12 Conductivity and Salinity Meter

Dr. Heena Rekhi Rekhi

  1. Description

The conductance of electricity through electrolytes is ionic in nature and through metallic conductors is electronic in nature. The difference in conductivity arises due to peculiar composition of these conductors. The free electrons in electrolytes will combine with molecules and cations present at the metal-electrolyte boundaries. Similarly the ions which move upto the metal under the effect of applied field cannot move into the metal. They will rather undergo an electron releasing or an electron accepting process.

 

Such situations lead to two types of electrolytic conduction.

 

  • The positive ions will move towards the cathode and negative ions towards the anode ; these effects being complementary to each other, sum up to give the total electrical conductance.
  • A characteristic energy of activation is required for chemical changes like other chemical reaction and these takes place only when experimental conditions are favourable for them. The activation energy is required for the exchange of electrons between the electrodes. If Inspite of the applied electric field no electrochemical changes take place then no electricity will be conducted through an electrolyte. With the increase in applied potential, the rate of electrochemical reactions enhances and new electrochemical changes induces. The potential drop takes place due to the resistances in the electrolyte and to compensate this higher potential is required. Alternating current is used to prevent significant changes in the concentration of an electrolyte and its composition during the conductance measurement instead of direct current.

 

The conductance of electricity through electrolytes and metallic conductors follow ohm’s law. According to this law, the current C, flowing is directly proportional to the electric potential applied across the electrolyte, at a given temperature V ~ C

 

Where V = electric potential applied across the electrolyte column.

 

C = electric current flowing through the electrolyte.

R = proportionally constant and is the resistance of the conductor.

Specific resistance and specific conductance:

The resistance of conductor at a given temperature is directly proportional to its length and inversely to its cross-sectional area. Thus

R ~ l/a

or R = ρ l/a

 

Where ρ is a constant and its value depends on the nature of material at specified temperature. ρ is also known as resistivity or specific resistance of the material. If l and a have unity values then specific resistance is defined as the resistance in ohms of conductor of a one cm in length and one square cm in cross section or it is also defined as the resistance between the two opposite faces of one cm cube of the conductor.

 

ρ = Ra/l

 

The reciprocal of resistance is known as conductance and the reciprocal of specific resistance or resistivity is known as specific conductance or conductivity. Thus specific conductance of a substance is defined as the conductance of one cm cube of the substance. It is generally denoted by the symbol κ (kappa). Thus κ = l/ρ

 

Equivalent Conductance and Molar Conductance:

 

The equivalent conductance of a solution is obtained by multiplying together the specific conductance of the solution and its volume in millimeters which contains 1 gram equivalent of the solute. Its value for a dilution volume v is represented by Lv.

 

Equivalent conductance, Lv = Specific conductance (κ) × volume in ml (v) per g

 

Factors effecting conductance:

 

Dilution

 

Both specific and equivalent conductance values of solution of electrolyte vary with dilution. The specific conductance values decreases as the solution are made dilutor mainly due to decrease in concentration of ions. However, the equivalent conductace of a solution increases with dilution as on dilution for every gram equivalent of electrolyte dissolved more ions are obtained. A limiting high value is eventually obtained which is called the equivalent conductance at infinite dilution or zero concentration.

 

Degree of Ionisation

 

For an electrolyte the conductance of any of its solution depends upon the concentration of the ions formed. Hence the degree of ionization of dissolved electrolyte may be obtained from the measured conductance of the solution.

 

Conductivity meter

 

The modern conductivity meter has a sophisticated electronic circuit which eliminates capacitance effect and faradic processes. It can be used for the measurement of wide range of conductivities 0.001 microsiemens per cm to 1300 microsiemens per cm. Moreover there is aprovision for automatic range switching by operation of a balancing control. The instrument is adjusted so that the constant of the cell in use is displayed on the digital meter that records the conductivity values. A platinum resistance thermometer is connected to the meter. This will automatically correct conductivities measured over a range of temperature to a value at 298 K. The clean conductivity cell is rinsed and charged with solution whose conductivity cell is to be measured. The required result is displayed on the meter.

 

Application of Conductivity meter

 

Purity

 

The purity of distilled and deionized water is commonly checked by conductimetric measurements. The conductivity of pure water is 5 × 10-8 Siemen per cm at 298 K. The smallest trace of ionic impurity leads to a large increase in conductivity. However, repeated distillation leads to constant value of conductivity.

 

Determination of Concentration

 

Solutions of strong electrolyte shows linear increase in conductance with increase in concentration for higher concentrations conductance passes through maximum (increases) because of increased hindrance produced by intraionic attractions for free motion of ions.

 

Measurement of salinity of sea water

 

Conductivity measurements are performed to determine the salinity values and give information regarding association and dissociation equilibria in aqueous solutions. The meter is calibrated against solutions of known concentration of suitable electrolyte for salinity readings.

 

Measurement of solubility of sparingly soluble salts

 

The extremely small solubility of sparingly soluble salts such as AgCl, BaSO4, PbSO4 etc. can be determined with the help of conductivity measurements. For example, the solubility(S) of AgCl can be determined by suspending the thoroughly washed precipitate of AgCl in conductivity water,warming, cooling to 298 K amd determining the specific conductivity of the filterate by means of a conductivity cell.

 

Determination of small amounts of ammonia in biological materials

 

Ammonia is removed from the sample by distillation or swept out with the current of air and absorbed in boric acid solution. Boric acid solution is used as absorbent and its small ionisation maintains the specific conductance of the solution, a linear function of the concentration of ammonium salt. The specific conductance of boric acid solution is determined and compared with standards already measured.

 

Elemental analysis

 

Hydrocarbons can be analysed for the Sulfur content by combustion of the sample followed by absorption of SO2 in water. The increase in conductance resulting from the H2SO4 that is produced can be related to the concentration of sulfur.

 

Moisture content of the wood / Ash content of cane

 

Special electrodes have been designed to determine conductometrically the moisture content of the wood. Before proceesing it into the pulp and paper. Conductometric determination of ash content of cane beet-sugar products has replaced tedious gravimetric procedure performed earlier.

 

Ion Chromatography

 

Conductivity cells can be coupled to ion chromatographic systems provide a sensitive tool for measuring the ionic concentration in the elute. Thus special micro-conductivity cells have been developed and placed in a thermostated enclosure. The sensitivity is improved by the use of a bipolar square-wave pulsed current which reduces polarization, capacitance effects and the changes in conductivity caused by heating effect of the current.

 

Salinity meter

 

Salinity is the accumulation of salt in soil and water. High salt levels can adversely affect plant growth, soil structure, water quality and infrastructure. Salinity is measure of number of grams of salts present per kilogram of sea water, which is measured in parts per thousand (ppt).

 

Salinity in ppt = grams of dissolved salts

1000 grams of sea water

 

Salinity of water and soil is measured by passing on electric current between the 2 electrodes of a salinity meter in a sample of water or soil. The electrical conductivity or EC of a soil water sample is influenced by the concentration and composition of dissolved salts. Ability of conductance of electric current through solution is increased after addition of salt. Therefore, higher value of electrical conductivity(EC) indicates higher salinity level. Determination of salinity is achieved through electrical measurement. Evaporation of the water followed by weighing the resultant residue gives the measure of inorganic dissolved matter and therefore gives salinity. As some hydrogen chloride and CO2 escape during the process of evaporation therefore it is a difficult process and some corrections must be made for this. In the past century only two major methods have been used in the oceanography for the measurement of the sea water samples salinity.There are some methods for determination of salt content of water.

 

Total dissolved solids

 

Measurement of TDS can be achieved through evaporation of known volume of water to complete dryness and therefore weighing the resultant solid residue. The units of measurement of TDS is mg/L.

 

Electrical conductivity (EC)

 

Passage of electric current between two metal electrodes suspended in the solution (water sample) and measurement of current gives the measure of electrical conductivity. The more dissolved salt in the water more will be the current flow and thus higher will be the electrical conductivity. Measurement of electrical conductivity gives estimate of TDS and is much quicker simpler as well as useful for field measurement. EC measures conductance of liquid in a measuring cell of specific dimension.

 

Light Refraction: Refractometers

 

Salts and other solids dissolved in water alter the way water refracts light that goes through it. Variations in the value of refractive index can be related to varied concentrations of solids in the water. The refractometer gives an idea about extent of salinity its units of measurement are g/L (ppt).

 

Specific Gravity: Hydrometers

 

Higher values of dissolved solids in water indicates higher values for the specific gravity or density of water. Measurement of specific gravity is carried using floating device called hydrometer which is a tube of glass with air sealed in it and is suspended in water upright due to a prominent weight at the bottom. The scale inside the tube gives indication of extent of submersion of tube in the water. However specific gravity values in the range of 1.000 to 1.070; are shown by some hydrometers . Conversion of these values into salinity is possible in the range of 0 – 100 g/l (ppt).

 

Titration of the chloride ion

 

Determination of sea water content of samples have been carried by chemical determination of content of halide ions by titration since years ago. This method also known as Mohr method (Mohr,1856) involves titrating a sample of sea water with silver nitrate solution of known concentration to the point where all halides (chloride plus a small amount of bromide) have been precipitated as silver halide, as detected by suitable indicator or electrode systems. A 15ml Knudsen pipette is used to measure sea water sample in to the titration vessel. After filling of Kundsen pipette volume is determined by rotating 3-way stop cock fitted at its upper end.

 

Boyle’s method for salinity determination by titration Determination  of  salinity  of  water  samples  by classical method was given by Robert Boyle in the Type of water sampleSalinity(ppt) early 1700. This method is based on two precipitation reactions. There occurs double displacement reaction between sodium chloride and silver nitrite resulting in precipitation of silver chloride. The idea involves the addition of silver nitrate in order to carry out the precipitation of all chloride ions present in the salt water as silver chloride. Quantitative determination of  precipitated  chloride  can  be  carried  out  by “spiking”  the  saltwater  samples  with potassium chromate. Additional reaction of potassium chromate with silver nitrate results in precipitation of red orange silver chromate. Silver ions preferentially combine with chloride ions in the presence of chromate ions. Thus, the silver chromate ppt. will disappear until all the chloride ions have been reacted. Determination of volume of silver nitrate which is required to reach the end point where firstly red orange precipitates appear and thereafter using simple proportion to determine chloride content in the sample and hence its salinity.

 

Measuring salinity in soil

 

Measurement of EC can be carried out in field or in a laboratory for a soil:water suspension (EC1:5). To measure EC1:5 in the field, put approximately 10ml of distilled water, rainwater or tank water into a jar, container or tube. Add small soil particles until the contents of the container increase by 5ml to bring the volume to 15ml. Add additional water to bring the total volume to 30ml. Shake intermittently

for five minutes and allow it to settle for five minutes. Dip an EC probe into the solution and take a reading. Washing of EC probe is required after its use. Salinity of soil gives the idea of the texture of soil which can also be interpreted from EC values. Salts are readily dissolved out of sandy soils whereas salts are more tightly held by clay soils. This means that the same amount of salt will have a greater impact on sandy soils then it will on clay soils. As a guide, sandy or loamy soils are moderately saline if EC1:5 is above 0.3 dS/m, and clay soils are moderately saline if EC1:5 is above 0.6 dS/m. As the EC1:5 is measured on a diluted sample, a more realistic measurement of the actual salt levels that a plant will encounter can be measured on a saturated extract (ECse). This can be done by some laboratories. As a guide, soils are generally considered saline if their ECse is greater than 2 – 4 dS/m.

 

Measuring salinity in water

 

Salinity in surface water and groundwater can be easily measured in the field by collecting a water sample, inserting an EC probe into the sample and reading the value shown on the meter. Alternatively, a water sample can be collected and forwarded to a laboratory for testing of salinity and chemical composition. The container should be entirely filled with the water sample to exclude air. Samples for laboratory analysis should be forwarded as quickly as possible. Delays and high temperatures will change the composition of salts in the sample, affecting the results.

 

Relationship of density-salinity

 

Figure below is the representation of the density-salinity relationship at several fixed temperatures. Salinity of any unknown sample can thus be found by measuring the density at specified temperature and extrapolating the graph the value of salinity.

Bibliography

  • B.D. Khosla; A. Gulati; V.C. Garg. Senior practical physical chemistry. r. chand & co.
  • S.I. Karaaslan; A.B. Tugrul. New approach to salinity determination & salinity dispersion along bosphorus.
  • Measuring salinity. Natural Resources and Water
  • Measurement of salinity by various experimental methods. Oceanography 3014Katzelson.
  • Conductivity/Salinity Measurement Principlesand Methods. DQM Information Paper.
  • D.A.  Skoog;  F.  J.  Holler,  T.A.  Nieman  (1998). Principles  of  Instrumental  Analysis,  5th  edition.
  • Orlando, FL: Harcourt Brace College Publishers.
  • H. Jeffery • J. Bassett • J. Mendham • R. C. Denney, Vogel’s textbook of quantitative chemical analysis fifth edition, John Wiley and sons, Newyork
  • Douglas A Skoog, Donald M, West Holler Thomson, Fundamentals of Analytical Chemistry, 8th Ed Galen W. Ewing, Instrumental Methods of Chemical Analysis
  • R.W. Murray, Analytical Chemistry is what analytical chemists do, Editorial, Anal. Chem., 66 (1994) 682A.