Why TDS Meters Do Not
Read Ionic Silver Correctly
- Oct 21, 2011 Copyright (c) CGCSForum.com -
Most people are under the mistaken impression that they can read the
concentration of silver in ppm in their colloidal silver with a Total
Dissolved Solids (TDS) meter. To understand why it will
not read correctly, it is necessary to understand how a TDS meter
works.
Inside of a TDS meter is an electrical circuit which measures the
resistance between its two electrodes when immersed in a liquid.
This circuit applies an AC voltage to the electodes, and then measures
the AC current which flows between the two electrodes. This
reading is then corrected for temperature (if its a high quality
instrument), and electrode geometry. The result is a measure of
Total Ionic Content. The name Total Dissolved Solids is
actually a misnomer, as a lot of dissolved solids will not read at all
on a
TDS meter since they do not ionize when they dissolve. Sugar is
such a substance. If you were to
dissolve a tablespoon of table sugar in a cup of distilled water and
read it with a TDS meter, the resulting reading would be zero.
The reason AC is used instead of DC voltage on the electrodes is to
keep the metals in the solution from plating onto the electrodes.
With AC, anything that plates onto the negative electrode will in
theory come off when the polarity reverses. AC also keeps the
ions from migrating from one electrode to the other, thereby keeping
the solution homogeneous.
Another thing which must be understood is the actual meaning of parts
per million (ppm). PPM means the weight of one substance
contained in a given weight
of another different substance. For example, if a million pounds
of bread
flour contained 1 pound of dirt, it would be 1 ppm dirt in the
flour. We are not interested in flour though, we are interested
in silver ions in water. For silver, one PPM means 1 gram of
silver in 1 million grams of water, which is equivalent to 1 milligram
of silver in 1 kilogram of water. Since water weighs 1 gram per
milliliter, this is equivalent to 1 milligram of silver per liter
of water.
The problem with measuring silver ions goes back to the what the TDS
meter actually measures - electrical current (conductivity). In a
solution,
electrical current (charge) is carried by ions moving through the
solution, and current is actually ions moving per time. It
doesn't matter whether it is a silver ion, or a sodium ion etc.
However,
not all ions are equal, each kind of ion has a different weight.
For example, a single sodium ion weighs 23, while a silver ion weighs
108, or 4.7 times as much as a sodium ion. Yet, the TDS meter
cannot distinguish between the two.
Another source of error is that ionic silver is not just silver, it is
silver oxide, or silver chloride or some other silver salt.
Therefore, the ppm of the colloidal silver will have a different value
for the same amount of silver depending on the silver salt
present. It is not possible to just have silver ions, there
will always be a matching anion present, like hydroxide, chloride,
nitrate, etc.
Most TDS meters are calibrated to read a mix of ions normally found in
ground and drinking water. This is done by adjusting the
circuitry to get a specific reading while the TDS meter electrodes are
immersed in a standardized solution of salt. The salt
solution is just used as a repeatable reference, and the TDS meter is
not
calibrated to read the actual ppm of the salt solution.
The result of these considerations is that the only TDS reading that is
accurate for silver is ZERO, and any other reading will be
incorrect. A TDS meter is not very useful for measuring silver,
but is useful for measuring the purity of the water used to make
colloidal silver.
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