260
6 Quantitative Aspects of Analytical Chemistry
(e) the mass of the vessel containing the weighed form.
The difference between (b) and (e) provides the mass of the weighed form. Through the
gravimetric factor. the mass of analyte - the proportion or concentration of which in the
sample is calcu lated from the sample mass or volume. (a) - is determined.
Gravimetric methods can rely on three different types of preliminary operations, namely:
(a) Chemical precipitation by addition of an organic or inorganic reagent to the
sample containing the analyte (see Box 6.6). This is the most common choice as
it allows the determination of both cations (by precipitation as oxide hydrates,
salts or metal chelates) and anions. One important practical consideration here
is whether the precipitated form coincides with the weighed form (see Box 6.7).
Box 6.7
Comparison of two Gravimetric Processes where the Weighed Form is the Same
as or Different from the Precipitated Form
/1 Precipitated form 7- weighed form /
/1 Precipitated form = weighed form /
GRAVIMETRIC DETERMINATION OF
GRAVIMETRIC DETERMINATION OF
ALUMINIUM AS AN OXIDE HYDRATE
NICKEL WITH DIMETHYLGLYOXIME
1 Acid sample + NH,c1 + phenol red (indicator)
1 Boiling of aCidified sa mple
2 Addition of 1 : 1 NH ! to Indicator colour change
2 Sequential addition of dlmethylglyoxime
3 Boiling and 15 min standing
(DMG) and 1: 1 NH,
4 Passing precipitate through filter paper
3 Standing for 1 h
5 Washing of precipitate with NH,NOJ
4 Taring of filtering plate and filtration
6 Taring crucible containing paper precipitate
5 Washing with hot water
7 Drying and burning of paper with Bunsen
6 Drying to constant weight in stove at 110"(
burner
8 Calcination in furnace at 1100"(
9 Cooling and rapid weighing (to avoid
F _ Ni
g -Ni(DMGh
atmospheric contact)
Fg=~
AIIO]
Note that the procedure in which the precipitated form coincides with the weighed form
involves substantially fewer steps and that such steps are more simple.
(b) Electrodeposition by use of an electrochemical assembly consisting of a constant electrical power supply establishing a potential difference between an
electrode (anode), where an oxidation takes place, and another (cathode),
where a reduction occurs, both electrodes being immersed in a stirred
sample solution. The cathode possesses a large surface area (typically that
provided by platinum gauze) that increases the efficiency of the reduction
reaction (e.g. M2+ + 2e- H M). The analyte gradually deposits onto the
electrode until it virtually disappears from the solution. The cathode is
6 Quantitative Aspects of Analytical Chemistry
(e) the mass of the vessel containing the weighed form.
The difference between (b) and (e) provides the mass of the weighed form. Through the
gravimetric factor. the mass of analyte - the proportion or concentration of which in the
sample is calcu lated from the sample mass or volume. (a) - is determined.
Gravimetric methods can rely on three different types of preliminary operations, namely:
(a) Chemical precipitation by addition of an organic or inorganic reagent to the
sample containing the analyte (see Box 6.6). This is the most common choice as
it allows the determination of both cations (by precipitation as oxide hydrates,
salts or metal chelates) and anions. One important practical consideration here
is whether the precipitated form coincides with the weighed form (see Box 6.7).
Box 6.7
Comparison of two Gravimetric Processes where the Weighed Form is the Same
as or Different from the Precipitated Form
/1 Precipitated form 7- weighed form /
/1 Precipitated form = weighed form /
GRAVIMETRIC DETERMINATION OF
GRAVIMETRIC DETERMINATION OF
ALUMINIUM AS AN OXIDE HYDRATE
NICKEL WITH DIMETHYLGLYOXIME
1 Acid sample + NH,c1 + phenol red (indicator)
1 Boiling of aCidified sa mple
2 Addition of 1 : 1 NH ! to Indicator colour change
2 Sequential addition of dlmethylglyoxime
3 Boiling and 15 min standing
(DMG) and 1: 1 NH,
4 Passing precipitate through filter paper
3 Standing for 1 h
5 Washing of precipitate with NH,NOJ
4 Taring of filtering plate and filtration
6 Taring crucible containing paper precipitate
5 Washing with hot water
7 Drying and burning of paper with Bunsen
6 Drying to constant weight in stove at 110"(
burner
8 Calcination in furnace at 1100"(
9 Cooling and rapid weighing (to avoid
F _ Ni
g -Ni(DMGh
atmospheric contact)
Fg=~
AIIO]
Note that the procedure in which the precipitated form coincides with the weighed form
involves substantially fewer steps and that such steps are more simple.
(b) Electrodeposition by use of an electrochemical assembly consisting of a constant electrical power supply establishing a potential difference between an
electrode (anode), where an oxidation takes place, and another (cathode),
where a reduction occurs, both electrodes being immersed in a stirred
sample solution. The cathode possesses a large surface area (typically that
provided by platinum gauze) that increases the efficiency of the reduction
reaction (e.g. M2+ + 2e- H M). The analyte gradually deposits onto the
electrode until it virtually disappears from the solution. The cathode is