Gravimetric Analysis and Precipitation Equilibria Dr A K

Gravimetric Analysis and Precipitation Equilibria Dr. A. K. M. Shafiqul Islam & Dr. Zarina Zakaria

Introduction n The term gravimetric pertains to a Weight Measurement. n Gravimetric method is one in which the analysis is completed by a weighing operation. n n 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.

There are two main types of gravimetric analyses A) Precipitation A chemical reaction causes the formation of a sparingly soluble substance that precipitates from solution is filtered, washed, purified (if necessary) and weighed. B) Volatilisation In this method the analyte or its decomposition products are volatilised 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: n n Calcium can be determined gravimetrically by precipitation of calcium oxalate and ignition of the oxalate ion to calcium oxide. Ca 2+ + C 2 O 42 - →Ca. C 2 O 4 → Ca. O + CO 2 + 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.

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

Solubility The product must be sufficiently insoluble to prevent the loss of weight. Filterability Precipitate formed should be adoptable to simple and rapid filteration 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 : - (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. (2) E. g. When Ag+ is precipitated out by addition of Cl. Ag+ + Cl- = Ag. Cl The low solubility of Ag. Cl is reduced further by the excess of Cl- which is added force to the reaction to proceed towards right side.

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 n Gravimetric analysis is potentially more accurate and more precise than volumetric analysis. n n n 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. 2. 3. 4. 5. 6. 7. 8. Preparation of the solution Precipitation Digestion Filtration Washing Drying or ignition Weighing Calculation

1. Preparation of analyte solution n n Gravimetric analysis usually involves precipitation of analyte from solution. May need to separate potential interferences before precipitating analyte May require p. H and/or other adjustments to maximize precipitate formation When possible, select precipitating agents that are selective (or specific, if possible).

2. Precipitation n The precipitate should n n Be insoluble, but not too insoluble Have large crystals n n Be free of contaminants n n n Easier to filter large crystals The smaller the surface area the better The smaller the solubility, S, the greater the tendency to form lots of small crystals that trap contaminants and are difficult to filter The greater the solubility, the more analyte is left unprecipitated n We need to strike a balance n (but not the analytical balance, please)

The von Weimarn Ratio n von Weimarn ratio = (Q – S)/S n A measure of relative supersaturation n The lower the better n n n Q = concentration of mixed reactants before precipitation n n If high, get excessive nucleation, lots of small crystals, large surface area If low, get larger, fewer crystals, small surface area Keep it low by using dilute solutions, stir mixture well, add reactants slowly S = solubility of precipitate at equilibrium n n Keep it high with high temperatures, adjusting p. H Can lower S later by cooling mixture after crystals have formed

Digestion of the Precipitate n Very small crystal with a large specific surface area have a higher surface energy and a higher apparent solubility than large crystals. This is an initial rate phenomenon and does not represent the equilibrium condition, and it is one consequence of heterogeneous equilibria. When the precipitate is allowed to stand in the presence of mother liquor (the solution from which it was precipitated), the larger crystal grow at the expanse of the small ones. This process is called digestion, or Ostwald ripening. The small particles tend to dissolve and precipitate on the surfaces of the larger crystals

Digestion of Crystalline Precipitates –(1) When a crystalline precipitate is allowed to stand in contact with the solution from which it was precipitated (mother liquor), it is more easily filtered and requires a less dense filtering medium than the freshly formed precipitate. This improvement in filterability is accelarated by heating, common practice is to treat crystalline precipitates in this manner prior to filtration. The process is called DIGESTION. Considerable experimental evidence indicates that the digestion process consists of a cementing together of crystals. The resulting aggregates are appreciably larger than the individual crystals and are therefore more easily retained.

Digestion of Crystalline Precipitates –(2) Ions making up a precipitate are in dynamic equilibrium with their counterparts in the solution. As a result, solution and reprecipitation of the solid take place constantly. The latter process can occur in such a way as to form bridges between adjacent solid particles thus leading to crystalline aggregates. At elevated temperatures, where the solubility is greater, the process would be expected to take place at an accelerated rate, this is in keeping with the experimental observation that the effectiveness of digestion is greater at higher temperatures.

Digestion of Crystalline Precipitates –(3) The improved filterability of precipitates after digestion has also been attributed wholly or in part to growth of large crystals at the expense of small ones. Such process might occur since theoretically small crystals should be more soluble than large ones. Therefore, digestion of a mixture of crystals results in the disappearance of the more soluble smaller particles and growth of the larger and more insoluble ones. There was however some experimental evidence that the very smallest crystals of the solid may have disappeared during the digestion.

Impurities in Precipitation n Precipitates tend to carry down from the solution constituents that are not normally soluble, causing the precipitate to become contaminated. This process is called coprecipitation.

Co-precipitation At least four types of co-precipitation can be recognized a. Occlusion and Inclusion b. Surface Adsorption c. Isomorphous Replacement d. Postprecipitation

a. Occlusion and Inclusion Occlusion is a type of coprecipitation in which the soluble impurity is enclosed or entrapped within the crystal structure of the solid. As such it acts as a seat of imperfection within the crystal. Generally, occluded substances are distributed in a non homogeneous fashion throughout the crystal. In addition, the amount of occluded substance is greatly influenced by the conditions and manner of precipitate formation.

a. Occlusion and Inclusion occurs when ions, generally of large size or charge, are trapped within the crystal lattice (isomorphorous inclusion, as with K+ in NH 4 Mg. PO 4 precipitation) Occlusion and inclusion impurities are difficult to remove. Digestion is effective in reducing the quantity of entrapped contaminant.

b. Surface Adsorption § All precipitates carry down soluble impurities by adsorption on their surfaces. § However, only where the specific surface areas are great does coprecipitation of this type have an appreciable effect on the weight of the solid. § Therefore contamination by surface adsorption will be important only when dealing with colloidal precipitates. § Colloidal particles formed under analytical conditions will always have adsorbed on their surfaces one of the constituent ions of the precipitate.

b. Surface Adsorption n Let’s use Ag. Cl as an example n Primary adsorption layer n Ion in excess adsorbs on the surface of the precipitate n n n Crystal becomes positive if Ag+ is in excess Crystal becomes negative if Cl- is in excess Counter-ion layer (counterlayer) n Charged crystal attracts ions of opposite charge n n n For example, NO 3 - attracted to positive crystal, results in a layer of silver nitrate around the Ag. Cl For example, Na+ attracted to negative crystal, results in a layer of Na. Cl around the Ag. Cl The more dilute the solution, the thicker is the counter-ion layer

Colloidal Precipitates (Such as Ag. Cl n When S is very small (very insoluble precipitates), cannot avoid high degree of nucleation and very small crystals n n Colloidal particles 1 to 100 μm in diameter Surface area as high as 3, 000 ft 2/g n n High potential for surface adsorption Other problems besides surface adsorption n Colloidal precipitates cause difficulties in filtering and washing as well

Colloidal Particle in Solution Positively charged primary adsorption layer on colloidal particle NO 3− Ag+ NO 3− H+ NO 3− Ag+ Cl− Ag+ Cl− Cl− Ag+ Cl− Ag+ Ag+ NO 3− H+ Ag+ NO 3− NO 3 − Counter-ion layer of solution with excess anions H+ Ag+ Colloidal solid Homogeneous solution (charges balanced) Coagulation: aggregation of colloidal particles into larger filterable masses

b. Surface Adsorption § Several techniques are adopted to reduce the magnitude of error arising from adsorption of contaminants on a colloidal precipitate. § Among these are the use of elevated temperatures and dilute solutions, digestion, washing and reprecipitation.

c. Isomorphous Replacement n n n Two compound are said to be isomorphous if they have the same type of formula and crystallize in similar geometric forms. When their lattice dimension are about same, one ion can replace another in a crystal, a resulting in a mixed crystal. This process is called isomorphous replacement. For example, in the precipitation of Mg+2 as magnesium ammonium phosphate, K+ has nearly the same ionic size as NH 4+ and can be replaces it to form magnesium potassium phosphate. Isomorphous replacement is very serious and very little can be done about it.

d. Post Precipitation A type of contamination wherein the impurity is an insoluble compound that precipitates after all or a major part of the analytical precipitate has formed. § Post precipitation arises only when the attempt is made to separate two ions on the basis of the rate at which they precipitate. § This will come down if the solution is allowed to stand too long before being filtered.

3. Digestion (Ostwald Ripening) n n Let precipitate stand in contact with solution, usually at high temperature Large crystals (small surface area) have lower free energy than small crystals (large surface area) n n Digestion makes larger crystals, reduces surface contamination, reduces crystal imperfections Digestion coagulates a colloid (causes the particles to agglomerate, or stick together), but surface area does not decrease as much as if larger crystals actually grew

Hydrophilic vs. Hydrophobic Colloids n Hydrophilic (water-loving) colloids form gels and do not coagulate well n n Hydroxides (Al(OH)3, Fe(OH)3, for example) often form gels and are very difficult to filter Hydrophobic (water-fearing) colloids coagulate readily n n Luckily, Ag. Cl is hydrophobic Gravimetric analysis of silver or chloride by Ag. Cl precipitation is effective in spite of colloidal nature of precipitate

4. Filtration n Sintered glass crucibles are used to filter the precipitates. These are marked as fine, medium and coarse porosities (marked f, m and c). 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. Colloidal precipitates present filtering problems if particles are too small n n Can plug filter paper or glass or pass right through the filter if not coagulated well Hydrophobic colloids generally filter better than hydrophilic colloids

5. Washing n n n Removes the mother liquor May also remove some of the coprecipitated compounds Can cause peptization of colloids n n n Diluting the counter-ion layer causes it to get larger, forcing coagulated colloidal particles apart And THERE THEY GO right through the filter Wash with a solution of a volatile electrolyte that will be removed in drying step

6. Drying or Igniting n n Dry the precipitate to remove water and volatile electrolytes from wash solution Ignition (very high-temperature drying) converts precipitates to compounds more suitable for weighing n n n Removes water of hydration Converts hygroscopic compound to non-hygroscopic compound e. g. , Mg. NH 4 PO 4 is decomposed to the pyrophosphate Mg 2 P 2 O 7 by heating at 900 o. C Sometimes it even converts the filter paper to ash

Obtaining a Final Known Composition n n Ignition is often used to drive off water or give an oxide form Thermogravimetry is a form of gravimetric analysis where mass is measured as a function of temperature:

Thermogravimetry

Combustion Analysis Absorbs water CO 2 Find the empirical formula for a 13. 72 mg organic sample that produced 6. 97 mg of water and 28. 44 mg of carbon dioxide

7. Weighing n n n Properly calibrated analytical balance Good weighing technique Avoid static electricity Review important points on p. 51 periodically And now, step 8, the calculations n Chem 105 stoichiometry, anyone?

Organic Precipitates n n 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 From the weight of the precipitate formed in a gravimetric analysis, we can calculate the weight of the analyte from the weight relationship of the precipitate and analyte. The weight of the analyte is calculated from the weight of the precipitate.

E. g. If we consider calculating the percentage of Cl 2 by weighing the Ag. Cl precipitate, then Cl 2 ----> 2 Ag. Cl (we get 2 moles of Ag. Cl from 1 mol Cl 2) Calculation of the corresponding weight of Cl 2 that would be contained in the Ag. Cl sample will be: gm Cl 2 / gm Ag. Cl = [F. Wt. Cl 2 ] / [F. Wt. Ag. Cl x [1 mol Cl 2 / 2 mol Ag. Cl] gm Cl 2 = [gm Ag. Cl] x [70. 906 gm Cl 2 / 1 mol Cl 2] x [1 mol Ag. Cl / 143. 32 gm Ag. Cl] x [1 mol Cl 2 / 2 mol Ag. Cl]

We can consider Gravimetric Factor (GF) as: - Then, gm Cl 2 = gm Ag. Cl x GF(gm Cl 2/gm Ag. Cl) GF = f. wt. of substance sought / f. wt. of substance weighed x a/b (mol sought / mole weighed)

#1 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 H 3 PO 4. The phosphate was precipitated with Mg 2+ followed by aqueous NH 3 to form Mg. NH 4 PO 4. 6 H 2 O. After being filtered and washed, the precipitate was converted to Mg 2 P 2 O 7 (FW=222. 57) by ignition at 1000º C. This residue weighed 0. 2161 gm. Calculate the percent P (FW = 30. 974) in the sample. Gravimetric factor, GF = f. wt. of substance sought / f. wt. of substance weighed x a/b (mol sought / mol weighed) GF = 30. 974 gm / 222. 57 gm x 2/1 = 0. 278 Amount of P = GF (0. 278) x Wt. of precipitate (0. 2161) = 0. 0601 gm % P = 0. 0601 gm P / 0. 3516 gm sample x 100 = 17. 11%

#2 A 0. 4960 gm sample of a Ca. CO 3 is dissolved in an acidic solution. The calcium is precipitated as Ca. C 2 O 4. H 2 O and the dry precipitate is found to weight 0. 6186 gm. What is the percentage of Ca. O in the sample? (FW Ca. O = 56. 08; FW Ca. C 2 O 4. H 2 O = 146. 2) GF Wt of Ca. O = 56. 08 gm / 146. 12 gm X 1/1 = 0. 3838 = Wt. of ppt. X GF = 0. 6186 gm X 0. 3838 = 0. 2374 gm % of Ca. O in Ca. CO 3 = 0. 2347 gm / 0. 4960 gm x 100 = 47. 87%

Precipitation Reactions n n Two fully dissolved and dissociated compounds are mixed and a “precipitate” (solid product) forms The mixture becomes opaque due to the formation of solid

Precipitation Reactions n Barium Iodate in Water: saturated solution contains excess solid n At Equilibrium: Ksp = [Ba 2+][IO 3−]2 n Solubility Product (in general): for salt Mx. Ay: Ksp = [M]x[A]y

Precipitation Reactions How many grams of Ba(IO 3)2 dissolve in 500 m. L of water At 25 C if Ksp = 1. 57 × 10− 9 S S 2 S Ksp = [Ba 2+][IO 3−]2 = [S][2 S]2 = 1. 57 × 10− 9 4 S 3 = 1. 57 × 10− 9 S (molar solubility of Ba(IO 3)2) = 7. 32 x 10− 4 mol L− 1 You must learn how to use your calculator!

Precipitation Reactions How many grams of Ba(IO 3)2 dissolve in 500 m. L of water At 25 C if Ksp = 1. 57 × 10− 9 S = 7. 32 × 10− 4 mmol Ba(IO 3)2 = S × Vml = (7. 32 × 10− 4)(500 m. L) = 0. 366 mmol Wt. Ba(IO 3)2 = 0. 366 mmol × 487 mg mmol− 1 = 178 mg 0. 178 g This could cause a significant error in a gravimetric analysis

Common Ion Effect The presence in solution of an excess of one ion from a slightly soluble ionic substance depresses its solubility (more complete precipitation). How many grams of Ba(IO 3)2 dissolve in 500 m. L of 0. 0200 M Ba(NO 3)2 at 25 C if Ksp = 1. 57 × 10− 9 S S + 0. 0200 2 S [Ba 2+][IO 3−]2 = (S + 0. 0200)(2 S)2 = 1. 57 × 10− 9 Assumption: S << 0. 0200 M (0. 0200)(2 S)2 = 1. 57 × 10− 9 S = 1. 40 × 10− 4 M Wt. Ba(IO 3)2 = 1. 40 × 10− 4 M × 500 m. L × 487 mg mmol− 1 = 34. 0 mg

Theory of Precipitation n Precipitation occurs when the ion product exceeds the solubility product For Mx. Ay: CMx × CAy > Ksp n Two Steps in Precipitation n Nucleation: small groups of ions form stable solid masses (nuclei) n Particle Growth: ions from solution add on to nuclei to form particles

Theory of Precipitation n The amount by which the ion product exceeds the solubility product is expressed qualitatively as supersaturation. Supersaturation = Q − S Q: instantaneous concentration S: equilibrium concentration (molar solubility) n Large supersaturation favors nucleation n Small supersaturation favors particle growth

Minimizing Supersaturation = Q − S 1. 2. 3. Elevated Temperature (increase S) Dilute Solution (decrease Q) Slow Addition, good stirring (decrease Q)

TUTORIAL n n Read Gary text book on 314 to 320 and understand the topics. Extract the objective/purpose of steps in gravimetrics analysis. Do questions number 16, 22, 27 and 32 complete with the calculations.