If you are taking readings at temperatures below 15°C or above 30°C, allow
more time for the temperature compensation to adjust and provide a stable
conductivity reading.
When you have finished using the Conductivity Probe, simply rinse it off with
distilled water and blot it dry using a paper towel or lab wipe. The probe can
then be stored dry.
If the probe cell surface is contaminated, soak it in water with a mild detergent
for 15 minutes. Then soak it in a dilute acid solution (0.1 M hydrochloric acid
or 0.5 M acetic acid works well) for another 15 minutes. Then rinse it well
with distilled water and blot dry.
Important:
Avoid scratching the inside
electrode surfaces of the elongated cell.
Some combinations of sensors interfere with each other when placed in the
same solution. The degree of interference depends on many factors, including
which combination of sensors is being used, which interface is being used, and
others. For more information, see
www.vernier.com/til/638/
Sampling in Streams and Lakes
It is best to sample away from shore and below the water surface, if possible. In
free-flowing streams, there will usually be good mixing of the water, so that
samples taken near the current will be quite representative of the stream as a
whole. If you are sampling an impounded stream or a lake, there will be very little
mixing; therefore, it is important to sample away from shore and at different
depths, if possible. Do not drop the Vernier Conductivity Probe so that the entire
electrode is submerged. The electrode is not constructed to withstand higher
pressures, so seepage into electronic components of the electrode will result.
Although it is better to take readings at the collection site, readings of total
dissolved solids or conductivity should not change significantly if you collect
samples and take readings at a later time. However, be sure that samples are
capped to prevent evaporation.
If sample bottles are filled brim full, then a gas such as carbon dioxide, which is
capable of forming ionic species in solution, is prevented from dissolving in the
water sample. Since the probe has built-in temperature compensation, you can do
your calibration in the lab. This means that even though you will be sampling in
water that has a different temperature than your calibration temperature, the probe
will take correct readings at the new sampling temperature.
Sampling in Ocean Water or Tidal Estuaries: Salinity
Salinity is the total of all non-carbonate salts dissolved in water, usually expressed
in parts per thousand (1 ppt = 1000 mg/L). Unlike chloride (Cl
-
) concentration,
you can think of salinity as a measure of the total salt concentration, comprised
mostly of Na
+
and Cl
-
ions. Even though there are smaller quantities of other ions
in seawater (e.g., K
+
, Mg
2+
, or SO
4
2-
), sodium and chloride ions represent about
91 percent of all seawater ions. Salinity is an important measurement in seawater
or in estuaries where freshwater from rivers and streams mixes with salty ocean
water. The salinity level in seawater is fairly constant, at about 35 ppt
(35,000 mg/L), while brackish estuaries may have salinity levels
between 1 and 10 ppt.
5
The salinity range of the Conductivity Probe is 0 to 10 ppt. Seawater has a salinity
of 35 ppt, so any seawater samples will need to be diluted before making
measurements with this sensor. We recommend that you dilute seawater samples
(or other samples that initially give readings above 10 ppt) to 1/4 of their original
concentration, then multiply their measured salinity reading by 4 to obtain a final
salinity value, in ppt. Brackish water in coastal estuaries is often in the range of
0 to 10 ppt, well within the high range of the probe.
Note:
Vernier also sells a
Salinity Sensor (order code SAL-BTA) with a range of 0 to 50 ppt.
Since there is no stored salinity calibration for a Conductivity Probe, perform a
two-point calibration using 5 ppt and 10 ppt salinity standards. Make sure your
sensor switch is on the high conductivity setting. You will need to prepare two
standard solutions to calibrate for salinity:
A low standard (5 ppt salinity), add 4.60 g of NaCl to enough distilled water
to prepare 1 liter of solution.
A high standard (10 ppt salinity), add 9.20 g of NaCl to enough distilled water
to prepare 1 liter of solution.
Determining the Concentration: Total Dissolved Solids
Because there is a nearly linear relationship between conductivity and
concentration of a specific ion or salt, the Conductivity Probe can be used to
determine the concentration of an ion. A curve can be obtained if you prepare or
purchase standard solutions. Note in this figure the 2:1 ratio between conductivity
in µS/cm and TDS concentration in mg/L.
Even though total dissolved solids is often defined in terms of this 2:1 ratio, it
should be understood that a TDS reading of 500 mg/L can have a different
meaning in a sample that is mostly NaCl than in another sample that is composed
primarily of hard water ions such as Ca
2+
and HCO
3
-
. The relationship between
conductivity and sodium chloride concentration is
approximately
a 2:1 ratio and
is very nearly a direct relationship. The table shows the relationship for sodium
chloride concentration in mg/L to TDS to conductivity.
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