|
Many properties of liquids are determined by knowing their pH, conductivity, dissolved solids concentration and Oxidation
Reduction Potential
pH is a measurement of the concentration of Hydrogen ions in fluids or semisolids. The scale ranges from 0 to 14 with 7
being a neutral solution. Above 7 is considered a base while below 7 is considered an acid. The further away from 7, the stronger the solution. For example a pH of 2 would
be a strong acid, while a pH of 6 would be mildly acidic.
Conductivity measures a solutions ability to conduct electrical current and is generally measured in micromho or microSiemen
which are equivalent to each other.
Total Dissolved Solids (TDS) is a measure of the solids in solution and is generally expressed in Parts Per Million (PPM) or
Parts Per Thousand (PPT).
Oxidation Reduction Potential (ORP) is a measure of the ratio of the loss of electrons (oxidation) to the gain of electrons
(reduction) in a solution. It is generally expressed in millivolts.
For general use, pH testing paper may be used to detect the general range of pH. The generic name for it is litmus
paper. A small strip of the paper is generally dipped in the solution. The paper changes color and the color change is compared to a chart to interpret pH. These generally
run under $10 for each range and with several feet on the roll can be used for many checks. The smaller the range, the more accurate the result.
For more accurate or continuous testing, electric or more common, electronic units are used. There are 3 main types of
electrodes used to sense the pH. Single junction glass used for “clean” liquids, Double junction glass used for thicker (more viscous) solutions or dirty liquids and Ion
Specific Field Effect Transistor (ISFET) for dirty liquids or semisolids. The ISFET units are more durable and easier to care for than the glass bulb units. The electrical or
electronic circuitry interprets the results of the probe and displays in meter or digital readout.
The meters for sensing pH, conductivity, TDS and ORP come in a wide variety of handheld, bench top, remote probe
styles. Additionally, some come with combinations of pH, conductivity, TDS and ORP.
 Costs for the small, single function, hand held units with integral probes range from around $30 to $125 depending on
temperature compensation, and accuracy. Typical accuracies for these pH units are from +/- 0.2 ph Units to +/- 0.02, conductivity +/- 2%, TDS +/- 2%, ORP +/- 5 Millivolts.
The cost range for the more accurate portable units with replaceable probes is in the $400 to $1200 range depending
on features and accuracies.  Bench top units which are typically used for high precision in laboratories cost in the $600 to $2000 range. A high end unit is capable of evaluating pH (accurate to +/-
0.002 units), conductivity (accurate to +/- 1.5 microSiemens), TDS (accurate to +/- 0.17%) and ORP (accurate to +/- 0.1 millivolt). The high end units typically
have serial communication and analog output capabilities.
A large variety of probes are available for specific
applications. Probe or instrument manufacturers should be contacted to assure the best probe for the application. The costs range from around $100
to $350 depending on the probe construction, precision and material to be sensed (liquid, dirty liquid, semi-liquid, moist solids).
The pH reading is directly effected by the temperature of the solution. So, the
temperature of the solution must be known and compensated for by use of tables or having the compensation built into the sensing instrumentation.
Accuracies of readings are also dramatically effected by the probe condition.
They should be rinsed between uses with distilled water. Glass pH probes should ideally be stored in pH 4 or 7 buffer solution and NEVER in distilled or
deionized water which cause ions to leach out of the glass bulb and make it useless. White Potassium Chloride crystals sometimes form on the glass.
Rinse these off and blot the probe dry before use.
pH Instrument calibration requires standard, buffer solutions of known pH.
These are generally in dry form in packets which are mixed into distilled water. The typical solutions are 4, 7 and 10.
|
