Nahita 903 Serie Manual De Instrucciones página 20

Enviroment: land, sludge, sediments, waters.
Biotechnology: fish farms, marine farmings, hydroponics
Food: milks, sauces, beverages, meat, canneds
Pharmaceutical / cosmetic: gels and shampoos, deodorants, creams
Petrochemical and general chemistry
2. THEORETICAL FUNDAMENTALS
The technique called direct potentiometry is the measure of the activity (or concentra-
tion) of a chemical species, measuring directly the potential with the one which is con-
nected, through a known logarithmic function called Nernst equation.
E = E 0 + 2,3 (RT/nF) log a i
E = measured potential (mV)
R = gas constant
n = valency of the ion
a i = ionic activity
In this equation "E" is the developed potential by the system, "E0" is the potential of
reference, "R" is the gas constant, "T" is the absolute temperature, "n" is the number of
electrons transferred and "F" is the Faraday constant.
ai represents the activity of the ion of interest. Ionic activity is related to the concentra-
tion (c i) by the activity factor (f i ).
a i = f i c i
at constant temperature, Nernst equation is:
E = E 0 + S ai
"S" is the slope of the electrode
From this equation we can deduce that making measurings of potential respect of a
reference electrode, we can know the activity and, therefore, the ion concentration. The
most known application of direct potentiometries is the use of what is known as Selected
Ions Electrodes (SIE).
To obtain valid analytical potentiometry measurements, one of the electrodes has to be
constant potential and it has not to suffer changes between experiments. Electrode that
satisfies this condition is known as reference electrode. Due to the stability of the referen-
ce electrode, any change in the potential of the system will be thanks to the contribution
of the other electrode, called working or indicator electrode.
Reference electrode can be an individual electrode or be incorporated into the indicator
electrode (combined electrode). See Appendix I
Version 3 August 09
E 0 = Potential of reference (mV)
T = absolute temperature (K)
F = Faraday constant
Instruction manual 50903000
ENGLISH
pH-meter has to be calibrated regularly in addition to the conditions indicated in
"Calibration" section, by calibration buffers.
"Life expectancy" of a pH electrode is about one year, this period varies depending on
the quality of the electrode and its use: according to number of daily measurements, mea-
surement at the extremes of pH scale and temperature, and especially, the maintenance.
If duration is too short, may not be using the appropriate electrode!
Additional liquid of combined reference electrode is a solution KCl 3 mol /L, that it can
be added by the hole on the upper electrode if it is refillable.
Electrodes must be kept into its protective storage with the corresponding electrolyte
(avoiding accidental breakages of glass body electrodes), keeping this way the bulb (the
most sensitive part of the electrode) always wet. We recommend KCl 4 M solution.
Regeneration of the electrode when it detects a decrease of the slope after a prolonged
use:
- Submerge the bult in HF 4% for 3~5s
- Clean it with distilled water
- Submerge it in HCl 0.1 M
Do not leave the electrode submerged in distilled water, protein or hydrofluoric acid
solution (HF) for so long.
The diaphragm that carries the electrode has not to be exposed to water-insoluble con-
taminant materials (organic oils, silicone, resins, pigments, etc), they can blocked the
diaphragm and prevent proper contact among solutions, being the main reason errors in
measurements.
When pH-meter is not being used, always place the protective device output BNC to
prevent that, in any case, the circuit is opened and damage the equipment.
Inspect the electrode and the wire. Wire used to connect to the pH-meter must be intact,
without cracks, like the electrode. If not, replace the electrode.Always use original parts
and components. Others may be similar but their use can damage the equipment.
Never dismantle the equipment, do not try to repair it by yourself, you will lose the
warranty and maybe you could suffer damages.
In case of failure, contact your supplier for repairing through Nahita Technical Service.
Cleaning
Important: each time you make a measurement you should clean the electrode with dis-
tilled water, avoiding this way that samples suffer contamination of previous solution.
Electrode has to be perfectly clean with distilled water. Do not use a cloth (because it
can be charge electrotatically), use a paper free of fuzz (filter paper, cellulose).
For cleaning different pieces of the equipment, we recommend use neutral products
that they do not contain abrasives agents, such as a few drops of soap for cleaning glass
lowered in water.
Do not spill the prepared on the equipment, you should wet a soft cloth with the pro-
duct.
Page 19
Page 28 Instruction manual 50903000
ENGLISH
Version 3 August-09