Page 11 - Modern Analytical Chemistry

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x Modern Analytical Chemistry

A Guide to Using This Text

. . . in Chapter

Representative Methods
Annotated methods of typical 246 Modern Analytical Chemistry
analytical procedures link theory with An additional problem is encountered when the isolated solid is non-
stoichiometric. For example, precipitating Mn 2+ as Mn(OH) 2 , followed by heating
practice. The format encourages to produce the oxide, frequently produces a solid with a stoichiometry of MnO x ,
where x varies between 1 and 2. In this case the nonstoichiometric product results
students to think about the design of from the formation of a mixture of several oxides that differ in the oxidation state
of manganese. Other nonstoichiometric compounds form as a result of lattice de-
the procedure and why it works. fects in the crystal structure. 6
Representative Method The best way to appreciate the importance of the theoreti-
cal and practical details discussed in the previous section is to carefully examine the
procedure for a typical precipitation gravimetric method. Although each method
has its own unique considerations, the determination of Mg 2+ in water and waste-
water by precipitating MgNH 4 PO 4 ×6H 2 O and isolating Mg 2 P 2 O 7 provides an in-
structive example of a typical procedure.
Margin Notes
Representative Methods not very selective, so a preliminary separation of Mg 2+ from potential interferents is
Margin notes direct students Method 8.1 Determination of Mg 2+ in Water and Wastewater 7
to colorplates located toward Description of Method. Magnesium is precipitated as MgNH 4 PO 4 ×6H 2 O using
(NH 4 ) 2 HPO 4 as the precipitant. The precipitate’s solubility in neutral solutions
the middle of the book (0.0065 g/100 mL in pure water at 10 °C) is relatively high, but it is much less soluble
in the presence of dilute ammonia (0.0003 g/100 mL in 0.6 M NH 3 ). The precipitant is
necessary. Calcium, which is the most significant interferent, is usually removed by
its prior precipitation as the oxalate. The presence of excess ammonium salts from
Mg(NH 4 ) 4 (PO 4 ) 2 , which is subsequently isolated as Mg(PO 3 ) 2 after drying. The
precipitate is isolated by filtration using a rinse solution of dilute ammonia. After
110 Modern Analytical Chemistry the precipitant or the addition of too much ammonia can lead to the formation of
filtering, the precipitate is converted to Mg 2 P 2 O 7 and weighed.
either case, the calibration curve provides a means for relating S samp to the ana- Procedure. Transfer a sample containing no more than 60 mg of Mg 2+ into a
lyte’s concentration. 600-mL beaker. Add 2–3 drops of methyl red indicator, and, if necessary, adjust the
volume to 150 mL. Acidify the solution with 6 M HCl, and add 10 mL of 30% w/v
(NH 4 ) 2 HPO 4 . After cooling, add concentrated NH 3 dropwise, and while constantly
EXAMPLE 5.3 stirring, until the methyl red indicator turns yellow (pH > 6.3). After stirring for
5 min, add 5 mL of concentrated NH 3 , and continue stirring for an additional 10 min.
Color plate 1 shows an example of a set of A second spectrophotometric method for the quantitative determination of Allow the resulting solution and precipitate to stand overnight. Isolate the
external standards and their corresponding Pb 2+ levels in blood gives a linear normal calibration curve for which precipitate by filtration, rinsing with 5% v/v NH 3 . Dissolve the precipitate in 50 mL
normal calibration curve. of 10% v/v HCl, and precipitate a second time following the same procedure. After
S stand = (0.296 ppb –1 ) ´C S + 0.003 filtering, carefully remove the filter paper by charring. Heat the precipitate at 500 °C
What is the Pb 2+ level (in ppb) in a sample of blood if S samp is 0.397? until the residue is white, and then bring the precipitate to constant weight at
1100 °C.
SOLUTION
Questions
To determine the concentration of Pb 2+ in the sample of blood, we replace
1. Why does the procedure call for a sample containing no more than 60 mg of
S stand in the calibration equation with S samp and solve for C A
S samp –.003 . 0 397 – .003
0
0
C A = ppb –1 = . 0 296 ppb –1 = .133 ppb
. 0 296
It is worth noting that the calibration equation in this problem includes an
extra term that is not in equation 5.3. Ideally, we expect the calibration curve to q y
give a signal of zero when C S is zero. This is the purpose of using a reagent There is a serious limitation, however, to an external standardization. The
blank to correct the measured signal. The extra term of +0.003 in our relationship between S stand and C S in equation 5.3 is determined when the ana-
calibration equation results from uncertainty in measuring the signal for the lyte is present in the external standard’s matrix. In using an external standardiza-
reagent blank and the standards. tion, we assume that any difference between the matrix of the standards and the
sample’s matrix has no effect on the value of k. A proportional determinate error
is introduced when differences between the two matrices cannot be ignored. This
An external standardization allows a related series of samples to be analyzed is shown in Figure 5.4, where the relationship between the signal and the amount
using a single calibration curve. This is an important advantage in laboratories of analyte is shown for both the sample’s matrix and the standard’s matrix. In
where many samples are to be analyzed or when the need for a rapid throughput of this example, using a normal calibration curve results in a negative determinate
l i iti l t i i l f th t l t d error. When matrix problems are expected, an effort is made to match the matrix
of the standards to that of the sample. This is known as matrix matching. When
Examples of Typical Problems the sample’s matrix is unknown, the matrix effect must be shown to be negligi-
ble, or an alternative method of standardization must be used. Both approaches
are discussed in the following sections.
Each example problem includes a matrix matching
Adjusting the matrix of an external
detailed solution that helps students in standard so that it is the same as the 5B.4 Standard Additions
matrix of the samples to be analyzed.
The complication of matching the matrix of the standards to that of the sample
applying the chapter’s material to method of standard additions can be avoided by conducting the standardization in the sample. This is known
A standardization in which aliquots of a as the method of standard additions. The simplest version of a standard addi-
practical problems. standard solution are added to the tion is shown in Figure 5.5. A volume, V o , of sample is diluted to a final volume,
sample. V f , and the signal, S samp is measured. A second identical aliquot of sample is
Bold-faced Key Terms with Margin Definitions
Key words appear in boldface when they are introduced within the text.
The term and its definition appear in the margin for quick review by the
student. All key words are also defined in the glossary.
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