562 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS
Potentiometric Instruments An instrument consisting of
an electrochemical cell containing indicating and reference
electrodes and electronic means for measuring cell poten-
tial to within 0.001 volt (depending on the accuracy desired)
may be optimized to measure concentrations of various ions
and molecules. The indicating electrode is the sensor which
gives the specific electrochemical detection of the analyte
in question, while, the function of the reference electrode
is to provide a stable reference potential for the indicating
electrode. The potential of the indicating electrode changes
as a function of the concentration of the analyte according to
the Nernst equation,E E^0 (0.0591/n)Log α (^) red / α (^) ox. (11)
The activities of the reduced and oxidized species are α (^) red
and α (^) ox , respectively, n is the number of electrons in the
redox reaction and E^0 is the standard reduction potential of
the redox couple when the activity is one.
For species, Sp, its activity, α (^) Sp , is a variable related to
its concentration in Moles/L. The symbol, [Sp], represents
the concentration of Sp in Moles/L. The α (^) Sp is defined by
the equation,
α (^) Sp f[Sp]. (12)
The activity coefficient, f, varies inversely with the ionic
strength of the solution. (Ionic strength is a function that
can be calculated from the concentration of ions in solution
and their ionic charges.) Therefore, at high concentrations
of electrolytes, the ionic strengths are high, and the activity
coefficients, small, less than one. Activity and concentration
are approximately equal when the ionic strength is low and f
is equal to values between 0.90 to 1.
TABLE 5
Metals Analysis by Spectroscopya
Metals Atomic Absorption Method Flame
Sb, Bi, Cd, Ca, Cs, Cr direct aspiration air-acetylene
Co, Cu, Au, Ir, Fe, Pb
Li, Mg, Mn, Ni, Pd, Pt
K, Rh, Ru, As, Na, Sr
Tl, Sn, Zn
Al, Ba, Be, Mo, Os direct aspiration N 2 O-acetylene
Rh, Si, Th, Ti, V
bAl, Sb, As, Ba, Be electrothermal none
Cd, Cr, Co, Cu, Fe, Pb
Mn, Mo, Ni, Se, Ag, Sn
cCd, Cr, Co, Cu, Fe chelationd/ air-acetylene
Pb, Mn, Ni, Ag, Zn extractione
cAl, Be chelationf/N 2 O-acetylene
extractione
Hg cold vapor none
As, Se hydride formation argon-hydrogen
or N 2 -hydrogen
Inductively Coupled Plasma (atomic emission)
Al, Sb, As, Ba, Be, B, Cd, Ca, Cr, Co, Cu, Fe,
Pb, Li, Mg, Mn, Ni, K, Se, Si, Ag, Na, Sr,
Tl, V, Zn
Flame Emission
Li, K, Na, Sr
Colorimetric/Spectrophotometric
Al, As, Be, B, Cd, Cr, Cu, Se, Si, Ag, V, Zn
a From Ref. (2); b microquantities; c low conc.; d ammonium pyrrolidinedithiocarbamate; e methyl isobutyl ketone;
f 8-hydroxyquinoline.
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