.molybdate = 1876.5], [F wt P2O5 = 141.95]

g P = 2.1771g x 30.97 x 1 mol 1876.5 1 mol = 0.0359g % P = 0.0359g x 100% 0.3588g = 10.01 %

g P2O5 = 2.1771 g x 141.95 x 1 mol 1876.5 2 mol = 0.0823g % P2O5 = 0.0823g x 100% 0.3588g = 22.94%

Precipitation Reactions Solubility of a compound = concentrations of a soluble species at equilibrium with its insoluble form. If the compound is sparingly soluble, it will produce cation & anion. Eg PbCl2 slightly dissolved in water. So PbCl2 has a specific solubility, s = solid phase aq = aqueous phase PbCl2 (s) Pb2+ (aq)+ 2Cl- (aq)

The equilibrium constant for the reaction is known as solubility product constant. Ksp (PbCl2) = [Pb2+][Cl-]2 Concentration of any solid (PbCl2) is constant and is combined in the equilibrium constant to give Ksp

Example Calculate the concentration of Ag+ and Cl- in a saturated solution of AgCl, and the molar solubility of AgCl. Ksp for AgCl at 25°C is 1.0 x 10-10.

Solution : AgCl Ag+ + Cl- s s s=solubility (M) Ksp = [Ag+] [Cl-] = (s)(s) = 1.0 x 10-10 s = 1.0 x 10-5 M = [Ag+] = [Cl-] Molar solubility of AgCl is equal to concentration of [Ag+] or [Cl-] so molar solubility of AgCl= 1.0 x 10-5 M

Common ion effect If there is an excess of one ion over the other : - the concentration of the other is suppressed - solubility of precipitate is decreased

Example Calculate the molar solubility of AgCl in (a) water and (b) in 0.10M KCl solution. Ksp for AgCl is 1.7x10-10.

Solution Solubility of AgCl in water AgCl Ag+ + Cl- s s s=solubility(M) Ksp = [Ag+ ] [Cl- ] = (s)(s) = 1.7x10-10 s = 1.3x10-5 M Solubility of AgCl in water = 1.3x10-5 M

b) Solubility of AgCl in 0.10M KCl AgCl Ag+ + Cl- s s+0.10 Ksp = [Ag+ ] [Cl- ] = (s)(s+0.10) = 1.7x10-10 Since s is small compared to 0.10M, it can be neglected Ksp = (s)(0.10) s = 1.7x10-9 M Solubility of AgCl in the presence of 0.10M KCl is 1.7x10-9

Precipitation only occur when ionic product greater than Ksp value If ionic product equal Ksp, all the ions will be in the solution without formation of precipitate