7 Gravimetric Analysis
Dr. Meenu
- Gravimetric Analysis
On macroscopic level gravimetric analysis is based on obtaining easily filterable precipitates that are free from contaminants. The process which involves conversion of analyte into sparingly soluble compounds of known composition, filtered, washed and dried is precipitation gravimetric analysis. The simple gravimetric analysis involves simple heating, precipitation, drying and separation of sample in order to find out volatile and non-volatile components. The analyte may be separated from the sample by converting it into gas and the mass of gas serves as measure of analyte concentration this process is known as volatilization gravimetry. Thermo gravimetric analysis method of analysis includes quantitative change in mass of analyte with reference to temperature. This technique is purely instrumental.
Advantages of gravimetric analysis:
- For macro quantitative analysis, this technique gives accurate results with the use of modern analytical balance.
- The process does not require any calibration or standardization as direct measurement of mass of analyte is done.
- The completion of reaction can be confirmed from the filtrate
- Except for certain procedures where Pt-crucible is required the process is cost effective.
- The results are reproducible within error of 0.3-0.5 % only.
2 Steps involved in gravimetric analysis
2.1 Sample preparation: For analysis of water soluble samples, solution is prepared in distilled water. Special treatment is required for analysis of elements from ores, minerals and alloys. Before addition of any precipitating agent the interfering agent if present in the solution must be removed. For example, during the analysis of PbCrO4, HNO3 interferes by forming precipitates of Pb(NO3)2 so the solution is treated with acetic acid and sodium acetate to remove nitrate ions from solution for accurate analysis. Any interfering agents if present in the solution must be removed with proper masking agent before analysis. The important factors for sample preparation for accurate gravimetric analysis includes pH, temperature, concentration of analyte, volume of solution, choice of precipitating agent.
For determination of silver ions in solution chloride ion as precipitating agent can be a better choice. However for determination of chloride ions in solution silver ion as precipitating agent are not a good choice as other halide ions also get precipitate along with silver chloride.
2.2 Precipitation: Among various insoluble components, choice of appropriate component be made that can be obtained from desired constituent.
For completion and specificity, the analyte should have low solubility. This can be judged from solubility product. The precipitating agent should be specific and selective. It should precipitate the desired analyte only.
The precipitates must be convertible into stable pure substance of definite chemical composition and the structure of precipitates should be such as to allow rapid filtration and washing. This can be achieved either by ignition or simple chemical operation such as evaporation. The simple heating of the filtered washed precipitates to appropriate temperature is termed as ignition. It helps to remove absorbed water, occluded water or water of hydration. The obtained precipitates must be in a state that can be converted to another analytically more favored compound. Another point of interest is that particle size should such that it should not pass through filtration medium and should not change by washing process. Hot solutions are preferred to increase precipitate solubility and decrease super saturation. The degree of super saturation can be minimized by adding dilute solution of precipitant slowly with thorough stirring. Digestion helps to collect perfect and pure crystalline material.
Among various ways to obtain filterable precipitates are digestion, temperature change, stirring and addition of suitable electrolyte. Weak electrolytes are preferred over water for washing as it may cause peptization sometimes i.e. precipitates are converted into colloidal state. Precipitates must be free from impurities and the particles from which precipitates are made should be homogeneous.
Inhomogeneity may be attributed to simultaneous precipitation that occurs when solubility product of another compound exceeds.
Co precipitation occurs when foreign material is brought down as surface impurity and post precipitation that occurs when two precipitates are formed at different rates. Proper experimental procedures are to be followed to avoid contamination of precipitates. During precipitation both surface adsorption and occlusion act as source of contamination. Adsorption of ions from the mother liquor over the precipitated particles is known as surface adsorption. This is more in smaller particles and can be removed by washing. Occlusion involves the entrapment of impurities within the lattice structure of crystalline precipitates. Both type of contamination can be reduced by maintaining a low degree of super saturation during precipitation and digestion.
2.3 Filtration: In quantitative analysis filtration must be conducted with much greater care than is sometimes given in qualitative analysis. Filtration should be done using gravimetric filter paper of suitable pore size. These papers have very small ash content. The filtration process must be such that it should retain smallest particles of precipitates with rapid filtration. The different filtering media used are (a) filter papers (b) sintered glass crucible (crucible fitted with permanent porous plates) (c) crucible for ignition of precipitates in filter paper.
2.3.1 Filter Papers: For gravimetric purpose ashless filter papers must be used depending upon the pore size. The filter papers are given different numbers. The Whatmann filter papers are most suitable for gravimetric work e.g. filter paper No. 41 is used for common purposes, filter paper No. 42 is used for the finest precipitates but the filtration is slow. Filter paper No. 40 with large pores is used for gelatinous precipitates. This paper is treated with HCl and HF acids during its manufacture. To recover the precipitates, the filter paper is burnt off. Burning of filter paper result into production of C and CO in the close vicinity of precipitates, that may cause reduction of precipitates. The precipitates which are reduction sensitive shall not be measured using filter paper.
The funnel with an angle 60° and stem of length about 15 cm should be used in order to promote rapid filtration. Set up filtration assembly. Start the filtration by decanting liquid into the funnel in order to avoid an immediate build up of solid particles that can clog the pores of the paper. The stem of the funnel should be vertical with its tip touching the sides of receiving beaker. The liquid is directed to the side of filter using a glass rod. The last trace of the precipitates is removed from the beaker using a jet of water from a wash bottle. The number of washings may be taken up to 9 or 10. The washing may be tested for the absence of ions.
2.3.2 Crucible with permanent porous plates: Sintered glass crucible is used for precipitates which require heating up to 200 oC. Sintered glass crucible is made of glass and has porous filter base. Crucible is mounted by means of a wide rubber tube and suction is applied. These crucibles are available in varying porosity for various types of precipitates. In corning pyrex crucible the letters C, M, F are written near the upper edge of crucible indicating coarse, medium and fine. G3 and G4 is indicated on the crucible meant for coarse and fine precipitates, respectively. The sintered filtering disc is attached by strong alkali. Such crucibles are not heated on flame directly. The precipitates are dried in electric oven. Porous porcelain crucibles are used for precipitates requiring high temperatures (>500oC). In this case also the crucible is not heated directly and the bottom is readily attacked by strong alkalies.
2.3.3 Crucible for ignition of precipitates in filter paper: Platinum, porcelain and fused silica glass crucibles are used for ignition of precipitates in filter paper. Though platinum crucible is most preferred, but their high cost limit utility. Porcelain crucible slightly change in weight on strong and prolong heating and are attacked by alkaline substances and HF, fused silica crucible are widely used as red hot crucible can be immediately dipped in heater without breaking and are prone to strong alkalies and HF acids.
The ideal washing liquid should satisfy the following conditions
1. It should not have dispersive action on the precipitates
2. It should be volatile at temperature of drying of the precipitates
3. It should not dissolve the precipitates.
4. It should contains no substances, which is likely to interfere with the determination of the precipitates
Ammonium salts are generally used for washing the precipitates e.g. NH4NO3 for washing Fe(OH)3
2.3.4 Drying, Ignition and Incineration of Precipitates: The precipitates after careful washing must be brought to constant composition before it can be weighed. Further treatment depends upon the nature of the filtering medium and of the precipitates. Drying is a process in which a precipitate is heated below 250°C. Ignition involves heating the precipitates from 250-1200°C. Drying may be done by keeping the precipitate in the electric oven maintained at 150°C. Conventionally drying of precipitates consists of placing the funnel containing the precipitate on a cone which is kept on a wire gauge or sand bath placed on a tripod stand. Heating is done with a burner placed below it. The stem of the funnel should not touch the hot sand bath, when drying is complete the filter paper will begin to separate from funnel. Avoid over heating the filter paper. Filter paper with the precipitate is taken out of funnel and opened in such a way that the fingers do not touch the precipitate. Take a dry and clean petri dish and remove as much as possible of precipitates. In another method the precipitates are collected over glazed paper, with the help of brush. Cover the precipitate with funnel and keep it safe. Fold the filter paper in cone shape. Catch the filter paper with tongs where no precipitates are stick. Heat the filter paper with burner.
In a previously constant weighed crucible, collect the ash. The paper should smoulder. The particles which fall on glazed paper swept from charring paper by violent activity of gases are collected with brush and are transferred to crucible. The crucible is heated strongly to brun off all carbon to white ash. The crucible should be reduced to flame from time to time.
Before weighing the precipitates finally the ash should be treated with suitable reagent as some of the precipitates may get reduced by carbon of paper. The crucible is cooled first and then one or two drops of reagent is added, heated gently avoiding the sputtering of precipitates. In case second reagent is added after cooling. The crucible, 1-2 drops are added and as removed as before. Heat the crucible strongly and then cool. Repeat the process till constant weight is obtained.
- Organic precipitating or chelating agents are used to form insoluble metal salts
- Some common representative chelating agents
4 Applications of gravimetric analysis
4.1 Determination of Nickel as Ni(DMG)2 in Nickrome alloy
Theory: Ni is precipitated as red colored Ni(DMG)2 complex in ethanoic acid-ethanoate buffer or in ammonical solution as the complex is soluble in mineral acids. During the reaction pH of solution is controlled. Lowering of pH may favor the reaction in backward reaction.
Procedure: Weigh out accurately 0.3-0.4 g ammonium nickel sulphate (NH4)2S04.NiSO4, 6H2O into a 500 mL beaker. To this add 25 ml of water. Add a 5% of dimethyl glyoxime reagent. Add dilute ammonia solution drop wise to the solution with constant stirring until precipitation takes place. Allow the solution to stand on the steam bath for 20-30 minutes. Test the supernatant liquid for complete precipitation by adding a few drops of DMG solution. Filter the cold solution through a sintered-glass crucible, previously heated to 110-120 OC and weigh after cooling in desiccators. Repeat the drying until constant weight is obtained.
4.2 Determination of sulphate as barium sulphate: Sulphate ions are precipitated from the solution as Barium sulphate with the addition of adding barium chloride Barium sulphate has solubility about 3 mg L-1 at the ordinary temperature in water. The solubility of barium sulphate increases in presence of mineral acids due to formation of hydrogen sulphate ion. The precipitation of barium sulphate is carried out in presence of weak acid to prevent the possible formation of the barium salts of anions such as chromate, carbonate, and phosphate, which are insoluble in neutral solutions. The precipitation is carried out at higher temperature as supersaturation is less at higher temperature. The solubility of the precipitate in the presence of barium chloride in 0.05 M HCl is negligible. Coprecipitation of sulphate with other metal ions such as aluminium, chromium, and iron can be removed by precipitation, and the influence of the other ions, if present, can be reduced by considerably diluting the solution and by digesting the precipitate. The precipitate of barium sulphates are easily reduced to sulphide at temperatures above 600 OC by the carbon of the filter paper. The reduction can be avoided by charring the paper without inflaming. Then carbon is burnt off slowly at a low temperature in presence of air. However if precipitates are reduced re-oxidation of precipitates can be done with sulphuric acid followed by volatilization of acid and heating.
Procedure: Weigh out accurately about 0.3 g of the solid and add 20-25 ml water. To this solution add 200 ml of .05 M HCl. Boil the solution and add drop wise 10-12 mL of warm 5 per cent barium chloride solution from a burette. Allow the precipitate to settle for a minute or two. Test the supernatant liquid for complete precipitation by adding a few drops of barium chloride solution. Add barium chloride till complete precipitation is observed. Decant the clear solution through an ashless filter paper (Whatman No. 40 or 540), and collect the filtrate in a clean beaker. Wash the precipitates 8-10 times with water. Fold the moist paper around the precipitate and place it in a porcelain crucible. Dry the paper by placing the loosely covered crucible upon a triangle several centimeters above a small flame. Do not allow the filter paper to burst in to flames. When charring is complete increase the temperature gradually and burn off the carbon in presence of excess air. Ignite the crucible for 10-15 minutes till it is red hot. Cool the precipitates and keep in the desiccators. Repeat the ignition process till constant weight is obtained.
Calculate the percentage of sulphate ions.
Bibliography
1. G. Marr and B.W. Rocket, ‘Practical Inorganic Chemistry’, University Science Books, 1999.
2. G. Pass and H. Sutcliffe, ‘Practical Inorganic Chemistry’, Chapman and Hall, London, 1968.
3 Vogel’s Textbook of Quantitative Chemical Analysis, Arthur Israel Vogel, Prentice Hall, 2000.
4 J. Mendham, R. C. Denney, J. D. Barnes, M. Thomas, ‘Vogel’s Textbook of Quantitative Analysis’, Pearson Education, 2006.