Page 317 - Modern Analytical Chemistry

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300 Modern Analytical Chemistry

precipitates and settles to the bottom of the container, allowing access to the
carbonate-free NaOH. Dilution must be done with water that is free from dissolved
CO 2 . Briefly boiling the water expels CO 2 and, after cooling, it may be used to pre-
pare carbonate-free solutions of NaOH. Provided that contact with the atmosphere
is minimized, solutions of carbonate-free NaOH are relatively stable when stored
in polyethylene bottles. Standard solutions of sodium hydroxide should not be
stored in glass bottles because NaOH reacts with glass to form silicate.

Inorganic Analysis Acid–base titrimetry is a standard method for the quantitative
analysis of many inorganic acids and bases. Standard solutions of NaOH can be
used in the analysis of inorganic acids such as H 3 PO 4 or H 3 AsO 4 , whereas standard
solutions of HCl can be used for the analysis of inorganic bases such as Na 2 CO 3 .
Inorganic acids and bases too weak to be analyzed by an aqueous acid–base
titration can be analyzed by adjusting the solvent or by an indirect analysis. For ex-
ample, the accuracy in titrating boric acid, H 3BO 3, with NaOH is limited by boric
acid’s small acid dissociation constant of 5.8 ´10 –10 . The acid strength of boric acid,
however, increases when mannitol is added to the solution because it forms a com-
–4
plex with the borate ion. The increase in K a to approximately 1.5 ´10 results in a
sharper end point and a more accurate titration. Similarly, the analysis of ammo-
+
nium salts is limited by the small acid dissociation constant of 5.7 ´10 –10 for NH 4 .
+
In this case, NH 4 can be converted to NH 3 by neutralizing with strong base. The
–5
NH 3 , for which K b is 1.8 ´10 , is then removed by distillation and titrated with a
standard strong acid titrant.
Inorganic analytes that are neutral in aqueous solutions may still be analyzed if
–
they can be converted to an acid or base. For example, NO 3 can be quantitatively
analyzed by reducing it to NH 3 in a strongly alkaline solution using Devarda’s alloy,
a mixture of 50% w/w Cu, 45% w/w Al, and 5% w/w Zn.
–
–
–
3NO 3 (aq) + 8Al(s) + 5OH (aq)+2H 2 O(l) ® 8AlO 2 (aq) + 3NH 3 (aq)
–
The NH 3 is removed by distillation and titrated with HCl. Alternatively, NO 3 can
be titrated as a weak base in an acidic nonaqueous solvent such as anhydrous acetic
acid, using HClO 4 as a titrant.
Acid–base titrimetry continues to be listed as the standard method for the de-
alkalinity termination of alkalinity, acidity, and free CO 2 in water and wastewater analysis. Al-
A measure of a water’s ability to kalinity is a measure of the acid-neutralizing capacity of a water sample and is as-
neutralize acid. – – 2–
sumed to arise principally from OH , HCO 3 , and CO 3 , although other weak
bases, such as phosphate, may contribute to the overall alkalinity. Total alkalinity is
determined by titrating with a standard solution of HCl or H 2 SO 4 to a fixed end
point at a pH of 4.5, or to the bromocresol green end point. Alkalinity is reported as
milligrams CaCO 3 per liter.
–
2–
–
When the sources of alkalinity are limited to OH , HCO 3 , and CO 3 , titra-
tions to both a pH of 4.5 (bromocresol green end point) and a pH of 8.3 (phe-
nolphthalein or metacresol purple end point) can be used to determine which
species are present, as well as their respective concentrations. Titration curves for
– – 2–
OH , HCO 3 , and CO 3 are shown in Figure 9.18. For a solution containing only
–
OH alkalinity, the volumes of strong acid needed to reach the two end points are
–
identical. If a solution contains only HCO 3 alkalinity, the volume of strong acid
needed to reach the end point at a pH of 8.3 is zero, whereas that for the pH 4.5 end
2–
point is greater than zero. When the only source of alkalinity is CO 3 , the volume
of strong acid needed to reach the end point at a pH of 4.5 is exactly twice that
needed to reach the end point at a pH of 8.3.