Page 246 - Modern Analytical Chemistry

1400-CH07 9/8/99 4:04 PM Page 229

Chapter 7 Obtaining and Preparing Samples for Analysis 229

17. Simpson, Apte, and Batley investigated methods for 24. A solute, S, has a distribution ratio between water and ether of
preserving water samples collected from anoxic (O 2 -poor) 7.5. Calculate the extraction efficiency if a 50.0-mL aqueous
environments containing high concentrations of dissolved sample of S is extracted using 50.0 mL of ether as (a) a single
sulfide. They found that preserving the water samples by portion of 50.0 mL; (b) two portions, each of 25.0 mL;
29
adding HNO 3 (which is a common preservation method for (c) four portions, each of 12.5 mL; and (d) five portions, each
aerobic samples) gave significant negative determinate errors of 10.0 mL. Assume that the solute is not involved in any
when analyzing for Cu . When samples were preserved by secondary equilibria.
2+
adding H 2 O 2 followed by HNO 3 , the concentration of Cu 2+ 25. What volume of ether is needed to extract 99.9% of the solute
was determined without a determinate error. Explain their in problem 24 when using (a) one extraction; (b) two
observations. extractions; (c) four extractions; and (d) five extractions.
18. In a particular analysis the selectivity coefficient, K A,I , is 0.816. 26. What must a solute’s distribution ratio be if 99% of the solute
When a standard sample known to contain an analyte-to- in a 50.0-mL sample is to be extracted with a single 50.0-mL
interferent ratio of 5:1 is carried through the analysis, the portion of an organic solvent? Repeat for the case where two
error in determining the analyte is +6.3%. (a) Determine the 25.0-mL portions of the organic solvent are used.
apparent recovery for the analyte if R I = 0. (b) Determine the –5
apparent recovery for the interferent if R A =1. 27. A weak acid, HA, with a K a of 1.0 ´10 , has a partition
coefficient, K D , between water and an organic solvent of 1200.
19. The amount of Co in an ore sample is to be determined using What restrictions on the sample’s pH are necessary to ensure
a procedure for which Fe is an interferent. To evaluate the that a minimum of 99.9% of the weak acid is extracted in a
procedure’s accuracy, a standard sample of ore known to have single step from 50.0 mL of water using 50.0 mL of the
a Co/Fe ratio of 10.2:1 is analyzed. When pure samples of Co organic solvent?
and Fe are taken through the procedure, the following
calibration relationships are obtained 28. For problem 27, how many extractions will be necessary if the
pH of the sample cannot be decreased below 7.0?
S Co = 0.786 ´number of grams Co 29. A weak base, with a K b of 1.0 ´10 , has a partition
–3
coefficient, K D , between water and an organic solvent of 500.
S Fe = 0.699 ´number of grams Fe What restrictions on the pH of the sample are necessary to
ensure that a minimum of 99.9% of the weak base is extracted
When 278.3 mg of Co is taken through the separation step, from 50.0 mL of water using two 25.00-mL portions of the
275.9 mg is recovered. Only 3.6 mg of Fe is recovered when a organic solvent?
184.9-mg sample of Fe is carried through the separation step. 30. A sample contains a weak acid analyte, HA, and a weak acid
Calculate (a) the recoveries for Co and Fe; (b) the separation interferent, HB. The acid dissociation constants and partition
factor; (c) the error if no attempt is made to separate the Co coefficients for the weak acids are as follows: K a,HA = 1.0 ´10 ,
–3
and Fe before the analysis; (d) the error if the separation step K a,HB = 1.0 ´10 , K D,HA = K D,HB = 500. (a) Calculate the
–7
is carried out; and (e) the maximum recovery for Fe if all the extraction efficiency for HA and HB when 50.0 mL of sample,
Co is recovered and the maximum allowed error is 0.05%.
buffered to a pH of 7.0, is extracted with 50.0 mL of the
20. The amount of calcium in a sample of urine was determined organic solvent. (b) Which phase is enriched in the analyte?
by a method for which magnesium is an interferent. The (c) What are the recoveries for the analyte and interferent in
selectivity coefficient, K Ca,Mg , for the method is 0.843. When a this phase? (d) What is the separation factor? (e) A
sample with a Mg/Ca ratio of 0.50 was carried through the quantitative analysis is conducted on the contents of the
procedure, an error of –3.7% was obtained. The error was phase enriched in analyte. What is the expected relative error
+5.5% when a sample with a Mg/Ca ratio of 2.0 was used. if the selectivity coefficient, K HA,HB , is 0.500 and the initial
(a) Determine the recoveries for Ca and Mg. (b) What is ratio of HB/HA was 10.0?
the expected error for a urine sample in which the Mg/Ca 31. The relevant equilibria for the extraction of I 2 from an
ratio is 10.0?
aqueous solution of KI into an organic phase are shown in the
21. Show that F is an effective masking agent in preventing a following diagram.
–
reaction of Al with EDTA. Assume that the only significant
3+
4–
–
forms of fluoride and EDTA are F and Y . I 2
22. Cyanide is frequently used as a masking agent for metal ions. Organic
–
The effectiveness of CN as a masking agent is generally better K D Aqueous
in more basic solutions. Explain the reason for this pH K f
dependency. I + I – I 3 –
2
23. Explain how an aqueous sample consisting of Cu , Sn , (a) Will the extraction efficiency for I 2 be greater for higher or
4+
2+
2+
2+
Pb , and Zn can be separated into its component parts by lower concentrations of I ? (b) Derive an expression for
–
adjusting the pH of the solution. the distribution ratio for this extraction.