水质分析仪表(图)

pH measurement
The pH value is a measure of the
alkalinity or acidity of an aqueous
solution. The scale reaches from 0
(highly acid) to 14 (highly alkaline).
On the basis of this scale, liquids are
referred to as acidic (e.g. hydrochloric
acid), neutral (e.g. distilled water)
and alkaline (e.g. sodium hydroxide).
The pH value can be measured by
means of electrochemical measuring
systems which consist of a measuring
and a reference electrode. In
the case of a single-rod probe, the
two electrodes are integrated in a
single housing. When the probe is
submerged in the solution to be
measured, a voltage is generated
between the measuring and the reference
electrode which is characteristic
of the corresponding pH value.
This voltage is detected by the
instrument, converted into the pH
value and displayed.
Application areas:
? Drinking water monitoring
? Pool water monitoring
? Municipal and industrial sewage
plants
? Production and process control
(e.g. chemical, paper, textile,
paint industries)
? Power plants (boiler feed water,
corrosion)
? Food production (dairies, sugar
plants, breweries, etc.)
Redox measurement
The term redox comes from the
two concepts of reduction and
oxidation, the designations for
chemical reactions of oxygen with
other substances. The term relates
to reactions in which one part gains
electrons (reduction) and one part
loses electrons (oxidation).
Everyday redox reactions:
? Iron rusts.
? Verdigris is generated when
copper is exposed to the
weather.
? Silver tarnishes.
In aqueous solutions, there is a redox
voltage between two electrodes
consisting of different metals.
It is a measure of the different
potential of the metals to lose
electrons. As a result, the (less
noble) metal is dissolved (oxidation)
and the nobler metal is separated
from the solution (reduction). The
redox voltage is measured by
means of a redox probe. Just like
the pH probe, it consists of a
measuring and a reference electrode
(single rod probe).
Application areas:
Redox measurements are performed
wherever chemical reactions have
to be monitored.
Examples include denitrification of
waste water (determination of redox
inflexion point), monitoring the
disinfection of cleaning agents or
decontamination of electroplating
solutions.
Oxygen measurement
More or less oxygen is dissolved in
virtually every liquid. For example,
water contains approx. 9 mg/l of
oxygen at a temperature or 20 °C
and an atmospheric pressure of
1013 mbar if it is saturated. Each
liquid takes up oxygen until the
partial pressures in the liquid and
in the air or gas in contact with the
liquid have reached equilibrium.
The oxygen concentration is determined
by means of an electrochemical
sensor. The simplest type of
oxygen sensor contains two electrodes.
Both electrodes are located
in an electrolyte system which is
separated from the sample by a
membrane. At one electrode,
oxygen is reduced to hydroxide
ions, at the other electrode, ions
are oxidised to oxygen. During this
electrochemical reaction, a current
flows between the two electrodes
in the sensor if they are connected.
The higher the oxygen concentration
in the solution, the greater the
current signal. The oxygen measuring
instrument uses a solubility
function to calculate the oxygen
concentration in the solution on the
basis of this signal.
Application areas:
? Waster water treatment, e.g.
optimisation of the cleaning
quality, monitoring of decomposition
processes
? Water monitoring, e.g. monitoring
the conditions of marine life, e.g.
fish and microorganisms
? Drinking water quality, evaluation,
prevention of corrosion damage