D. Harvey - Modern Analytical Chemistry (794078), страница 77
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Thermal decomposition or combustion is accomplishedusing a Bunsen or Meker burner, a laboratory oven or a muffle furnace, with thevolatile products vented to the atmosphere. The weight of the sample and the solidresidue are determined using an analytical balance.Constant-temperature decomposition or combustion, followed by trappingand weighing the volatilized gases, requires more specialized equipment. Decomposition of the sample is conducted in a closed container, and the volatilizedgases are carried by a purge-gas stream through one or more selective absorbenttraps.In a thermogravimetric analysis, the sample is placed in a small weighingboat attached to one arm of a specially designed electromagnetic balance andplaced inside an electric furnace.
The temperature of the electric furnace isslowly increased at a fixed rate of a few degrees per minute, and the sample’sweight is monitored.Representative Method Although each volatilization gravimetric procedure has itsown unique characteristics, the following indirect method for the determination ofSi in ores and alloys by formation of volatile SiF4 provides an instructive example ofa typical procedure.2571400-CH08 9/9/99 2:18 PM Page 258Modern Analytical ChemistryRepresentative Methods258Method 8.2Determination of Si in Ores and Alloys9Description of Method.
Silicon is determined by dissolving the sample in acid.Dehydration of the resulting solution precipitates silicon as SiO2. Because a variety ofother insoluble oxides also form, the precipitate’s mass does not provide a directmeasure of the amount of silicon in the sample. Treating the solid residue with HFresults in the formation of volatile SiF4. The decrease in mass following the loss ofSiF4 provides an indirect measure of the amount of silicon in the original sample.Procedure.
Transfer a sample of between 0.5 and 5 g to a platinum crucible alongwith an excess of Na2CO3, and heat until a melt is formed. After cooling, dissolve theresidue in dilute HCl. Dehydrating silicon to SiO2 is accomplished by evaporating thesolution to dryness on a steam bath and heating the residue for 1 hour at 110 °C.Moisten the residue with HCl, and repeat the dehydration. Remove any acid-solublematerials from the residue by adding 50 mL of water and 5 mL of concentrated HCl.Bring to a boil, and filter through #40 filter paper. Wash the residue with hot 2% v/vHCl followed by hot water.
Evaporate the filtrate to dryness twice, and, followingthe same procedure, treat to remove any acid–soluble materials. Combine the twoprecipitates, and dry and ignite to a constant weight at 1200 °C. After cooling, add 2drops of 50% v/v H2SO4 and 10 mL of HF. Remove the volatile SiF4 by evaporating todryness on a hot plate.
Finally, bring the residue to constant weight by igniting at1200 °C.Questions1. According to the procedure, the sample should weigh between 0.5 and 5 g. Onwhat basis should a decision on the amount of sample be made?In this procedure the critical measurement is the decrease in mass following thevolatilization of SiF4. The reaction that occurs isSiO2(s) + 4HF(aq) → SiF4(g) + 2H2O(l)The water and any excess HF are removed during the final ignition and do notcontribute to the change in mass. The loss in weight, therefore, is equal to thegrams of SiO2 present after the dehydration step. For every 0.1 g of Si in theoriginal sample, a weight loss of 0.21 g is expected.
The amount of sample usedis determined by how much Si is present. If a sample is 50% w/w Si, a 0.5-gsample will give a respectable weight loss of 0.53 g. A 0.5-g sample that is only5% w/w Si, however, will give a weight loss of only 0.053 g. In this case, a largersample is needed.2. Why are acid-soluble materials removed before the dehydrated residue istreated with HF?Any acid-soluble materials present in the sample will react with HF or H2SO4. Ifthe products of these reactions are volatile or decompose at the ignitiontemperature of 1200 °C, then the change in weight will not be due solely to thevolatilization of SiF4. The result is a positive determinate error.3. Why is H2SO4 added with the HF?Many samples containing silicon also contain aluminum and iron. Afterdehydration, these metals are present as Al2O3 and Fe2O3.
These oxides arepotential interferents since they also are capable of forming volatile fluorides.In the presence of H2SO4, however, aluminum and iron form nonvolatilesulfates. These sulfates decompose back to their respective oxides when ignitedto 1200 °C. As a result, the change in weight after treating with HF and H2SO4 isdue only to the loss of SiF4.1400-CH08 9/9/99 2:18 PM Page 259Chapter 8 Gravimetric Methods of Analysis8C.2 Quantitative ApplicationsUnlike precipitation gravimetry, which is rarely used as a standard method of analysis, gravimetric methods based on volatilization reactions continue to play an important role in chemical analysis.
Several important examples are discussed in thefollowing sections.Inorganic Analysis Determining the inorganic ash content of organic materials,such as polymers and paper, is an example of a direct volatilization gravimetricanalysis. The sample is weighed, placed in an appropriate crucible, and the organicmaterial is carefully removed by combustion. The crucible containing the residue isthen heated to a constant weight using either a burner or an oven.Another example of volatilization gravimetry is the determination of dissolvedsolids in water and wastewater.
In this method a sample of the water is transferredto a weighed dish and dried to a constant weight at either 103–105 °C, or at 180 °C.Samples dried at the lower temperature retain some occluded water and lose somecarbonate as CO2. The loss of organic material, however, is minimal.
At the highertemperature, the residue is free from occluded water, but losses of carbonate aregreater. In addition, some chloride, nitrate, and organic material are lost throughthermal decomposition. The residue remaining after drying at either temperaturecan be ignited to constant weight at 500 °C. The loss in weight on ignition providesan indirect measure of the amount of volatile solids in the sample, and the weight ofthe remaining residue gives the amount of fixed solids.Indirect analyses based on the weight of the residue remaining after volatilization are commonly used in determining moisture in a variety of products and in determining silica in water, wastewater, and rocks.
Moisture is determined by drying apreweighed sample with an infrared lamp or in a low-temperature oven. The difference between the original weight and the weight after drying equals the mass ofwater lost.The determination of silicon is commonly encountered in metallurgical andmining laboratories responsible for the analysis of ores, slags, and alloys. Thevolatilization gravimetric method, which is appropriate for samples containing highconcentrations of silicon, was described earlier in Method 8.2.As a final example, the determination of carbon in steels and other metal alloyscan be determined by heating the sample.
The carbon is converted to CO2, which iscollected in an appropriate absorbent trap, providing a direct measure of theamount of C in the original sample.Organic Analysis The most important application of volatilization gravimetry tothe analysis of organic materials is an elemental analysis. When burned in a streamof pure O2, many elements, such as carbon and hydrogen, are released as gaseouscombustion products, such as CO2 and H2O. The combustion products are passedthrough preweighed tubes containing appropriate absorbents.
The increase in themass of these tubes provides a direct indication of the mass percent of carbon andhydrogen in the organic material.Alkaline metals and earths in organic materials can be determined by addingH2SO4 to the sample before combustion. Following combustion, the metal remainsbehind as a solid residue of metal sulfate. Silver, gold, and platinum can be determined by burning the organic sample, leaving a metallic residue of Ag, Au, or Pt.Other metals are determined by adding HNO3 before combustion, leaving a residueof the metal oxide.2591400-CH08 9/9/99 2:18 PM Page 260260Modern Analytical ChemistryVolatilization gravimetry is also used to determine biomass in water and wastewater.
Biomass is a water quality index, providing an indication of the total mass oforganisms contained within a sample of water. A known volume of the sample ispassed through a preweighed 0.45-µm membrane filter or a glass-fiber filter anddried at 105 °C for 24 h. The residue’s mass provides a direct measure of biomass. Ifsamples are known to contain a substantial amount of dissolved inorganic solids,the residue can be ignited at 500 °C for 1 h, thereby removing all organic materials.The resulting residue is wetted with distilled water to rehydrate any clay mineralsand dried to a constant weight at 105 °C. The difference in weight before and afterignition provides an indirect measure of biomass.Quantitative Calculations When needed, the relationship between the analyte andthe analytical signal is given by the stoichiometry of any relevant reactions.
Calculations are simplified, however, by applying the principle of conservation of mass.The most frequently encountered example of a direct volatilization gravimetricanalysis is the determination of a compound’s elemental composition.EXAMPLE 8.5A 101.3-mg sample of an organic compound known to contain Cl is burned inpure O2 and the combustion gases collected in absorbent tubes. The tube usedto trap CO2 increases in mass by 167.6 mg, and the tube for trapping H2Oshows a 13.7-mg increase. A second sample of 121.8 mg is treated withconcentrated HNO 3 producing Cl 2 , which subsequently reacts with Ag + ,forming 262.7 mg of AgCl.
Determine the compound’s composition, as well asits empirical formula.SOLUTIONApplying the principle of conservation of mass to carbon, we writeMoles C = moles CO2Converting from moles to grams, and rearranging to solve for milligrams ofcarbon givesg CO2 × AW C × 1000 mg/gFW CO2=0.1676 g × 12.011 g/mol × 1000 mg/g44.011 g/mol= 45.74 mg CThus, the %w/w C in the sample ismg C45.74 mg× 100 =× 100 = 45.15% w/w Cmg sample101.3 mgThe calculation is then repeated for hydrogenMoles H = 2 × moles H2O2 × g H2O × AW H × 1000 mg/g2 × 0.0137 g × 1.008 g/mol × 1000 mg/g=FW H2O18.015 g/mol= 1.533 mg Hmg H1.533 mg× 100 =× 100 = 1.51% w/w Hmg sample101.3 mg1400-CH08 9/9/99 2:18 PM Page 261Chapter 8 Gravimetric Methods of Analysisand for chlorineMoles Cl = moles AgClg AgCl × AW Cl × 1000 mg/g0.2627 g × 35.453 g/mol × 1000 mg/g=FW AgCl143.32 g/mol= 64.98 mg Clmg Cl64.98 mg× 100 =× 100 = 53.35% w/w Clmg sample121.8 mgAdding together the weight percents for C, H, and Cl gives a total of 100.01%.The compound, therefore, contains only these three elements.
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