Conductivity Technical Information

Definitions

• Activity:

  The activity is a measure of the amount of ions chemically active in a concentration of the ions in solution. This means that the participation of ions in a chemical reaction is not only determined by the concentration but also by the presence of other ions in the solution. In concentrated solution the activity of the ions is considerably less than the total concentration.

• Activity Coefficient:

  The activity coefficient (F) shows the ratio between the active concentration and the total concentration.
  
  F = active concentration / total concentration

• Anions:

  Anions are negatively charged ions, e.g. CL-, NO-, OH-.

• Automatic temperature compensation:

  Automatic control of the sensitivity of the measuring converter to compensate for the reaction of the chemical equilibrium.

• Capacity:

  The capacity of a conductor is the quotient of the charge and, as a result of this charge, the generated voltage of a conductor.

• Cell constant:

  The cell constant of a conductivity measuring cell is determined by the ratio of the distance between the electrodes and the area of the electrodes.
  
  C = L / O (cm -1; m -1)

• Concentration:

  The concentration of a solution is determined by the quantity of matter dissolved per volume or per weight of the solution.

  1. Grams per dm³ (liter) or kilograms/m3:
     The weight in grams per dm³ (liter) solution.
     
     
  2. Grams per kilogram:
     The weight in grams of matter per kilogram of solution.
     
     
  3. Milligrams per kilogram:
     The weight in milligrams of matter per kilogram of solution.
     NOTE: This is commonly expressed as ppm (parts per million).
     
     
  4. Micrograms per kilogram:
     The weight in micrograms of matter per kilograms of solution.
     NOTE: This is commonly expressed as ppb (part per billion).
     
     
  5. Mol per liter or molar:
     The weight in grams corresponding with the molecular weight per liter of solution.
     
     
  6. Mol per kilograms or molal:
     The weight in grams corresponding with the molecular weight per kilograms of solution.
     
     
  7. Weight percents:
     The weight of dissolved matter per 100 grams of solution.
     
     
• Condensate:

  The name of a solution after condensation of steam.

• Current density:

  The number of the ratio between the current value and the area of the conductor.

• Dissociation of ions:

  Dissociation is separating into positive and negative ions.

• Dissociation constant:

  The number gives the ratio between the concentration of the separated ions in a matter and the concentration of the unseparated matter.
  
  AB = A / B
  
  K = [A] [B] / [AB]

• Electrolyte:

  An electrolyte is a matter that separates ions in an aqueous solution. Weak electrolytes partly dissociate, strong electrolytes dissociate almost completely.

• Equivalent conductivity:

  The equivalent conductivity of a solution is the specific conductivity of this solution when 1 kilogram equivalent/m³ or 1 gram equivalent/dm3 of the solid matter is dissolved.

• Hydration:

  Hydration is the surrounding of ions with molecules of water.

• Ion strength:

  The strength of ions in a solution is determined by both the concentrations of the ions in the solution and the nature of these ions. The strength of ions determines the activity of each ion in the solution. In an equation, the strength of ions is:
  
  I = 0.5 (C)(Z²)
  
  C = concentration
  
  Z² = square of the charge of the ion

• Polarization effect:

  Screening of the electrodes of a conductivity measuring cell, e.g. by an incomplete discharge of ions.

• Rhondinising of electrodes:

  The addition of a rhondium layer on the electrode surfaces for the increase of these surfaces.

• Scale factor (f):

  The number of the ratio between the conductivity value at the process temperature and the conductivity value at the reference temperature.

• Temperature coefficient:

  Indicates the temperature influence on the chemical equilibrium (e.g. dissociation) of the solution.

Practical Considerations For Conductivity and TDS (Total Dissolved Solids) Measurement

When using a meter to measure either the ppm of total dissolved solids or conductivity of a liquid, it is necessary to periodically calibrate the meter using a calibration standard solution. There are, however, special considerations to be given to each type of calibration. Whereas conductivity is an absolute measurement with calibrations that are transferrable from one type of solution to another, ppm total dissolved solids calibrations are specific to one type of dissolved solids solution and must not be transferred from one type of dissolved solids solution to the next. Doing this will result in some serious errors in measurement.

Although the basis for testing ppm of total dissolved solids is the conductivity of the solution, it is not correct to assume that this measurement is absolute. It is always necessary to calibrate all total dissolved solids meters with a parts per million total dissolved solids standard calibration solution that contains the same type of salts or mixtures of salts as the solution to be tested. Failure to do this will result in serious errors in the measurement of total dissolved solids. This is because total dissolved solids meters are calibrated by correlating the conductivity of the solution to the ppm dissolved solids and this correlation varies considerably from one species of dissolved solids to the next.

One similar conclusion can be made for all types of dissolved solids. Most pre-formulated parts per million total dissolved solids standard calibrated solutions are formulated with calcium carbonate (CaCO³), sodium chloride (NaCl), potassium chloride (KCl), or the 442 (40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride) natural water formulation. If your test solution's major dissolved solids components are the same as any of these, you may want to choose the pre-made formulation that best approximates your test solution. Generally, CaCO³ is used for boiler waters, NaCl is used for brines, and the 442 formulation is used for lakes, streams, wells, and boilers. Alternatively, if the contents of the ppm standard calibration solution used for calibration are known, it is possible to cross reference from existing calibration curves to curves for different types of dissolved solids solutions. Curves and tables are available in various reference books.

The previous discussion and references are based on standard conditions of temperature (25°C). When measuring conductivity or total dissolved solids in other than standard conditions, certain corrections for these variations must be accounted for before going on to determine the final values of conductivity and total dissolved solids. Without some sort of correction for standard temperature, conductivity or total dissolved solids measurements at various temperatures are meaningless because they cannot be compared. Many meters overcome this by incorporating temperature sensing elements and temperature sensing circuitry into the meter so that the value given is corrected for standard temperature. Using a meter that does not have temperature compensation will require the operator to use look-up tables or formulas to correct for the temperature effect. A good discussion of the effect of temperature on conductivity and total dissolved solids testing can be found on pages 6 and 7 in the article "Theory and Application of Electrolyte Conductivity Measurement", Copyright 1982 by the Foxboro Company.

This discussion should prove useful to all users of conductivity and dissolved solids testing procedures. It is to be considered a "rule-of-thumb" guideline for using conductivity and dissolved solids testing equipment. Fine tuning of the standard curves and formulas for your specific application is recommended. We hope this discussion helps you better understand the process.

Conductivity Measurement Tips

1. For greatest accuracy, ensure that no particulate matter is suspended in the test solution. If necessary, filter or allow particles to settle.
   
   
2. Ensure that no air bubbles are trapped in the cell when making measurements.
   
   
3. Ensure that the cell plates are completely immersed in the test solution.
   
   
4. Do not immerse the cell to the very bottom of the sample container if there is any possibility of a sedimentary layer existing there.
   
   
5. Ensure that the cell plates are thoroughly rinsed in deionized water when the cell is removed from solution. Leaving the cell immersed in deionized water is the preferred method for short-term storage.
   
   
6. Although it is not critical to store the conductivity cell with the plates in a wetted condition, if they are allowed to dry out completely, it may take some minutes for stability to be achieved upon usage as the plates become wetted.
   
   
7. Ensure that no deposits of dried salts or particulate matter are allowed to build up around the cell plates or on the body of the cell. This may produce a conductivity path lower than that through the solution.
   
   
8. Glass bodied cells must be used for solvents and strong acid or alkaline solutions.
   
   
9. Standard units and cells are calibrated to a reference temperature of 25°C.