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Electrolyte Solution

In order to understand the complicated ideas about the study of electrolyte solutions we need to explore the kinetic entities in solution, the introduction of a dissolved electrolyte opens up the avenue of many solute entities like ions, solvated or un-solvated electrostatically associated groups of ions and covalently bound molecules and complex ions.

The concept of weak and strong electrolyte is more relevant if the associated and non-associated electrolytes are discussed as well.

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What is an Electrolyte?

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Electrolyte solutions are solutions that can conduct electricity and the colligative properties like lowering of vapour pressure, depression of freezing point, elevation of the boiling point and osmotic pressure all depend on the number of individual particles present in solution. For some of the solutes it is found that the number of particles actually present in solution is higher than what is expected from the formula given in compound.

The self-ionisation of water which result in $H_{3}O^{+}$ (aq) and $OH^{-}$ (aq) always observed in aqueous electrolyte solution. The measured resistance or conductance is a composite quantity made up from the contribution of the ions of electrolyte and the $H_{3}O^{+}$ (aq) and $OH^{-}$ (aq). These can be obtained by measuring the resistance or conductance of $H_{2}O$ (l) used while making the electrolyte solutions.

The conductivity is subtracted from the conductivities found for each electrolyte solution.
  • In the study of electrolyte solutions two types of solute can be distinguished
  • The number of particles present is an integral number of times the number of particles expected on the basis of stoichiometric unit. This ratio does not change with change in concentration
  • The number is greater than that corresponding to the stoichiometric unit which are much less than the values found in category ( ratio of actual number of particles to the stoichiometric number of stoichiometric units increase dramatically with decrease in concentration.

How to Make Electrolyte Solution?

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Based on the basic characteristics of the solutes and solvent some basic factors are used to create the ground rules which helps in make electrolyte solutions.

The electrolytic solution can be made based on the following basic characteristics which are as follows:
  • Molecular units: non conducting normal colligative properties, x ray structures showing discrete molecular units in the solid
  • Molecular units plus ions: weakly conducting, colligative properties showing slightly more than the expected numbers of particles present, x ray structures showing discrete molecular units in the solid
  • Ions: highly conducting, colligative properties, considerably greater than expected, x ray structure showing a giant ionic lattice.
Solutes in solution:
Molecular units are called non-electrolyte
Molecular units plus ions are called weak electrolytes
Ions only are called strong electrolytes \ygyk

Electrolyte Solution for Batteries

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Batteries like the conventional lithium ion based the Lithium ions are formed at one electrode ion the considered battery and injected into the electrolyte at electrode / electrolyte interface. These Li+ ions are removed from the electrolyte at the other electrode and allowed to enter the electrode through the electrolyte / electrode interface.

Both the ions are transported inside the electrolyte under the influence of an electrical field through migration. The influence of convection is neglected while the ionic conductivity of the non-aqueous electrolyte used in Li ion batteries is relatively low as opposed to the electrolyte used in other forms like Nickel cadmium batteries. The total current that flows through electrolyte is the sum of the diffusion and migration contribution of both ion types as defined for chemical domain.

The electrical field inside the electrolyte cannot be neglected in Lithium ion model as it could be done in Nickel cadmium model. The electrolyte solutions are essential for all kinds of batteries as they help in transferring the respective ions from one electrode to another and also help in the working of these units.

In case we are using zinc electrodes in copper zinc batteries, the zinc ions move into the solution and at the same time anions from the other copper cell into the zinc unit to compensate the charge and finally neutralize the effects of these ions.

$Zn(s) \rightarrow Zn^{2+}(aq) + 2 e^{-}$

Solution of electrolyte is always required in batteries, even in dry cells. The simplest battery consists of two electrodes. The figure here illustrates a copper-zinc battery. The left hand is a zinc electrode. The zinc atoms have a tendency to become ions leaving the electrons behind.
$Cu^{2+}(aq) + 2 e^{-} \rightarrow Cu(s)$

In case of dry cells, these electrolytic solutions are replaced by fused paste in order to prevent the leakage from the dry cell units.

Electrolyte Solution Examples

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For any electrolytic solution the two main categories are strong and weak electrolytes. For a strong electrolyte the resultants are strong acids, strong bases and corresponding salts. For any weak electrolyte the resultants are weak acids and weak bases.

Amongst strong electrolytes, the strong acids could be hydrogen chloride (HCl), Hydrogen bromide (HBr), Hydrogen sulphate $(H_{2}SO_{4})$, Nitric acid $(HNO_{3})$, $HClO_{3}$ etc.

The strong bases are sodium hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH), barium hydroxide Ba $(OH)_{2}$ etc.

Amongst the weak electrolytes, the weak acids are hydrogen fluoride (HF), acetic acid $(CH_{3}COOH)$, carbonic acid $(H_{2}CO_{3})$, phosphoric acid $(H_{3}PO_{4})$ etc. The weak bases are ammonia solution, pyridine etc.

For any weak electrolyte the partially dissociation into ions at moderate concentration increase the fraction of ions as the concentration decrease. For any strong electrolyte the ions present show no significant amount of molecular species present and the molar conductivity is independent of concentration.

Electrolyte Solution Experiment

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Prior to the electrolyte experiment the mass M and inner volume V of the empty column are determined. The sample amount $m_{(AR)}$ of counter ion matric AR form the ion exchanger (A is the counter ion and R is the matrix). The ion exchanger is covered with distilled water and water is periodically run through to total swelling of the ion exchanger while the operation was carried out at a constant volume of the system or rather the ion exchanger solution.

Once the equilibrium is achieved the column is weighed again and the mass of water is calculated. The ion exchanger was alternatively taken to equilibrium either with water or electrolyte solutions changing for each experiment concentration and also for the type of electrolyte that is considered.

For each of these experiments the effective volume is always equal to $\frac{Q}{c}$ and is calculated for the electrolyte in case its concentration is equal to concentration ā€˜cā€™ of the external solution, while the volume is basically the similitude of the retention volume in the liquid.
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