In this post by Jack Dugan of Jenco Instruments, we provide background information on conductivity, resistivity, TDS and salinity.

Background

Conductivity, resistivity, totally dissolved solids (TDS) and salinity are all measurements of the alternating electrical current (AC) of liquid.  These parameters are measured by analytical instruments commonly called electrical conductivity meters, EC meters, or just conductivity meters.  Conductivity meters measure the alternating electrical current conducted between two or four electrical conducting plates of a conductivity electrode and convert the resulting conductivity reading into resistivity, TDS and salinity.  Some conductivity meters display only a single variable while others are combination meters and may display in several formats.

Conductivity

Conductivity is a gross, indirect measurement of the concentration of ions present in a liquid solution.  The unit of measure most commonly used for conductivity is the Siemen. Micro-Siemens, uS/cm, (10-6 ) is Micro-Siemens per cubic centimeter.  Milli-Siemens, mS/cm, (10-3 ) is Milli-Siemens per cubic centimeter and for highly conductive solutions there is the Siemen.  Conductivity is measured without regard to temperature.  Compensated Conductivity is measured at 25° Celsius or some other selected reference temperature.  Conductivity = 1/Resistivity.

Resistivity

Resistivity’s unit is the Ohm. Resistivity is the inverse of conductivity.  Resistivity = 1/Conductivity.The normal unit of Resistivity is the mega ohm, abbreviated M? = 1,000,000 ?.1uS = 1u ohm and 1mS =1m ohm.  Resistivity is an older unit of measure generally superseded by the Siemen but is often used for measuring deionized water especially for compliance with the regulations of governmental agencies.

TDS

TDS or totally dissolved solids is a measurement of the total dry mass of dissolved solids in water. TDS is measured by grams per liter, g/L and is based on compensated conductivity where a temperature coefficient is selected and a TDS calculated coefficient is selected based on the types of salts in the solution.

Salinity

Salinity is a measurement of dissolved salt and is measured in ppt, parts per thousand or ppm, parts per million.  Salinity is based on the conductivity and the temperature.  In general ppm times a factor of two equals the conductivity for common salts like NaCL.  Example: 1000 ppm = 2000 uS/cm.

Welcome to our inaugural blog post!  This week we take a detailed look at the origin of the pH meter and the technical innovations of the earliest pH meters.  A special thanks to Jack Dugan of Jenco Instruments for putting together this informative post.

The history of pH is ancient

For millennium people knew that there were sour foods such as vinegar and lemons and other foods like egg whites, baking soda and lime that were not sour and were slippery. However it was not until the modern era that scientist began to understand why this was so. In the early 1800’s an Irish chemist Robert Boyle coined the terms acids and bases to define these two different types of substances and invented litmus paper. In the late 1800’s a Swedish scientist Svante Arrhenius proposed that water can dissolve compounds into Ions and proposed that acids in water release Hydrogen Ions, H+ and bases release Hydroxide Ions, OH-. The pH scale was devised in 1909 by a Dr. Soren Sorensen while working in the Carlsberg laboratories in Denmark as an aid to making beer more consistently. As to the meaning of pH the H stands for Hydrogen and the “p” is generally considered to mean “Power” though on this point there is some controversy, hence pH is the power of Hydrogen. At pH 7.00 there is a balance between Hydrogen Ions, H+ and Hydroxide Ions, OH- so pH 7.00 is pH Neutral.

The first pH meter

The first pH meter is attributed to Arnold Beckman who in 1934 produced what is considered the first proto type of an actual practical pH meter. However the first pH glass electrode was made by the collaboration of two men Frits Haber and Zygmunt Klemensiewicz in 1906. In order to use their glass pH electrode they had to overcome the great electrical resistance of the glass which is a natural insulator similar to ceramic insulators. This required elaborate apparatus and a galvanic measuring device that was not practical except for laboratory use.

Arnold Beckman solved this problem by applying the use of a vacuum tube amplifier to the glass electrode and electrochemical cell and added batteries and calibrations dials and measuring gauges and packaged it all in a nice wooden box about the size of a men’s hat box. Beckman named it the Model “G” pH Meter, perhaps adapting a marketing method of Henry Ford and his famous Model “T”, and began marketing the Model “G” in 1936. It remained in production with improvements to the 1950’s

Modern day pH meters

Today there are many manufacturers of pH meters using glass electrodes but all are based on the same science. There is an external pH electrode or combination pH electrode with built in reference that outputs a very small voltage signal into a high impedance, high gain amplifier with the necessary calibration dials or microprocessor chip and either an analog dial or a digital display. All pH meters are basically a relatively inexpensive Digital Voltage Meter, DVM, with an Algorithm to convert the voltage signal to the pH logarithmic scale or in a pH/mV, (mille-volt), meter you can also read pH or ORP, Oxidation Reduction Potential, directly as a voltage.

Jenco Instruments introduced the world’s first small, economical digital pH and pH/mV, (mille-volt), meters with discrete integrated circuits in the early 1970’s and marketed them through the now famous Cole-Parmer Catalog Company. Today, Jenco Instruments still quietly serves the market by producing the best quality pH meters at reasonable prices.