Electrolytic conductance

• Conductivity is an extremely useful measurement in a variety of • Industries • Commonly we think of conductivity as a measure of water ‘purity’ although conductivity alone cannot determine this • It’s also used as a safety measure in the fuel industry • Static electricity may be gained by fuel through movement or air friction (Aircraft) where the conductivity must be extremely low to avoid the potential of fuel ignition

Any substance that produce ions when dissolved in a solvent (usually water) is an electrolyte

• Strong electrolyte • Weak electrolyte • Non-electrolyte

• Strong electrolytes are substances that only exist as ions in solution • They completely dissociate to their ions when dissolved in solution • Ionic compounds are typically strong electrolytes • Strong acids, strong bases and salts are strong electrolytes • They conduct electricity when in aqueous solution Example: Hydrochloric acid, Sodium chloride HCl + H2O → H3O+ + Cl − NaCl + H2O → Na+ + Cl−

• A weak electrolyte only partially dissociates in solution and produces relatively few ions (exist in water as a mixture of individual ions and in contact molecules) • Weak acids and weak bases are weak electrolytes • They conduct electricity weakly Example: Acetic acid, ammonia CH3COOH + H2O → CH3COO- + H+ NH3 + H2O → NH4+ + OH-

• A non-electrolyte does not dissociate at all (present entirely as intact molecules) in solution and therefore does not produce any ions • Non-electrolytes are typically polar covalent substances that do dissolve in water as molecules instead of ions • They do not conduct electricity at all Example: Sugar C12H22O11 + H2O → C12H22011

• Occurs when a voltage is applied, ions of the electrolyte move and electric current flows through the electrolytic solution • This power of the electrolyte to conduct electricity is known as conductance or conductivity

Ohms Law

• It states that the current flowing through a conductor (wire) is directly proportional to the potential difference across it V = IR V = applied potential (V) I = current measured (A) R = solution resistance (Ω) between the two electrodes

• The increase in the [ions] present in the solution, the lower the solution resistance, R, will be • A strong electrolyte like KCl or NaCl dissolve in water, the no. of ions per unit volume increases and the solution resistance, R, is lowered • Thus increasing the current measured for a particular applied potential

• Conductance is the property of an object dependent of its amount/mass or physical shape and size • While conductivity is the inherent property of the material that makes up the object • No matter how the object changes in terms of shape/size/mass, as long as it is made of the same material and the temperature remains the same, its conductivity does not change

• The reciprocal of solution resistance (1/R) is called Conductance, G • Conductance is expressed as Siemens (S) or ohm-1 (Ω-1) G = 1 / R = KA / L Where, A = surface area of each electrode l = distance between electrodes κ = conductivity

• Values of conductivity,κ, increases with T and concentration • The conductivity of a solution of water is highly dependent on its concentration of dissolved salts and sometimes other chemical species which tend to ionize in the solution

As an indicator of how salt free or impurity free the sample is; the purer the water, the lower the conductivity

It is defined as the conducting power of all the ions produced by dissolving one mole of an electrolyte in solution

• The conductivity of a solution is measured in a cell called conductance cell or conductivity cell • K = 1 / R ( L / A ) • Since l and A are difficult to measure, the usual procedure is to treat L /A as a cell constant, Kcell thus : K = 1 /R ( Kcell ) = GKcell

The conductance of all the ions produced by one gram equivalent of an electrolyte in a given solution

• Λeq = 1000 x K (specific conductance ) / c [or Normality] = (Ω-1 cm-1) • The Λeq increase with increase in dilution, but eventually becomes constant • According to definitions : Conductance = Specific conductance (K) = Equivalent conductance (Λ) • In general (if the soln. is diluted), Λ = K x V, where V is the volume (mL) containing 1g equivalent of the electrolyte

N = Weight of solute in gram / Equivalent mass x Volume in litre N x Eq. wt = Molarity x Molar mass N = Molarity x Valency N = Molarity x NU,ber of H+ or OH- ions

• The resistance of the solution between two parallel electrodes is determined by using it • To this day, the Wheatstone bridge remains the most sensitive and accurate method for precisely measuring resistance values

• Nature of the electrolyte • Concentration of the solution • Temperature • Migration of ions

• The higher the concentration of ions in solution, the higher its conductivity will be • Whether a material is a strong electrolyte (e.g. potassium nitrate, KNO3), a weak electrolyte (e.g. acetic acid, CH3COOH) or a non-electrolyte (e.g. alcohol, oil) the concentration of ions in solution will be different in each case • Strong electrolytes form ions easily • Weak electrolytes do not form ions easily and • Non-electrolytes do not form ions in solution eg ) When you workout and sweat heavily, you feel tired because your body loses electrolytes like sodium and potassium These need to be replaced to maintain the concentration of electrolytes in your bodily fluids That is why, energy and sports drinks have a high electrolyte content in them

• Electricity is carried out through the solution of an electrolyte by migration of ions Therefore : 1) Ions move toward oppositely charged electrodes at different speeds 2) Concentration of the electrolyte changes around the electrode due to difference in the speed of the ions 3) Loss of concentration around any electrode is proportional to the speed of the ion that moves away from the electrode, • So, loss around anode/Loss around cathode = Speed of cation/Speed of anion • The relation is valid only when the discharged ions do not react with atoms of the electrodes

• An increase in temperature results in an increase in conductivity, but depending on the specific ions in solution and their concentration, the extent of the temperature effect will vary dramatically • The temperature effect varies dramatically depending both on temperature and quality of the water • Just 1 ppm (1,000 ppb) change in salt concentration changes the temperature compensation slope from approximately 5%/°C for pure water to 2.5%/°C for 1 ppm NaCl • Therefore, it is critical that all conductivity readings be referenced to a particular temperature, typically 25° C