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Ionic Strength Formula aims at measuring the concentration of ions that is present in a solution. In general, ions are referred to as the charged particles which form several compounds or say that these charged particles are present in most of the organic compounds. This ionic strength formula takes into account multiple factors related to an ion, therefore, has faces influence on these, such as the valency or the molar concentration of the ions, etc. Further, the complete information about the Ionic strength, its formula and derivation and their applications has been elaborated here.
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Key Terms: Ionic Strength, Ions, Solution, Concentration, Dissociation Constant, Valency, Ideal Solution, Molarity, Compounds, Charged Particles
What is Ionic Strength?
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The concept of Ionic strength was postulated by Lewis and Randall in 1921. It is usually referred to as a way of measuring the concentration of ions in a solution. The compounds which are rich in ions or are made up of ions are water-soluble. After dissolving in water, these compounds dissociate into ions. And this forms the ionic strength of the solution in which it has dissolved. Thus, it could be said that ionic strength is a major characteristic feature of the solution. The electrolytic concentration in a solution affects the properties like the solubility of salts or the dissociation constant. The electrically charged ions effectively influence their behaviour of attraction or repulsion. Therefore, making it necessary to analyze the ionic strength.
Also Check: Classification of Organic Compounds
Derivation of Ionic Strength Formula
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The sum of the molar concentration of every ion in a solution to the times of its valency which is squared collectively forms the ionic strength formula.
I = 1 / 2 ∑ni = 1 ci zi2
Here,
- 1 / 2 represents the ions as cation and anion.
- c represents the concentration in molar units’ mol/L.
- z represents the charge of ions.
\(I = \frac{1}{2} \sum_{ni} = 1 c_i z_i^2\)
Ionic Strength Formula
Considerations for Ionic Strength
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There are a few solutions which are known for not exactly following the ideal behaviour and such solutions are referred to as the non-ideal solutions. For instance, there are solutions where the consideration of the interaction forces becomes important along with the application of the unit of molality (mol/kg of H2O) rather than the molarity.
Also Check: Relation between molarity and molality
Importance of Ionic Strength
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- The importance of the ionic strength is majorly seen in the theory of Debye – Huckle which takes into account to describe the strong deviations from the ideality that occurs in the ionic solutions.
- Another major importance of ionic strength is seen in the theory of double layer and related electroacoustic and electrokinetic phenomena. Here, various heterogeneous systems and colloids are taken into account.
- This simply states that the Debye length comes to be inversely proportional to that of the square root of the ionic strength.
- Thus, it could be said that the Debye length is one of the most significant characteristics of the double layer thickness.
- Moreover, it has been found that the compression of the double layer and the escalation of the electrical potential gradient can be enhanced by raising the valence or concentration of the counterions.
- The stability constant determination also involves the usage of the media of high ionic strength. This determination assists in the minimization of changes during titration at lower concentrations, that too, majorly in the activity quotient of solutes.
High & Low Ionic Strength
Things to Remember
- Ionic strength is more about the property of the solution that is influenced by the nature of ions in the solution and furthermore, other factors like valency, molar concentration, etc. also impacts the ionic strength.
- It has been found that the normal waters or say seawater and mineral water mainly have the non - negligible ionic strength.
- This non - negligible ionic strength states the presence of dissolved salts in the water and we all know that the seawater is saline.
- However, this salinity and solubility effectively impact the characteristics of the solution and its ionic strength.
- Molecular biology and biochemistry are also a few of the fields that have a majority of the usage of ionic strength.
Solved Questions
Ques. Find the ionic strength of a solution of potassium chloride 3M. (4 Marks)
Ans. Since, the dissociation = KCl → K+ + Cl -
Therefore, the concentration of each ion is the same as the concentration of the salt, 3 mol/L.
Then, the equation shall be,
I = 1/2 [(3 mol / L) (+1)2 + (3 mol / L) (-1)2] = 3 M
For, a solution of potassium chloride 1 M and magnesium sulfate of 0.2 M, the ionic strength shall be,
First, the dissociation needs to be drawn,
KCl → K+ + Cl- and MgSO4 → Mg2+ + SO42-
Now, the new equation shall be,
I = 1/2 [(1 mol / L) (+1)2 + (1 mol/L)(-1)2 + (0.2 mol/L)(+2)2 + (0.2 mol/L)(-2)2 ] = 3.6 M
Ques. What will be the ionic strength of KCl if its concentration is 2 M? (3 Marks)
Ans. Given,
| Ions | Z | C | Z2 |
|---|---|---|---|
| K+ | 1 | 2 | 1 |
| Cl– | 1 | 2 | 1 |
The ionic strength is given by,
I = 1/2 [(2 × 1) + (2 × 1)]
= 2
Ques. What is the impact of ionic strength on pH? (2 Marks)
Ans. Ionic strength has a major impact on the pH of any solution. It has been found that the pH optimum moves towards lower pH values when the ionic strength is increased. However, this only happens when z = - 1 or z = 0. On the other hand, the pH is known to remain the same if z = + 1 and simultaneously, Ph shifts to a higher value for z = +2.
Ques. Calculate the ionic strength of a solution of potassium chloride 3M. (3 Marks)
Ans. Since the dissociation will be KCL→K++Cl−.
Therefore, the concentration of each ion is the same as the concentration of the salt, 3 mol / L.
Now, on applying the equation,
I = 1/2 [(3 mol/L)(+1)2 + (3 mol/L)(-1)2] = 3 M
Hence, the ionic strength is 3 M.
Ques. Illustrate the importance of ionic strength. (3 Marks)
Ans. The importance of ionic strength is majorly seen in the theory of Debye–Huckle. This theory is mainly concerned with the strong deviations from the ideality that occurs in the ionic solutions. Another major importance of ionic strength is related to the theory of double layer and related electroacoustic and electrokinetic phenomena.
Ques. What will be the ionic strength of K2SO4, if the following are given. (3 Marks)
Ans. Given,
| Ions | Z | C | Z2 |
|---|---|---|---|
| K+ | 1 | 2 | 1 |
| SO4-2 | 2 | 1 | 4 |
The ionic strength is given by
I = 1/2 [(2 × 1) + (1 × 4)]
= 3
Ques. What will be the ionic strength of a solution containing 0.2 M NaCl an 0.2 M Na2CO3. (3 Marks)
Ans. µ = 1/2 ΣCiZi2
µ = 1 / 2 (CNa + Z2 Na+ + CCl- Z2 Cl- + CCO3 2- Z2 CO32-)
µ = 1 / 2 (0. 6 x 12 + 0. 2 x 12 + 0. 2 x 22)
= 0.8
Ans. The Debye–Hückel theory was postulated by Peter Debye and Erich Hückel. It aims at imparting the theoretical explanation for departures from ideality in solutions of electrolytes and plasmas. It is mainly a model of linearized Poisson – Boltzmann , which assumes an extremely simplified model of electrolyte solution. However, it gave accurate predictions of mean activity coefficients for ions in dilute solution.
Ques. Find the ionic strength of KCl if its concentration is 2 M? (3 Marks)
Ans. Given,
| Ions | Z | C | Z2 |
|---|---|---|---|
| K+ | 1 | 2 | 1 |
| Cl– | 1 | 2 | 1 |
The ionic strength is given by,
I = 1212 [(2 ×× 1) + (2 ×× 1)]
= 2
Ques. What is the ionic strength of a 0.1 M NaCl? (3 Marks)
Ans. µ = 1/2 ΣCiZi2
µ = 1/2 (CNa + Z2Na+ + CCl- Z2Cl-)
µ = 1/2 (0. 1 x 12 + 0. 1 x 22)
= 0. 1
Ques. Assume that αNa + and αSO42- are equal to 3Ao, each. Then, what will be the activity coefficients of Na+ and SO42- in a solution containing 0. 03 M Na2SO4? (4 Marks)
Ans. µ = 1 / 2 ΣCiZi2
µ = 1 / 2 (C Na+ Z2 Na+ + CSO42- Z2 SO42-)
µ = 1 / 2 (0. 06 x 12 + 0. 03 x 22)
= 0. 09
- log fi = (0. 51 Zi 2 µ 1 / 2) / (1 + µ 1 / 2)
For sodium ion we have
- log f Na + = (0. 51 x 12 * 0. 09 1 / 2) / (1 + 0. 09 1 / 2)
= 0. 118
f Na + = 10 -0. 118 = 0. 76
For sulfate we have
- log fSO42- = (0. 51 x 22 * 0. 091 / 2) / (1 + 0. 091 / 2) = 0.47
fSO42- = 10 -0. 47 = 0. 34
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