Page 284 - Modern Analytical Chemistry

1400-CH08 9/9/99 2:18 PM Page 267

Chapter 8 Gravimetric Methods of Analysis 267

8H PROBLEMS

1. Starting with the equilibrium constant expressions for Experiment 2. When equal volumes of 1.5 M Ba(SCN) 2 and
reactions 8.1, and 8.3–8.5, verify that equation 8.7 is correct. 1.5 M MgSO 4 are mixed, a curdy precipitate immediately
forms. Individual particles of BaSO 4 can be seen as points
2. Equation 8.7 shows how the solubility of AgCl varies as a
–
function of the equilibrium concentration of Cl . Derive a under magnification at 1500´.
similar equation to describe the solubility of AgCl as a Experiment 3. When equal volumes of 0.0005 M Ba(SCN) 2
+
function of the equilibrium concentration of Ag . Graph the and 0.0005 M MgSO 4 are mixed, the complete precipitation
resulting solubility function and compare it with that shown of BaSO 4 requires 2–3 h. Individual crystals of BaSO 4 obtain
in Figure 8.1. lengths of approximately 0.005 mm.
3. Derive a solubility diagram (solubility versus pH) for 7. Aluminum can be determined gravimetrically by precipitating
Zn(OH) 2 that takes into account the following soluble zinc as Al(OH) 3 and isolating as Al 2 O 3 . A sample containing
+
–
2–
hydroxide complexes: Zn(OH) , Zn(OH) 3 , Zn(OH) 4 . approximately 0.1 g of Al is dissolved in 200 mL of H 2 O and
What is the optimum pH for the quantitative precipitation of 5 g of NH 4 Cl and a few drops of methyl red indicator is added
Zn(OH) 2 ? (methyl red is red at pH levels below 4 and yellow at pH levels
above 6). The solution is heated to boiling, and 1:1 NH 3 is
4. For what pH range will the following precipitates have their
added dropwise till the indicator turns yellow, precipitating
lowest solubility?
Al(OH) 3. The precipitate is held at the solution’s boiling point
a. CaC 2 O 4
for several minutes, filtered, and washed with a hot solution
b. PbCrO 4
of 2%, w/v NH 4 NO 3. The precipitate is then ignited at
c. BaSO 4
1000–1100 °C, forming Al 2O 3.
d. SrCO 3
(a) Cite two ways in which this procedure has been designed
e. ZnS
to encourage the formation of larger particles of precipitate.
5. When solutions of 1.5 M KNO 3 and 1.5 M HClO 4 are mixed, (b) The ignition step must be carried out carefully to ensure
–
a white precipitate of KClO 4 is formed. If traces of MnO 4 are
that Al(OH) 3 is quantitatively converted to Al 2O 3 . What effect
present, an inclusion of KMnO 4 is possible. Impure
would an incomplete conversion have on the reported %w/w
precipitates of KClO 4 are colored purple by the included
Al? (c) What role do NH 4Cl and methyl red indicator play in
KMnO 4 . Following are the descriptions and results for two
this procedure? (d) An alternative procedure involves
experiments in which KClO 4 is precipitated in the presence of isolating and weighing the precipitate as the 8-
–
MnO 4 . Explain why the two experiments lead to different
hydroxyquinolate, Al(C 9 H 6ON) 3. Why might this be a more
results (see Color Plate 6).
advantageous form of Al for a gravimetric analysis?
Experiment 1. Place 1 mL of 1.5 M KNO 3 in a test tube, add
8. Calcium is determined gravimetrically by precipitating it as
3 drops of 0.1 M KMnO 4 , and swirl to mix. Add 1 mL of 1.5
CaC 2 O 4× H 2 O, followed by isolating the precipitate as CaCO 3 .
M HClO 4 dropwise, agitating the solution between drops.
The sample to be analyzed is dissolved in 10 mL of water and
Destroy the excess KMnO 4 by adding 0.1 M NaHSO 3
15 mL of 6 M HCl. After dissolution, the resulting solution is
dropwise. The resulting precipitate of KClO 4 has an intense
heated to boiling, and a warm solution of excess ammonium
purple color.
oxalate is added. The solution is maintained at 80 °C, and 6 M
Experiment 2. Place 1 mL of 1.5 M HClO 4 in a test tube, add NH 3 is added dropwise, with stirring, until the solution is
3 drops of 0.1 M KMnO 4, and swirl to mix. Add 1 mL of 1.5 faintly alkaline. The resulting precipitate and solution are
M KNO 3 dropwise, agitating the solution between drops. removed from the heat and allowed to stand for at least 1 h.
Destroy the excess KMnO 4 by adding 0.1 M NaHSO 3 After testing the solution for completeness of precipitation, the
dropwise. The resulting precipitate of KClO 4 is pale purple or sample is filtered, washed with 0.1% w/v ammonium oxalate,
white in color. and dried at 100–120 °C for 1 h. The precipitate is then
6. When solutions of Ba(SCN) 2 and MgSO 4 are mixed, a transferred to a muffle furnace where it is converted to CaCO 3
precipitate of BaSO 4 forms. Following are the descriptions by drying at 500 ± 25 °C until a constant weight.
and results for several experiments in which only the (a) Why is the precipitate of CaC 2O 4× H 2O converted to
concentrations of Ba(SCN) 2 and MgSO 4 are different. Explain CaCO 3? (b) In the final step, if the sample is heated at too
why these experiments produce different results. high of a temperature, some CaCO 3 may be converted to
Experiment 1. When equal volumes of 3.5 M Ba(SCN) 2 and CaO. What effect would this have on the reported %w/w Ca?
3.5 M MgSO 4 are mixed, a gelatinous precipitate immediately (c) Why is the precipitant, (NH 4) 2C 2O 4, added to a hot, acidic
forms. solution rather than to a cold, alkaline solution?