D. Harvey - Modern Analytical Chemistry (794078), страница 76
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The stoichiometry of the redox reactionin which Na3PO3 is a reactant and Hg2Cl2 is a product was given earlier in thechapter. It also can be determined by using the conservation of electrons.Phosphorus has an oxidation state of +3 in PO33– and +5 in PO43–; thus,oxidizing PO33– to PO43– requires two electrons. The formation of Hg2Cl2 byreduction of HgCl2 requires 2 electrons as the oxidation state of each mercurychanges from +2 to +1. Since the oxidation of PO33– and the formation ofHg2Cl2 both require two electrons, we haveMoles Na3PO3 = moles Hg2Cl2Converting from moles to grams leaves us withg Na3 PO3g Hg 2 Cl 2=FW Na3 PO3FW Hg 2 Cl 22531400-CH08 9/9/99 2:18 PM Page 254254Modern Analytical Chemistrywhich can be solved for g Na3PO3 and %w/w Na3PO3 in the sample.g Na3 PO3 =g Hg 2 Cl2 × FW Na3 PO3FW Hg 2 Cl2=0.4320 g × 147.94 g /mol472.09 g /mol= 0.1354 gg Na3 PO30.1354 g× 100 =× 100 = 97.27% w/w Na3 PO3g sample0.1392 g8B.3 Qualitative ApplicationsPrecipitation gravimetry can also be applied to the identification of inorganic andorganic analytes, using precipitants such as those outlined in Tables 8.1, 8.2, 8.4,and 8.5.
Since this does not require quantitative measurements, the analytical signalis simply the observation that a precipitate has formed. Although qualitative applications of precipitation gravimetry have been largely replaced by spectroscopicmethods of analysis, they continue to find application in spot testing for the presence of specific analytes.88B.4 Evaluating Precipitation GravimetryScale of Operation The scale of operation for precipitation gravimetry is governedby the sensitivity of the balance and the availability of sample.
To achieve an accuracy of ±0.1% using an analytical balance with a sensitivity of ±0.1 mg, the precipitate must weigh at least 100 mg. As a consequence, precipitation gravimetry is usually limited to major or minor analytes, and macro or meso samples (see Figure 3.6in Chapter 3). The analysis of trace level analytes or micro samples usually requiresa microanalytical balance.Accuracy For macro–major samples, relative errors of 0.1–0.2% are routinelyachieved. The principal limitations are solubility losses, impurities in the precipitate,and the loss of precipitate during handling. When it is difficult to obtain a precipitatefree from impurities, an empirical relationship between the precipitate’s mass andthe mass of the analyte can be determined by an appropriate standardization.Precision The relative precision of precipitation gravimetry depends on theamount of sample and precipitate involved.
For smaller amounts of sample orprecipitate, relative precisions of 1–2 ppt are routinely obtained. When workingwith larger amounts of sample or precipitate, the relative precision can be extended to several parts per million. Few quantitative techniques can achieve thislevel of precision.Sensitivity For any precipitation gravimetric method, we can write the followinggeneral equation relating the signal (grams of precipitate) to the absolute amount ofanalyte in the sampleGrams precipitate = k × grams of analyte8.13where k, the method’s sensitivity, is determined by the stoichiometry between theprecipitate and the analyte.
Note that equation 8.13 assumes that a blank has beenused to correct the signal for the reagent’s contribution to the precipitate’s mass.1400-CH08 9/9/99 2:18 PM Page 255Chapter 8 Gravimetric Methods of Analysis255Consider, for example, the determination of Fe as Fe2O3. Using a conservationof mass for Fe we write2 × moles Fe2O3 = moles FeConverting moles to grams and rearranging yields an equation in the form of 8.13g Fe2 O3 =1 FW Fe2 O3×× g Fe2AW Fewhere k is equal tok =1 FW Fe2 O3×2AW Fe8.14As can be seen from equation 8.14, we may improve a method’s sensitivity in twoways. The most obvious way is to increase the ratio of the precipitate’s molar massto that of the analyte.
In other words, it is desirable to form a precipitate with aslarge a formula weight as possible. A less obvious way to improve the calibrationsensitivity is indicated by the term of 1/2 in equation 8.14, which accounts for thestoichiometry between the analyte and precipitate. Sensitivity also may be improvedby forming precipitates containing fewer units of the analyte.Selectivity Due to the chemical nature of the precipitation process, precipitantsare usually not selective for a single analyte. For example, silver is not a selectiveprecipitant for chloride because it also forms precipitates with bromide and iodide.Consequently, interferents are often a serious problem that must be considered ifaccurate results are to be obtained.Time, Cost, and Equipment Precipitation gravimetric procedures are time-intensiveand rarely practical when analyzing a large number of samples.
However, since muchof the time invested in precipitation gravimetry does not require an analyst’s immediate supervision, it may be a practical alternative when working with only a few samples. Equipment needs are few (beakers, filtering devices, ovens or burners, and balances), inexpensive, routinely available in most laboratories, and easy to maintain.8C Volatilization GravimetryA second approach to gravimetry is to thermally or chemically decompose a solidsample.
The volatile products of the decomposition reaction may be trapped andweighed to provide quantitative information. Alternatively, the residue remainingwhen decomposition is complete may be weighed. In thermogravimetry, which isone form of volatilization gravimetry, the sample’s mass is continuously monitoredwhile the applied temperature is slowly increased.8C.1 Theory and PracticeWhether the analysis is direct or indirect, volatilization gravimetry requires that theproducts of the decomposition reaction be known. This requirement is rarely aproblem for organic compounds for which volatilization is usually accomplished bycombustion and the products are gases such as CO2, H2O, and N2.
For inorganiccompounds, however, the identity of the volatilization products may depend on thetemperature at which the decomposition is conducted.thermogravimetryA form of volatilization gravimetry inwhich the change in a sample’s mass ismonitored while it is heated.1400-CH08 9/9/99 2:18 PM Page 256256Modern Analytical Chemistry25.00Mass (mg)20.0015.0010.005.000.00Figure 8.9⋅01002003005006007008009001000Temperature (°C)Thermogram for CaC2O4 H2O.thermogramA graph showing change in mass as afunction of applied temperature.400Thermogravimetry The products of a thermal decomposition can be deduced bymonitoring the sample’s mass as a function of applied temperature. (Figure 8.9).The loss of a volatile gas on thermal decomposition is indicated by a step in thethermogram. As shown in Example 8.4, the change in mass at each step in a thermogram can be used to identify both the volatilized species and the solid residue.EXAMPLE 8.4The thermogram in Figure 8.9 shows the change in mass for a sample ofcalcium oxalate monohydrate, CaC2O4 H2O.
The original sample weighed24.60 mg and was heated from room temperature to 1000 °C at a rate of 5 °Cmin. The following changes in mass and corresponding temperature rangeswere observed:⋅Loss of 3.03 mg from 100–250 °CLoss of 4.72 mg from 400–500 °CLoss of 7.41 mg from 700–850 °CDetermine the identities of the volatilization products and the solid residue ateach step of the thermal decomposition.SOLUTIONThe loss of 3.03 mg from 100–250 °C corresponds to a 12.32% decrease in theoriginal sample’s mass.3.03 mg× 100 = 12.32%24.60 mgIn terms of CaC2O4⋅ H O, this corresponds to a loss of 18.00 g/mol.20.1232 × 146.11 g/mol = 18.00 g/mol1400-CH08 9/9/99 2:18 PM Page 257Chapter 8 Gravimetric Methods of AnalysisThe product’s molar mass, coupled with the temperature range, suggests thatthis represents the loss of H2O.
The residue is CaC2O4.The loss of 4.72 mg from 400–500 °C represents a 19.19% decrease in theoriginal mass of 24.60 g, or a loss of0.1919 × 146.11 g/mol = 28.04 g/molThis loss is consistent with CO as the volatile product, leaving a residue ofCaCO3.Finally, the loss of 7.41 mg from 700–850 °C is a 30.12% decrease in theoriginal mass of 24.60 g. This is equivalent to a loss of0.3012 × 146.11 g/mol = 44.01 g/molsuggesting the loss of CO2. The final residue is CaO.Once the products of thermal decomposition have been determined, an analytical procedure can be developed. For example, the thermogram in Figure 8.9 showsthat a precipitate of CaC2O4 H2O must be heated at temperatures above 250 °C,but below 400 °C if it is to be isolated as CaC2O4.
Alternatively, by heating the sample to 1000 °C, the precipitate can be isolated as CaO. Knowing the identity of thevolatilization products also makes it possible to design an analytical method inwhich one or more of the gases are trapped. Thus, a sample of CaC2O4 H2O couldbe analyzed by heating to 1000 °C and passing the volatilized gases through a trapthat selectively retains H2O, CO, or CO2.⋅⋅Equipment Depending on the method, the equipment for volatilization gravimetry may be simple or complex. In the simplest experimental design, the weight of asolid residue is determined following either thermal decomposition at a fixed temperature or combustion.
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