Gravimetric analyses

Gravimetric analysis is a type of quantitative analysis in which the amount of a species in a material is determined by converting the species to a product that can be isolated completely and weighed. Precipitation reactions are frequently used in gravimetric analyses. In these reactions, you determine the amount of an ionic species by precipitating it from solution. The precipitate, or solid formed in the reaction, is 

As an example of a gravimetric analysis, consider the problem of determining the amount of lead in a sample of drinking water. Lead, if it occurs in the water, probably exists as the lead(II) ion, Pb. Lead(II) sulfate is a very insoluble compound of lead(II) ion. When sodium sulfate, Na2S04, is added to a solution containing Pb , lead(II) sulfate precipitates (that is, PbS04 comes out of the solution as a fine, crystalline solid). If you assume that the lead is present in solution as lead(ll) nitrate, you can write the following equation for the reaction  

OOO-L sample of polluted water was analyzed for lead(ll) ion, Pb, by adding an excess of sodium sulfate to it. The mass of lead(ll) sulfate that precipitated was 229.8 mg. What is the mass of lead in a liter of the water Give the answer as milligrams of lead pCT liter of solution. 

All of the lead in the water solution is precipitated as lead(ll) sulfate, PbS04. If you determine the percentage of lead in PbS04, you can calculate the quantity of lead in the water sample. 

Following Example 3.7, you obtain the mass percentage of Pb in PbS04 by dividing the molar mass of Pb by the molar mass of PbS04, then multiplying by 100%  

Each of the alkaline-earth metals can be determined gravimetrically by at least one method but a specific precipitant is used for each. In the three examples described below, the conditions required in each case are described. 

FIGURE 18.4 Filter crucible used to collect and weigh precipitates for gravimetric analysis. 

The versatile nature of organic chemistry led to a number of organic reagents used to form precipitates with analytes, especially metals. One widely used reagent was 8-hydroxyquinoline, which is shown forming a precipitate with zinc ion in the following reaction  

Certain aqueous reactions are useful for determining how much of a particular substance is present in a sample. For example, if we want to know the concentration of lead in a sample of water, or if we need to know the concentration of an acid, knowledge of precipitation reactions, acid-base reactions, and solution stoichiometry will be useful. Two common types of such quantitative analyses are gravimetric analysis and acid-base titration. 

Gravimetric analysis is an analytical technique based on the measurement of mass. One type of gravimetric analysis experiment involves the formation and isolation of a precipitate, such as AgCl(s)  

This reaction is often used in gravimetric analysis because the reactants can be obtained in pure form. The net ionic equation is 

Gravimetric analysis is a highly accurate technique, because the mass of a sample can be measured accurately. However, this procedure is applicable only to reactions that go to completion or have nearly 100 percent yield. In addition, if AgCl were soluble to any significant degree, it would not be possible to remove all the Cl ions from the original solution, and the subsequent calciilation would be in error. Sample Problem 4.12 shows the calculations involved in a gravimetric experiment. 

8633-g sample of an ionic compound containing chloride ions and an unknown metal cation is dissolved in water and treated with an excess of AgNO,. If 1.5615 g of AgQ precipitate forms, what is the percent by mass of Cl in the original compound  

Think About It Pay close attention to which numbers correspond to which quantities. 

we must dilute 102 mL of the 8.61 M H2SO4 solution with sufficient water to give a final volume of 5.00 X 10 mL in a 500-mL volumetric flask to obtain the desired concentration. 

Check The initial volume is less than the final volume, so the answer is reasonable. 

Practice Exercise How would you prepare 2.00 X 10 mL of a 0.866 M NaOH solution, starting with a 5.07 M stock solution  

What is the final concentration of a 0.6 M NaCl solution if its volume is doubled and the number of moles of solute is tripled  

Now that we have discussed the concentration and dilution of solutions, we can examine the quantitative aspects of reactions in aqueous solution, or solution stoichiometry. Sections 4.6-4.S focus on two techniques for studying solution stoichiometry gravimetric analysis and titration. These techniques are important tools of quantitative analysis, which is the determination of the amount or concentration of a substance in a sample. 

A rough measure of the yield of polymerisation is obtained by weighing the dry polymer after removal of unreacted monomer, solvent and template by continuous extraction and comparing the weight to the total weight of added monomer [13]. 

---

Total sulfur NF T 60-109 ASTM D 129 Oxidation in pressurized bomb and gravimetric analysis... 

Many precipitation reactions that are useful as separation techniques for gravimetric analysis fail to meet one or both of two requirements for titrimetry ... 

Analytical scheme outlined by Fresenius for the gravimetric analysis of Ni in ores. 

Analytical scheme outlined by Hillebrand and Lundelh for the gravimetric analysis of Ni in ores (DMG = dimethylgloxime). The factor of 0.2031 in the equation for %Ni accounts for the difference in the formula weights of Ni(DMG)2 and Ni see Chapter 8 for more details. 

Snow, N. H. Dunn, M. Patel, S. Determination of Crude Pat in Pood Products by Supercritical Pluid Extraction and Gravimetric Analysis, /. Chem. Educ. 1997, 74, 1108-1111. 

A precipitation gravimetric analysis must have several important attributes. Eirst, the precipitate must be of low solubility, high purity, and of known composition if its mass is to accurately reflect the analyte s mass. Second, the precipitate must be in a form that is easy to separate from the reaction mixture. The theoretical and experimental details of precipitation gravimetry are reviewed in this section. 

In some situations the rate at which a precipitate forms can be used to separate an analyte from a potential interferent. For example, due to similarities in their chemistry, a gravimetric analysis for Ca + may be adversely affected by the presence of Mg +. Precipitates of Ca(01T)2, however, form more rapidly than precipitates of Mg(01T)2. If Ca(01T)2 is filtered before Mg(01T)2 begins to precipitate, then a quantitative analysis for Ca + is feasible. 

Quantitative Calculations When needed, the relationship between the analyte and the 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 gravimetric analysis is the determination of a compound s elemental composition. 

In an indirect volatilization gravimetric analysis, the change in the sample s weight is proportional to the amount of analyte. Note that in the following example it is not necessary to apply the conservation of mass to relate the analytical signal to the analyte. 

In a gravimetric analysis a measurement of mass or change in mass provides quantitative information about the amount of analyte in a sample. The most common form of gravimetry uses a precipitation reaction to generate a product whose mass is proportional to the analyte. In many cases the precipitate includes the analyte however, an indirect analysis in which the analyte causes the precipitation of another compound also is possible. Precipitation gravimetric procedures must be carefully controlled to produce precipitates that are easily filterable, free from impurities, and of known stoichiometry. 

When the analyte is already present in a particulate form that is easily separated from its matrix, then a particulate gravimetric analysis may be feasible. Examples include the determination of dissolved solids and the determination of fat in foods. 

Carmosini, N. Ghoreshy, S. Koether, M. C. The Gravimetric Analysis of Nickel Using a Microwave Oven, ... 

The %w/w fat in candy bars is determined by an indirect particulate gravimetric analysis. Supercritical GO2 is used to extract the fat from the sample, and the change in the sample s weight is used to determine the fat content. 

Determine the uncertainty for the gravimetric analysis described in Example 8.1. (a) How does your result compare with the expected accuracy of 0.1-0.2% for precipitation gravimetry (b) What sources of error might account for any discrepancy between the most probable measurement error and the expected accuracy ... 

Erdey, L. Gravimetric Analysis, Pergamon Oxford, 1965. Steymark, A. Quantitative Organic Microanalysis, The Blakiston Go. New York, 1951. 

This experiment describes the use of a fractional factorial design to examine the effects of volume of HNO3, molarity of AgN03, volume of AgN03, digestion temperature, and composition of wash water on the gravimetric analysis for chloride. 

Tetraphenylarsonium chloride (107,108) has also been used for the precipitation of the perchlorate ion in gravimetric analysis. 

Analysis. Excellent reviews of phosphate analysis are available (28). SoHds characterization methods such as x-ray powder diffraction (xrd) and thermal gravimetric analysis (tga) are used for the identification of individual crystalline phosphates, either alone or in mixtures. These techniques, along with elemental analysis and phosphate species deterrnination, are used to identify unknown phosphates and their mixtures. Particle size analysis, surface area, microscopy, and other standard soHds characterizations are useful in relating soHds properties to performance. SoHd-state nmr is used with increasing frequency. 

Another step in laboratory automation to be achieved is the conversion of standard chemical procedures such as titrations or thermal gravimetric analysis, into unit laboratory operations. A procedure could then be selected from these laboratory operations by an expert system and translated by the system to produce a set of iastmctions for a robot. The robot should be able to obey specific iastmctions, such as taking a specified sample aliquot and titrating it using a specified reagent. 

Thermal Properties. Spider dragline silk was thermally stable to about 230°C based on thermal gravimetric analysis (tga) (33). Two thermal transitions were observed by dynamic mechanical analysis (dma), one at —75° C, presumed to represent localized mobiUty in the noncrystalline regions of the silk fiber, and the other at 210°C, indicative of a partial melt or a glass transition. Data from thermal studies on B. mori silkworm cocoon silk indicate a glass-transition temperature, T, of 175°C and stability to around 250°C (37). The T for wild silkworm cocoon silks were slightly higher, from 160 to 210°C. 

The heavy metal salts, ia contrast to the alkah metal salts, have lower melting points and are more soluble ia organic solvents, eg, methylene chloride, chloroform, tetrahydrofiiran, and benzene. They are slightly soluble ia water, alcohol, ahphatic hydrocarbons, and ethyl ether (18). Their thermal decompositions have been extensively studied by dta and tga (thermal gravimetric analysis) methods. They decompose to the metal sulfides and gaseous products, which are primarily carbonyl sulfide and carbon disulfide ia varying ratios. In some cases, the dialkyl xanthate forms. Solvent extraction studies of a large number of elements as their xanthate salts have been reported (19). 

Determination of the thermal decomposition temperature by thermal gravimetric analysis (tga) defines the upper limits of processing. The tga for cellulose triacetate is shown in Figure 11. Comparing the melt temperature (289°C) from the dsc in Figure 10 to the onset of decomposition in Figure 11 defines the processing temperature window at which the material can successfully be melt extmded or blended. 

Imperforate Bowl Tests The amount of supernant hquid from spin tubes is usually too small to warrant accurate gravimetric analysis. A fixed amount of shiny is introduced at a controlled rate into a rotating imperforate bowl to simulate a continuous sedimentation centrifuge. The liquid is collected as it overflows the ring weir. The test is stopped when the solids in the bowl build up to a thickness which affects centrate quality. The solid concentration of the centrate is determined similarly to that of the spin tube. 

---