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Listed below are some technical terms that you may find throughout our electrode pricing pages and specifications:

ACCURACY - Correctness. The closeness of a measured value to the known "true" value.

AUTOMATIC TEMPERATURE COMPENSATION - Automatic instrumental adjustment of displayed pH to correct for variations in electrode slope due to temperature changes. The output of a thermistor probe regulates the correction factor. Automatic temperature compensation normally is used only with pH and conductivity electrodes, not with specific ion electrodes, since the correction is not sufficiently accurate.

AUTOMATIC TEMPERATURE COMPENSTOR (ATC) - A thermal element imbedded either into the tip of a pH or conductivity electrode, or as a separate element (probe) that can is inserted into the fluid to compensate for temperature fluctuations. ATC's are typically connected to instrumentation (bench meters or process transmitters) via two or three-wire configuration. Consult your meter or transmitter manual for the recommended temperature compensator value if required i.e. 100 ohm, 1000 ohm, 3 Kohm, etc. If in doubt, contact us!

BLANK - A solution that is similar in composition to the sample solution but does not contain the species being measured. Blanks are run to eliminate systematic error caused by the background.

BUFFERED SOLUTION - Also known as buffer solution or buffer, it is a solution buffered with respect to a given species when that species is in equilibrium with roughly equal levels of both a free complexing agent and a 1:1 complex of the species. The activity of the buffered species tends to remain constant if the solution is diluted or concentrated, or if small amounts of the buffered species are added or removed. Buffering solutions can be made for many species besides hydrogen ion (pH). Typical however, pH buffers are available as 4.01, 7.01, and 10.01 pH, with a +/- 0.02 pH accuracy.

CALIBRATION CURVE - A plot, generally drawn on semi-logarithmic graph paper, of electrode potential versus ion level in two or more standardizing solutions. Unknown samples are determined by converting measured electrode potential to ionic activity or concentration using the curve. Other methods of plotting calibration curves include the Liter Beaker Method and Serial Dilution Method.

CATION - A positively charged ion (Na+, Ca++, NH4, etc.)

CELL CONSTANT - For a conductivity electrode, the ratio of the distance between the electrodes and the area of the electrodes. C=L/O(cm-1;m-1)

COATING - In some applications, the nature of the process stream may cause the electrode to become coated during use. Coating makes them less responsive and necessitates periodic removal from the process for cleaning. Partial coating will not affect pH measurement. Full coating can be a problem. Applications requiring frequent cleaning of buildup should merit the consideration of wet-tap style electrode holders, where the electrode can easily be removed and cleaned without the need to shut down the process.

COMBINATION ELECTRODE - A sensing electrode and reference electrode housed in one body.

CONCENTRATION - The actual amount of a substance in a given volume of the solution. When measuring ionic concentrations by electrode, a distinction is made between the concentration of the free, unbound ion, and total concentration, which includes ions bound to complexing agents.

DECOMPLEXING AGENT - A species that is added to a solution to free bound ions. The decomplexing agent is a strong complexing agent for the species binding the ions, and is added in sufficient excess to free all of the complexed ions.

DILUTION - Changing the concentration of a species in solution by addition of solvent.

DIRECT MEASUREMENT - Determination of sample concentration of activity by directly relating the electrode potential to the level of the species being measured. The level can be read from a calibration curve or the log scale of a specific ion meter. Ionic activity can be determined by direct measurement if activity standards are used, and ionic concentration if concentration standards are used. For total concentration measurements, use of reagents to free bound ions and to adjust the ionic strength is usually necessary before the direct measurement is performed.

DOUBLE JUNCTION - A term used to describe a reference electrode that has a dual salt bridge chamber (typically filled with KCl) connecting the reference (Ag/AgCl) wire to the process solution. This style of reference is used when there is a risk of poisoning from substances such as sulfides, silicones, or other silver-reacting compounds.

DRIFT - Slow changes in the potential of the electrode pair maintained in a solution of constant composition and temperature.

ELECTRODE - A general term for those parts of the electro-chemical measuring system which are in contact with the sample.

ELECTRODE CALIBRATION - Determining the relationship between the potential developed by an electrode in two or more standardizing solutions and the sample solution activity or concentration.

ELECTRODE PAIR - A sensing electrode and a reference electrode; the reference electrode may be separate or combined in one body with the sensing electrode.

ELECTROLYTE - A substance which ionizes in aqueous solution; also, a solution containing ions. Electrolyte is used to fill reference pH, ORP, and ISE electrodes, and in fact 'closes the circuit' between the measuring and reference electrodes. Electrolyte is also used in membrane type dissolved oxygen electrodes.

END POINT - The apparent equivalence point of a titration. When a titration curve is plotted on standard coordinate paper, the end point in generally assumed to be the inflection point. On Gran's paper the end point is the intersection on the horizontal axis.

EQUITRANSFERENCE - Equal diffusion rates of the positively and negatively charged ions of an electrolyte across a liquid junction.

FILLING SOLUTION - The solution inside a sensing or reference electrode which is replenished periodically. Solutions that are permanently sealed within the electrode (like the buffer inside a pH electrode) are usually called internal reference solutions to differentiate them from filling solutions. INTERNAL FILLING SOLUTION OF SENSING ELECTRODE - In liquid membrane electrodes, an aqueous internal filling solution contacts the internal reference element and the membrane, which is saturated with ion exchanger. The filling solution contains a fixed level of chloride and of the ion for which the electrode was designed; the concentration of this ion determines the zero potential point of the electrode. In addition, the filling solution is saturated with silver chloride to prevent the silver chloride of the internal reference element from dissolving. REFERENCE ELECTRODE FILLING SOLUTION - A concentrated salt solution contacting the internal reference element and the sample solution. The composition of the filling solution is chosen to maximize stability of the potentials developed at the internal reference element/filling solution interface and the filling solution/sample junction.

FLOW EFFECTS - Liquid flow can have effects ranging from stress fracture to abrasive wear on the electrode tip (with abrasive fluids). When possible, piping should be arranged so that the electrode will stay wetted even if the process liquid flow is stopped. Also of great importance is to position the electrode far enough downstream of chemical injection(s) to ensure adequate mixing before contacting the electrode.

FLOW-THRU ELECTRODES - Specific ion and reference electrodes designed for anaerobic measurements. The two electrodes are connected by plastic tubing to a syringe or pump, and the sample is pumped through the electrodes at at constant rate.

GAS SENSING ELECTRODE (POTENTIOMETRIC) - An electrode which uses a gas-permeable, ion-permeable membrane to separate the sample solution from an internal filling solution. A dissolved gaseous species diffuses through the membrane causing a change in the level of an ion in the internal filling solution. The change is detected by an internal ion-sensing element.

GLASS ELECTRODE - An electrode whose sensing element is made of a thin layer or bulb of a special glass. Glass electrodes for hydrogen ion (pH) and sodium ion show good selectivities.

GX-1 / GX-2 / GX-3 / GX-4 - Various glass types available for different pH electrode ranges and temperatures. For most applications, a GX-2 or GX-3 glass is ideal. GX-2 glass will measure pH ranges from 0-12 pH and temperatures from -5 to 100 degrees Celsius. GX-3 glass is suitable for pH levels ranging from 0 to 14 pH with a temperature range of 0 to 100 degrees Celsius.

INTERFERENCE - Any species in the sample solution that causes either a positive or negative measurement error.

INTERNAL REFERENCE ELEMENT - The second sensing element in combination and gas sensing electrodes which provides the reference potential.

IONIC STRENGTH ADJUSTOR - A solution of high ionic strength used to dilute samples and standardizing solutions before measurement. The ISA "swamps out" differences in ionic strength from solution to solution, making the activity coefficient of the ion being sensed about the same in all solutions. Multi-purpose ISA's can contain pH adjusters, decomplexing agents, or species that remove interferences. Examples of special ISA's are TISAB, for fluoride measurement, and SAOB, for sulfide measurement.

JUNCTION POTENTIAL - The portion of the total observed potential between the sensing and reference electrodes that is formed at the liquid/liquid junction between the reference filling solution and the sample solution.

KARL FISCHER TITRATION - A titration method for determining the water content of samples using a pair of platinum electrodes and a reagent containing iodine, sulfur dioxide, pyridine, and methanol. A polarizing current is applied to the electrodes and the resultant potential is measured.

KNOWN ADDITION - The determination of original sample concentration from the change in electrode potential on the addition of a known volume of reagent containing a known concentration of the ion being measured. Also known as standard addition.

LIFE-TIME - The length of time that an electrode functions usefully. Life-time of solid-state and glass electrodes is limited by mechanical failure of the electrode body or chemical attack on the sensing membrane, and can range from a few days (if the electrode continuously in hot or abrasive flowing streams), to several years under normal laboratory or potable water type applications. The life-time of liquid membrane electrodes is limited by loss of ion exchanger with use, and is generally 1-3 months. Operation can be restored by replacing the membrane, ion exchange solution, and aqueous filling solution.

LOWER LIMIT OF DETECTION - The lowest concentration of an ion that can be distinguished from zero concentration. The lower limit of detection for solid-state and liquid ion exchange electrodes is determined by the solubility of the sensing element. The lower limit for glass electrodes is determined by response to interfering ions.

MEMBRANE - A thin piece of material covering a structure or separating solutions and permitting selective transport of a chemical species between the two species. There are typically three types of membranes: liquid ion exchange membrane, gas electrode membrane, and crystal or solid-state electrode membrane.

NOISE - Abrupt, random, small changes in displayed electrode potential, usually due to the pickup of strong static charges. Noise may be caused by air bubbles, poor connections in the sensing electrode, reference electrode, or instrument, solutions containing organic solvents which are poor conductors, or high electrical resistance somewhere in the circuit. Some older pH meters/transmitters are sensitive to fluctuations in line voltage, and this may appear as noise.

pH ELECTRODE - A specific ion electrode, made of glass or other materials, that responds to hydrogen activity.

pH UNIT - A tenfold change in hydrogen activity, expressed as the negative logarithm of the hydrogen ion activity. Thus a solution of pH 7 is 10-7 M in hydrogen activity, and a pH 8 solution is 10-8 M.

POISONING - The chemical conversion of the surface of a solid-state electrode to a form that is unresponsive to changes in ion level. In many cases, electrode function can be restored by removing a thin layer of the sensing element or by reversing the poisoning reaction chemically.

PRECISION - A measure of the reproducibility of a method, when multiple measurements are made on the same sample under the same conditions. The observed values may differ from the true values without affecting the precision.


RESPONSE TIME - The length of time necessary to obtain a stable electrode potential when the electrode is removed from one solution and placed in another of different concentration. Response time depends on the electrode type, the magnitude and direction of the concentration change, temperature, and the presence of electrode interferences, if any. Under most conditions, an electrode will exhibit a value of at least 90% of the final value after one minute.

SALT BRIDGE - A technique for isolating the reference element of a reference electrode from the sample solution by use of an intermediate solution containing non-interfering ions. In the Double Junction reference electrode, the outer filling solution serves this purpose.

SELECTIVE ION ELECTRODE - A term used for specific ion electrode. It is derived from permselective, which signifies that the electrode responds only to ions of positive charge or only to ions of negative charge.

SENSING ELECTRODE - A device which develops an electrical potential in response to a chemical species in solution.

SINGLE JUNCTION - A term used to describe a reference electrode that has a single salt bridge chamber (typically filled with KCl) connecting the reference (Ag/AgCl) wire to the process solution. This style of electrode can be used in general purpose applications where there is little risk of silver reacting compounds contaminating (poisoning) the junction.

SLOPE - The angle between the horizontal axis and the line formed by plotting electrode potentials against ion level on semi-logarithmic graph paper. By analogy, slope has also come to be a measure of electrode response to the ion being detected. Theoretical Nernstian slope, which is temperature dependent, is 59.16 mV at 25 degrees Celsius for a ten-fold change in the activity of a monovalent ion and 29.58 mB for a divalent ion. Interferences and liquid electrodes requiring renewal can provide low slope. Greater than theoretical slopes are rare and indicate more than one process occurring.

SOLUBILITY - The concentration of a given species in a given solvent when the solution is saturated with respect to that species. Solubility depends of the nature of the solute (dissolved species), the nature of the solvent, temperature, and for gases, pressure. Solubility is conveniently expressed in grams per liter of water at 25 degrees Celsius.

SPECIFIC ION ELECTRODE - See also Selective Ion Electrode - A device that develops an electrical potential proportional to the logarithm of the activity of an ion in solution. The term "specific" signifies that the electrode is specified for a particular ion, and prefers it over other ions which are typically found in the sample.

STANDARDIZING - Determining the potential developed by and electrode in one standardizing solution so that potentials developed in unknown solutions can be converted to concentration by comparison. Standardization is less accurate than calibration, which uses two or more standards. It is generally used to correct for electrode drift after an initial calibration.

STANDARDIZING SOLUTION - A solution of known concentration used to calibrate or standardize an electrode. The electrode potential developed in an unknown solution is compared to the potential developed in one or more standardizing solutions in order to ascertain the concentration of the unknown.

STREAMING POTENTIAL - A potential that is generated in solutions moving at high velocity through a chamber. Liquid junction potential problems are often wrongly confused with streaming potentials.

TEMPERATURE COMPENSATOR - A thermistor type temperature sensing element, usually imbedded at the electrode tip or in the electrode housing, by which meters can compensate for temperature fluctuations during pH, Conductivity, and Dissolved Oxygen measurements. Temperature compensators are available in many values, typically measured in resistivity, to connect with various meters and transmitters. They are normally connected via 2-wire or 3-wire configuration. In some meters, and if the temperature value be does not fluctuate by a significant amount, the temperature value can be manually entered, eliminating the need for automatic compensation.

TEMPERATURE EFFECTS - Changes in potential developed between a sensing and reference electrode due to changes in temperature. The slope of a sensing electrode varies with temperature, as does the potential developed by the internal reference element in the reference electrode. In addition, the solubility of some salts changes with temperature. It is therefore advisable to measure all samples and standardizing solutions at the same temperature.

TISAB - Total Ionic Strength Adjustment Buffer - A reagent used in measuring total fluoride concentration. It contains a sodium acetate/acetic acid buffer to free fluoride bound to hydrogen, and CDTA or citrate to free fluoride from aluminum, iron, an other cations, and it fixes the fluoride ion activity coefficient by its high ionic strength.

TRACE AMOUNT - Properly speaking, an amount of a substance too low to be quantitatively measured.

WATER HARDNESS - A measure of the ability of water to form insoluble precipitates when boiled or when soap is added. Hardness is caused by the presence of Calcium, Magnesium, and Iron, and to a lesser extent, other divalent cations expressed in ppm of Calcium Carbonate. Hardness can be measured quantitatively with the water hardness electrode, which exhibits almost equal response to all divalent cations. Because most water hardness is due to Calcium and Magnesium, some definitions assume that the effect of other divalent cations can be neglected.

ZERO POTENTIAL POINT - The solution concentration at which an electrode pair develops a potential of 0 mV. In many instances, it is also the isopotential point.


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