44
River and Stream Sediments
Hydrofluoric acid had to be eliminated by evaporation and perchloric acid was a
serious interferent. A technique for the reduction of chemical interference in lead and
nickel determinations was recommended, which consisted of matrix modification by
ammonium dihydrogen phosphate and ascorbic acid. Rapid heating was recommended for cadmium determinations.
Microwave Digestion Techniques. In addition to the bomb digestion techniques
mentioned above, the technique of digesting sediments in PTFE lined bombs in a
microwave oven has also been discussed [56, 57]. Mahan et al. [56] carried out
sequential extractions of calcium, iron, chromium, manganese, lead, and zinc and
obtained comparable results to conventional techniques in less time.
It has been demonstrated [57] that closed vessel microwave sediment digestions
can reduce sample dissolution times, be applicable to a wide variety of sample
matrices, allow for dissolution flexibility, and produce analytical data comparable to
those obtained by conventional digestion procedures. Digestion at 200°C with 70 %
nitric acid in a 120-ml closed vessel produces an internal pressure of 120 psig, whilst
digestion with 37 % hydrochloric acid at 153°C produces an internal pressure of
100 psig.
Figure 2.5 a is a temperature-pressure plot obtained for the simultaneous heating of
six closed vessels, each containing 20 ml of concentrated nitric acid, at 600 W power.
As can be seen, the temperature and pressure rise smoothly to 193°C and 100 psig in
12 min.
Figure 2.5 b is a temperature-pressure plot obtained for the heating of six closed
vessels, each containing 20 ml of concentrated hydrochloric acid, at 600 W power. For
this acid, the curve also increases smoothly to 140°C and 101 psig in about
4.5 min.
At such elevated temperatures, these and other acids become more corrosive.
Materials that digest slowly, or will not digest at the atmospheric boiling points of the
acids, become more soluble, so dissolution times are greatly reduced.
The closed vessel system used and shown in Fig. 2.6 consists of a vessel body, safety
pressure relief valve, vessel cap, venting nut and tubing, all of which are constructed
of Teflon PFA material. Teflon PFA is transparent to microwave energy, allowing
microwaves to pass through the vessel and couple directly with a digesting acid, thus
making it possible to obtain elevated temperatures very rapidly. This vessel system is
designed to operate at internal pressures up to 120 psig. Above 120 psig, the safety
valve will open, allowing the system to vent into a collection container, thus lowering
the pressure inside the vessel. The valve then reseals, allowing pressure to increase
again. This automatic venting is a safety feature of the closed vessel system to ensure
that vessel rupture, due to excessive pressure, will not occur.
Figures 2.7 a,b shows the temperature-pressure development plots obtained for 1 g
of NBS SRM 1645 standard river sediment during the first 30 min of digestion, respectively with the stated quantities of 50 % nitric acid and nitric acid-hydrogen peroxide.
Excellent agreement with reference values were obtained using either method of
digestion (Table 2.24).
Bando et al. [53] discussed analytical errors associated with iron, manganese,
copper, chromium, and zinc determinations in particulate sedimentary matter by
River and Stream Sediments
Hydrofluoric acid had to be eliminated by evaporation and perchloric acid was a
serious interferent. A technique for the reduction of chemical interference in lead and
nickel determinations was recommended, which consisted of matrix modification by
ammonium dihydrogen phosphate and ascorbic acid. Rapid heating was recommended for cadmium determinations.
Microwave Digestion Techniques. In addition to the bomb digestion techniques
mentioned above, the technique of digesting sediments in PTFE lined bombs in a
microwave oven has also been discussed [56, 57]. Mahan et al. [56] carried out
sequential extractions of calcium, iron, chromium, manganese, lead, and zinc and
obtained comparable results to conventional techniques in less time.
It has been demonstrated [57] that closed vessel microwave sediment digestions
can reduce sample dissolution times, be applicable to a wide variety of sample
matrices, allow for dissolution flexibility, and produce analytical data comparable to
those obtained by conventional digestion procedures. Digestion at 200°C with 70 %
nitric acid in a 120-ml closed vessel produces an internal pressure of 120 psig, whilst
digestion with 37 % hydrochloric acid at 153°C produces an internal pressure of
100 psig.
Figure 2.5 a is a temperature-pressure plot obtained for the simultaneous heating of
six closed vessels, each containing 20 ml of concentrated nitric acid, at 600 W power.
As can be seen, the temperature and pressure rise smoothly to 193°C and 100 psig in
12 min.
Figure 2.5 b is a temperature-pressure plot obtained for the heating of six closed
vessels, each containing 20 ml of concentrated hydrochloric acid, at 600 W power. For
this acid, the curve also increases smoothly to 140°C and 101 psig in about
4.5 min.
At such elevated temperatures, these and other acids become more corrosive.
Materials that digest slowly, or will not digest at the atmospheric boiling points of the
acids, become more soluble, so dissolution times are greatly reduced.
The closed vessel system used and shown in Fig. 2.6 consists of a vessel body, safety
pressure relief valve, vessel cap, venting nut and tubing, all of which are constructed
of Teflon PFA material. Teflon PFA is transparent to microwave energy, allowing
microwaves to pass through the vessel and couple directly with a digesting acid, thus
making it possible to obtain elevated temperatures very rapidly. This vessel system is
designed to operate at internal pressures up to 120 psig. Above 120 psig, the safety
valve will open, allowing the system to vent into a collection container, thus lowering
the pressure inside the vessel. The valve then reseals, allowing pressure to increase
again. This automatic venting is a safety feature of the closed vessel system to ensure
that vessel rupture, due to excessive pressure, will not occur.
Figures 2.7 a,b shows the temperature-pressure development plots obtained for 1 g
of NBS SRM 1645 standard river sediment during the first 30 min of digestion, respectively with the stated quantities of 50 % nitric acid and nitric acid-hydrogen peroxide.
Excellent agreement with reference values were obtained using either method of
digestion (Table 2.24).
Bando et al. [53] discussed analytical errors associated with iron, manganese,
copper, chromium, and zinc determinations in particulate sedimentary matter by