Chapter 2
Volumetric Techniques
Standard Solution
The solution that is having exact and known concentration is called standard solution. Standard solutions can be prepared with the help of primary standards.
Primary Standards
Primary standards are those chemical reagents having high percent purity, stability toward air, having high molecular mass, readily solubility in the solvent, having medium cost and readily availability. A few primary standards are as follow,
· Potassium hydrogen phthalate (KHP) is used as a primary standard for standardization of NaOH
· Na2CO3 is used as a primary standard for standardization of HCl
· Na2C2O4 is used as a primary standard for standardization of KMnO4
· Zn pellets or Mg ribbons are used as a primary standard for standardization of EDTA
Standardization
Standardization is a process used for preparation of solutions of known concentrations. In this process the solutions of primary standard as well as analyte are prepared. Then known volume from standard solution with the help of pipette is taken in the titration flask. In order to locate the end point of this titration one drop of indicator solution is also added to this solution in the titration flask and it is titrated against analyte solution that is taken in the burette. The end point will be that point at which indicator shows color change and at that point titration should be stopped and the volume used from burette should be measured and then using the relationship of millimoles or milliequivalent the unknown concentration can be calculated as bellow,
Millimoles of analyte = Millimoles of primary standard solution
M1V1 = M2V
Secondary Standard Solution
Secondary standard solution is that solution used for further standardization of solution having not exact known concentration. The preparation of secondary standard solution is done with the help of primary standard solution. After standardization with primary standard then we call it secondary standard solution and can be used for further standardization. For example the NaOH solution after standardization against KHP is now a secondary standard solution and this solution can be used for standardization of HCl solution.
Calculation of pH
Acids are those chemical reagents on dissolution in water produce hydrogen ion (H+). Acids may be strong acids like HCL, H2SO4, and HNO3 or may be weak acids like acetic acid etc. The strong acid is that which completely dissociate into hydrogen ion while the weak acid is that which partially dissociate into hydrogen ion. For acidic solution its strength of acidity can be calculated in term of pH. Experimentally pH can be measured by using an instrument called pH-meter.
In mathematics p is meant negative log
So pH can be defined as negative log of hydrogen ion concentration i.e.
For strong acidic solution the pH can be calculated as
Example: Calculate the pH for 0.1M HCl solution
Solution:
As we know HCl is strong acid and it completely dissociate into hydrogen ion and produce 0.1M of H+
In this example the hydrogen ion concentration is 0.1M so putting the value in above expression As we know for strong acid pH
So putting values
Example: Calculate the pH of 0.1N H2SO4 solution.
Solution:
As we know in case of strong acid,
As H2SO4 is strong acid completely dissociate and producing 0.1N hydrogen ion concentration so putting the magnitude of hydrogen ion concentration in the above pH expression as
pH calculations for weak acids
Weak acids are those acids that dissociate partially into hydrogen ion when dissolve in water. For example acetic acid
In case of weak acid we cannot write pH as
For example acetic acid it does dissociate as below
The above reaction shows that acetic acid dissociates partially and equilibrium is established between acetic acid and hydrogen ions and acetate ion. This equilibrium condition can be expressed by equilibrium constant as
Re arranging the above expression as
As at equilibrium state the hydrogen ion concentration is equal to acetate ion concentration i.e.
So putting this acetate ion concentration in the above expression as below
Or rearranging the above expression as
As we know
Or
Example: Calculate the pH for 0.001N CH3COOH solution of 250ml volume. Ka value for acetic acid is 1.75 x 105- at 25 Co
Solution:
As we know CH3COOH is a weak acid and in case of weak acid pH can be calculated by using the expression as given below
So substituting the values in this expression
Calculation of pOH
So pOH can be defined as negative log hydroxyl ion concentration i.e.
For basic solution its strength of basicity can be measured in term of pOH. Bases are those chemicals that can produce hydroxyl ion (-OH) in water. As we know bases may be strong bases like NaOH, KOH, etc or may be weak bases like Ammonium hydroxide etc. The strong base is that which completely dissociate into hydroxyl ion while the weak base is that which partially dissociate into hydroxyl ion. For strong basic solution the pOH can be calculated as
Example: Calculate the pOH and pH for 0.01N NaOH of 250ml volume.
Solution:
As we know
So
As we know for water dissociate as below
But the amount of [ H2O] is too much remain nearly constant so we can write it as
So
Now taking negative log on both sides of the above expression
As we know
So the above expression can be written as
As we know Kw value for water is 1 x 10-14 at 25 Co
So
So for this calculation
Calculate the pH of 0.01N NH4OH solution its Kb is 1.75 x 10-5 at 25C0.
Ans:
As we know
pOH = -log [ OH-]
But
In case of Weak base
Substituting the values in above expression
[ OH-] = 4.18 x 10-4 M
pOH = -log [ OH-]
So
pOH = -log [4.18 x 10-4]
pOH = 3.31
But for pH calculation
Rearranging the above expression
Buffer solutions
Buffer solutions are those solutions when they are added to some other solution they resist to change in pH if small amount of strong acid or strong base is added to that solution. Or
We can say the buffer solution has the property to control the pH of that solutions to which it has been added.
Chemical Composition of a buffer solution
Buffer solution are chemically made of either weak acid or weak base and the salt derived from that weak acid, or weak base e.g. buffer solution of CH3COOH/CH3COONa and the other one NH3OH/NH4Cl
Buffer action
The mechanism how the buffer solution control the pH is that as we know buffer solution containing two components one is weak acid or weak base and the other is its salt.
For example if an acidic buffer solution has been added a few ml of strong acid, so it will react with the salt component of the buffer solution and will change it back to weak acid so its effect on the pH change will be less and negligible i.e.
If few ml of strong base like NaOH has been added to this buffer solution so that will react with the acidic component of the buffer solution and will produce salt that is again neutral solution i.e.
So we can see that the effect of strong acid or strong base that has been added to buffer solution has been neutralized either by reacting with salt component or with acidic or basic component of that buffer solution.
Criteria for Selection a buffer solution of a required pH
In order to prepare buffer solution for a required pH range, it is required that we should select either a buffer mixture of weak acid or weak base and their pKa or pKb values should be having values very close to the required pH range in that we want to have control the pH of a solution. For example we need to control pH of a solution near to 5 pH. As we know this pH range is on acidic side so first we should select weak acid and its salt for preparation of this buffer mixture. As acetic acid is having pKa value i.e. pKa = 4.76, so we will select acetic acid and sodium salt mixture for preparation of this buffer solution of pH 5.
Calculation the pH of a buffer solution
The pH of a buffer solution can be calculated by using Handerson Hasselbalch equation i.e.
In case of acidic buffer solution the Handerson Hasselbalch equation is written as
And for basic buffer solution the Handerson Hasselbalch equation is written as
Example: Calculate the pH of a solution that is 0.01N in CH3COOH and 0.02N in CH3COONa. Ka value for acetic acid is 1.75 x 105- at 25 Co
Solution:
This can be solved by using Handerson Hasselbalch equation. As for weak acid the Handerson Hasselbalch equation is
For acetic acid
Putting the values in the above expression
As
So
Example: Calculate the pH of a solution that is 0.200M in NH3 and 0.300M in NH4Cl. The dissociation constant vale kb for NH3 is kb= 1.75 x 10-5 at 25 Co
Solution:
This can be solved by using Handerson Hasselbalch equation. As NH3 is weak base and for basic buffer solution the Handerson Hasselbalch equation is written as
So for NH3
Putting the values in the above expression
As
So
But for calculation of pH
Rearranging the above expression
Buffer Capacity
Buffer capacity is defined as the number of moles of a strong acid or a strong base that causes the changes of pH of liter buffer solution by 1 pH unit.
So a buffer solution that is composed of concentrated mixture solution of both the components so It means its buffering capacity will be also high and will be neutralize high amount of strong acid or strong base to bring change in pH.
Titration
Volumetric techniques are also called titrations which are a group of classical quantitative analytical techniques used to determine the unknown amount or concentration of an analyte solution. Titrations are based on volume measurement therefore named as volumetric techniques.
Procedure of Titrations
Titrations involve the slow addition of one solution where the concentration is known called standard solution to a known volume of another solution where the concentration is unknown called analyte until the reaction reaches a desired level. The point at which all the amount of analyte is consumed and the amount of standard solution becomes equal to the amount of analyte solution is called the equivalence point. In order to locate the equivalent point indicators or instruments are used. The point at which the indicators give color change is called end point. At end point the volumes of both analyte and standard solutions are measured by using volume measuring equipments like pipettes and burettes. Not every titration requires an indicator. In some cases, either the reactants or the products are strongly colored and can serve as the "indicator". For example, a redox titration using potassium permanganate (pink/purple) as the titrant does not require an indicator. When the titrant is reduced, it turns colorless. After the equivalence point, there is excess titrant present. The equivalence point is identified from the first faint persisting pink color (due to an excess of permanganate) in the solution being titrated.
Calculations in Titrations
Using the relationship of millimoles or millieqvivalent the unknown amount of analyte is calculated like this
As units of amount are millimole or millieqvivalent and we know number of millimoles or number of millieqvivalent are calculated by the relationship as
And the
Putting the values in the above expression as below
Or to calculate weight or mass in grams of analyte the following relationship is used as bellow
Equipments used in titration
During titration several types of volumetric glass wares are used. They all are designed to help measure volume of a liquid.
These are volumetric flasks, burettes and pipettes. They are characterized by a high accuracy and repeatability of measurements. Volumetric flask is used to dilute original sample to known volume, so that it contains exact volume. Pipettes are used to transfer know volume of the solutions. Burettes are used to add known volume of titrant to the titrated solution and they have scale on the sides, so that you can precisely measure volume of the added solution. Burette is similar to the pipette, as it is designed to measure volume of the delivered liquid, but it can measure any volume of the solution.
Two other types of volumetric glass are graduated pipettes and graduated cylinders. These are too designed to deliver requested amount of solution and they have a scale on the side. However, their accuracy is usually much lower than the accuracy of volumetric glass described above. They are used to measure amounts of auxiliary reagents, like buffers.
Usually when measuring the volumes of transparent solution, the bottom of the concave meniscus must be precisely read on a calibration mark. To make reading of the meniscus position easier we can use piece of paper with a horizontal black stripe, about an inch and half wide. If paper is hold half an inch behind a burette with a stripe about a half an inch below meniscus, solution surface seems to be black and is much easier to see.