![Orthophosphate (POOrthophosphate (PO44
3-3-
) is determined by) is determined by
weighing as ammonium phosphomolybdateweighing as ammonium phosphomolybdate
(NH(NH44)PO)PO44.12MoO.12MoO33. Calculate the percent P. Calculate the percent P
and the percent Pand the percent P22OO55 if 2.1771g precipitateif 2.1771g precipitate
(ppt) were obtained from a 0.3588g sample.(ppt) were obtained from a 0.3588g sample.
[F wt P = 30.97], [F wt P.molybdate = 1876.5],[F wt P = 30.97], [F wt P.molybdate = 1876.5],
[F wt P[F wt P22OO55 = 141.95]= 141.95]](https://image.slidesharecdn.com/chemmaths1-150629222436-lva1-app6892/85/Chem-maths1-27-320.jpg)
Chem maths1
- 2. IntroductionIntroduction The term gravimetric pertains to aThe term gravimetric pertains to a WeightWeight Measurement.Measurement. Gravimetric method is one in which the analysisGravimetric method is one in which the analysis is completed by a weighing operation.is completed by a weighing operation. Gravimetric Analysis is a group of analyticalGravimetric Analysis is a group of analytical methods in whichmethods in which the amount of analyte isthe amount of analyte is determined by the measurement of the mass ofdetermined by the measurement of the mass of a pure substance containing the analyte.a pure substance containing the analyte. Gravimetric Methods can also be defined asGravimetric Methods can also be defined as quantitative methodsquantitative methods based on the determiningbased on the determining the mass of a pure compound to which thethe mass of a pure compound to which the analyte is chemically related.analyte is chemically related. Analyte = constituents to be determined
- 3. There are two main types of gravimetric analyses A) Precipitation B) Volatilization 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. 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.
- 4. Example for Precipitation:-Example for Precipitation:- Calcium can be determined gravimetrically byCalcium can be determined gravimetrically by precipitation of calcium oxalate and ignition ofprecipitation of calcium oxalate and ignition of the oxalate ion to calcium oxide.the oxalate ion to calcium oxide. CaCa2+2+ + C+ C22OO44 2-2- →CaC→CaC22OO44 CaCCaC22OO44 → CaO + CO→ CaO + CO22 + CO+ CO The precipitate thus obtained are weighed andThe precipitate thus obtained are weighed and the mass of calcium oxide is determined.the mass of calcium oxide is determined.
- 6. Example for Volatilization:-Example for Volatilization:- The analyte or its decomposition products are volatilised atThe analyte or its decomposition products are volatilised at a suitable temperature. The volatile product is thena suitable temperature. The volatile product is then collected and weighed, i.e. the mass of the product iscollected and weighed, i.e. the mass of the product is indirectly determined from the loss in mass of the sample.indirectly determined from the loss in mass of the sample. ExampleExample Water can be separated from most inorganic compounds byWater can be separated from most inorganic compounds by ignition, the evolved water can then be absorbed on anyignition, the evolved water can then be absorbed on any one of several solid desiccants. The weight of water evolvedone of several solid desiccants. The weight of water evolved may be calculated from the gain in weight of the absorbent.may be calculated from the gain in weight of the absorbent.
- 7. Gravimetric AnalysisGravimetric Analysis Gravimetric analysis is potentially more accurateGravimetric analysis is potentially more accurate and more precise than volumetric analysis.and more precise than volumetric analysis. Gravimetric analysis avoids problems withGravimetric analysis avoids problems with temperature fluctuations, calibration errors,temperature fluctuations, calibration errors, and other problems associated withand other problems associated with volumetric analysis.volumetric analysis. But there are potential problems withBut there are potential problems with gravimetric analysis that must be avoided togravimetric analysis that must be avoided to get good results.get good results. Proper lab technique is criticalProper lab technique is critical
- 8. Steps in a Gravimetric AnalysisSteps in a Gravimetric Analysis 1. Preparation of the solution1. Preparation of the solution 2. Precipitation2. Precipitation 3. Digestion3. Digestion 4. Filtration4. Filtration 5. Washing5. Washing 6. Drying or ignition6. Drying or ignition 7. Weighing7. Weighing 8. Calculation8. Calculation
- 9. 1.1. Preparation of analyte solutionPreparation of analyte solution Gravimetric analysis usually involvesGravimetric analysis usually involves precipitation of analyte from solutionprecipitation of analyte from solution.. 11stst step – prepare the analyte solutionstep – prepare the analyte solution May need preliminary separation to separateMay need preliminary separation to separate potential interferences before precipitatingpotential interferences before precipitating analyteanalyte May need adjustment of solution condition toMay need adjustment of solution condition to maintain low solubility of precipitatemaintain low solubility of precipitate May require pH/temp/volume (of sol.) and/orMay require pH/temp/volume (of sol.) and/or other adjustments to maximize precipitateother adjustments to maximize precipitate formation. Eg Calcium oxalate insoluble in basicformation. Eg Calcium oxalate insoluble in basic mediummedium
- 10. 2. Precipitation2. Precipitation The precipitating reagent is added at aThe precipitating reagent is added at a concentration that favors the formation of a "good"concentration that favors the formation of a "good" precipitate.precipitate. This may require low concentration, extensiveThis may require low concentration, extensive heating (often described as "digestion"), or carefulheating (often described as "digestion"), or careful control of the pH.control of the pH. The precipitate shouldThe precipitate should Be sufficiently insolubleBe sufficiently insoluble Have large crystals (Easier to filter largeHave large crystals (Easier to filter large crystals)crystals) Be free of contaminantsBe free of contaminants
- 11. Precipitation process :Precipitation process : When solution of precipitating agent (AgNOWhen solution of precipitating agent (AgNO33)) added into testing solution (chloride solution) toadded into testing solution (chloride solution) to form AgCl precipitate,form AgCl precipitate, SupersaturationSupersaturation : the solution phase contains: the solution phase contains more dissolved salt than at equilibrium. Themore dissolved salt than at equilibrium. The driving force will be for the system to approachdriving force will be for the system to approach equilibrium (saturation).equilibrium (saturation). - Nucleation- Nucleation : initial phase of precipitation. A min: initial phase of precipitation. A min number of particle precipitate will gathernumber of particle precipitate will gather together to form a nucleus of particle precipitatetogether to form a nucleus of particle precipitate (solid phase). Higher degree of supersaturation,(solid phase). Higher degree of supersaturation, the greater rate of nucleationthe greater rate of nucleation Ag+ Cl- Ag+ Cl-Ag+Cl- Ag+ Cl-Ag+Cl-Ag+Cl- Ag+Cl-Ag+Cl-Ag+ Cl-
- 12. nucleation involves the formation of ion pairs andnucleation involves the formation of ion pairs and finally a group of ions formed.finally a group of ions formed. -- Particle growthParticle growth : particle enlargement process.: particle enlargement process. Nucleus will grow and forming a crystal of aNucleus will grow and forming a crystal of a specific geometric shape.specific geometric shape. Von weimarn discover – the particle size ofVon weimarn discover – the particle size of precipitates is inversely proportional to theprecipitates is inversely proportional to the relative supersaturation of the sol. during therelative supersaturation of the sol. during the precipitation process.precipitation process.
- 13. 3. Digestion of the Precipitate3. Digestion of the Precipitate Let precipitate stand in contact with motherLet precipitate stand in contact with mother liquor (the solution from which it wasliquor (the solution from which it was precipitated), usually at high temp.precipitated), usually at high temp. This process is calledThis process is called digestiondigestion, or, or OstwaldOstwald ripeningripening. The small particles tend to dissolve. The small particles tend to dissolve and precipitate on the surfaces of the largerand precipitate on the surfaces of the larger crystalscrystals Large crystal (small surface area) have lowerLarge crystal (small surface area) have lower free energy than small crystal (large surfacefree energy than small crystal (large surface area)area) Digestion make larger crystals, reduce surfaceDigestion make larger crystals, reduce surface contamination, reduce crystals imperfectioncontamination, reduce crystals imperfection
- 15. 4. Filtration4. Filtration Sintered glass crucibles are used to filter theSintered glass crucibles are used to filter the precipitates.precipitates. The crucibles first cleaned thoroughly and thenThe crucibles first cleaned thoroughly and then subjected to the same regimen of heating andsubjected to the same regimen of heating and cooling as that required for the precipitate.cooling as that required for the precipitate. This process is repeated until constant massThis process is repeated until constant mass has been achieved, that is, until consecutivehas been achieved, that is, until consecutive weighings differ by 0.3 mg or less.weighings differ by 0.3 mg or less. (you may see the proper technique of filtration in(you may see the proper technique of filtration in chapter 2)chapter 2)
- 17. 5. Washing5. Washing Coprecipitated impurities esp those onCoprecipitated impurities esp those on surface can be removed by washing thesurface can be removed by washing the precipitateprecipitate Wet precipitate with mother liquor andWet precipitate with mother liquor and which will also be remove by washingwhich will also be remove by washing Need to add electrolyte to the wash liquidNeed to add electrolyte to the wash liquid bcoz some precipitate cannot be washbcoz some precipitate cannot be wash with pure water, peptization occur.with pure water, peptization occur. Eg HNOEg HNO33 for AgCl precipitatefor AgCl precipitate
- 18. 6) Drying or ignition6) Drying or ignition To remove solvent and wash electrolytesTo remove solvent and wash electrolytes Done by heating at 110 to 120°C for 1 to 2Done by heating at 110 to 120°C for 1 to 2 hrs.hrs. Converts hygroscopic compound to non-Converts hygroscopic compound to non- hygroscopic compoundhygroscopic compound May used high temp if precipitate must beMay used high temp if precipitate must be converted to a more suitable form beforeconverted to a more suitable form before weighingweighing Eg MgNHEg MgNH44POPO44 convert to pyrophosphateconvert to pyrophosphate MgMg22PP22OO77 by heating at 900°C.by heating at 900°C.
- 19. 7) Weighing7) Weighing After the precipitate is allowed to coolAfter the precipitate is allowed to cool (preferably in a desiccator to keep it from(preferably in a desiccator to keep it from absorbing moisture), it is weighed (in theabsorbing moisture), it is weighed (in the crucible).crucible). Properly calibrated analytical balanceProperly calibrated analytical balance Good weighing techniqueGood weighing technique
- 20. 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 Analysis: WeightGravimetric Analysis: Weight RelationshipRelationship
- 21. 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
- 22. 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
- 23. 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 1 107.8 2 = 0.32g
- 24. percent composition by weight of the analytepercent composition by weight of the analyte in the sample :in the sample : % A = g% A = gAA x 100%x 100% ggsamplesample ggAA – grams of analyte (the desired test substance)– grams of analyte (the desired test substance) ggsamplesample – grams of sample taken for analysis– grams of sample taken for analysis
- 25. 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.
- 26. g P = 0.2161 g x 30.974 x 2g P = 0.2161 g x 30.974 x 2 222.57 1222.57 1 = 0.0601g= 0.0601g % A = g% A = gAA x 100%x 100% ggsamplesample = 0.0601 g x 100%= 0.0601 g x 100% 0.3516 g0.3516 g = 0.1709 %= 0.1709 % GF = f wt analyte (g/mol) x a (mol analyte/mol precipitate) f wt precipitate (g/mol) b = g analyte / g precipitate
- 27. Orthophosphate (POOrthophosphate (PO44 3-3- ) is determined by) is determined by weighing as ammonium phosphomolybdateweighing as ammonium phosphomolybdate (NH(NH44)PO)PO44.12MoO.12MoO33. Calculate the percent P. Calculate the percent P and the percent Pand the percent P22OO55 if 2.1771g precipitateif 2.1771g precipitate (ppt) were obtained from a 0.3588g sample.(ppt) were obtained from a 0.3588g sample. [F wt P = 30.97], [F wt P.molybdate = 1876.5],[F wt P = 30.97], [F wt P.molybdate = 1876.5], [F wt P[F wt P22OO55 = 141.95]= 141.95]
- 28. g P = 2.1771g x 30.97 x 1 molg P = 2.1771g x 30.97 x 1 mol 1876.5 1 mol1876.5 1 mol = 0.0359g= 0.0359g % P = 0.0359g x 100%% P = 0.0359g x 100% 0.3588g0.3588g == 10.01 %10.01 %
- 29. g Pg P22OO55 = 2.1771 g x 141.95 x 1 mol= 2.1771 g x 141.95 x 1 mol 1876.5 2 mol1876.5 2 mol = 0.0823g= 0.0823g % P% P22OO55 = 0.0823g x 100%= 0.0823g x 100% 0.3588g0.3588g == 22.94%22.94%