Page 227 - Modern Analytical Chemistry

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210 Modern Analytical Chemistry

(b) Thermometer

(a) Thermometer

Distillation Condenser
adaptor

Distillation Condenser
adaptor Fractionating
column
Distillation
flask
Figure 7.14 Receiving flask
Typical equipment for a Distillation
(a) simple distillation; and a Receiving flask flask
(b) fractional distillation.

Another approach for purifying solids is recrystallization. The solid is dissolved
in a minimum volume of solvent, for which the analyte’s solubility is significant
when the solvent is hot, and minimal when the solvent is cold. The interferents
must be less soluble in the hot solvent than the analyte, or present in much smaller
amounts. A portion of the solvent is heated in an Erlenmeyer flask, and small
amounts of sample are added until undissolved sample is visible. Additional heated
solvent is added until the sample is again dissolved or until only insoluble impuri-
ties remain. The process of adding sample and solvent is repeated until the entire
sample has been added to the Erlenmeyer flask. If necessary, insoluble impurities
are removed by filtering the heated solution. The solution is allowed to cool slowly,
promoting the growth of large, pure crystals, and then cooled in an ice bath to min-
imize solubility losses. The purified sample is isolated by filtration and rinsed to re-
move soluble impurities. Finally, the sample is dried to remove any remaining
traces of the solvent. Further purification, if necessary, can be accomplished by ad-
ditional recrystallizations.

Changes in Chemical State Distillation, sublimation, and recrystallization use a
change in physical state as a means of separation. Chemical reactivity also can be
used in a separation by effecting a change in the chemical state of the analyte or in-
terferent. For example, SiO 2 can be separated from a
sample by reacting with HF. The volatile SiF 4 that forms
Table 7.7 Selected Examples of the is easily removed by evaporation. In other cases distilla-
Application of Distillation to the tion may be used to remove a nonvolatile inorganic ion
Separation of Inorganic Ions after chemically converting it to a more volatile form. For
+
example, NH 4 can be separated from a sample by mak-
Analyte or Interferent Treatment a
ing the solution basic, resulting in the formation of NH 3 .
2– 2– +
CO 3 CO 3 +2H 3 O ® CO 2 +3H 2 O The ammonia that is produced can then be removed by
+ + –
NH 4 NH 4 +OH ® NH 3 +H 2 O distillation. Other examples are listed in Table 7.7.
2– 2– +
SO 3 SO 3 +2H 3 O ® SO 2 +3H 2 O Other types of reactions can be used to chemically
+
2–
S 2– S +2H 3 O ® H 2 S+2H 2 O separate an analyte and interferent, including precipita-
tion, electrodeposition, and ion exchange. Two impor-
a Underlined species is removed by distillation. tant examples of the application of precipitation are the