Introduction - Determination of Anions in Natural and Treated Waters - page 19

Non saline waters: arsenate, arsenite, iodate, iodide, nitrate, nitrite, phosphate, selenate,
selenite, sulphide, also nitriloacetate.
Seawater: bromate and sulphate.
Aqueous precipitation: nitrate.
Surface and ground
waters:

bromide and arsenate.

Potable water: iodide, nitrate, nitrite, perchlorate and silicate.

Waste waters: chromate, dichromate and sulphite.

Trade effluents: free cyanide, selenate and selenite.

Differential pulse polarography has found limited applications.
Three basic techniques of polarography are of interest and the basic principles of these
are outlined below.

Universal: Differential Pulse (DPN, DPI, DPR)
In this technique a voltage pulse is superimposed on the voltage ramp during the last
40ms of controlled drop growth with the standard dropping mercury electrode; the drop
surface is then constant. The pulse amplitude can be preselected. The current is measured
by integration over a 20ms period immediately before the start of the pulse and again for
20ms as the pulse nears completion. The difference between the two current integrals
(1 2 -1 1 ) is recorded and this gives a peak-shaped curve. If the pulse amplitude is
increased, the peak current value is raised but the peak is broadened at the same time.

Classical Direct Current (DCT)
In this direct current method, integration is performed over the last 20ms of the controlled
drop growth (Tast procedure): during this time, the drop surface is constant in the case of
the dropping mercury electrode. The resulting polarogram is step-shaped. Compared with
classical DC polarography according to Heyrovsky, ie with the free-dropping mercury
electrode, the DCT method offers great advantages; considerably shorter analysis times,
no disturbance due to current oscillations, simpler evaluation and larger diffusion-
controlled limiting current.

Table 1.4 Inductively coupled plasma optical emission spectrometers available on the
market

Supplier Model System Number

of
elements
claimed

Maximum
analysis
rate
(elements
min L −1 )

Micro-
processor

Auto-
sampler

Range
(nm)

Perkin-Elmer Plasma II Optimized 70 Up to 50 Yes Yes 160-80