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This article throws light upon the three expressions of concentration. The three expressions of concentration are:
(1) Molarity (2) Molality and (3) Normality.
1. Molarity:
Molarity is defined as the number of moles of solute per litre of solution and is denoted by M. This is the most common expression for the solutions of the solids with known molecular weight. Molarity usually describes solutions of accurate concentrations; solutes are weighed on a analytical balance and solution volumes are measured in volumetric flasks. A more common concentration range for solutions in biochemistry is milimolarity, denoted by mM.
2. Molality:
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Molality is defined as the number of moles of solute per 1000 grams of solvent and is denoted by m. Molality can be more precise than molarity, since both solute and solvent are weighed. (A 1.00 millimolal solution contains 1.00 millimole of solute dissolved in 1000 grams of solvent.)
3. Normality:
Normality is defined as the number of gram-equivalent weights of solute per litre of solution, and is denoted by N. this designation is useful for solutions of acids and bases. One gram-equivalent weight of acid is the quantity of the acid that can donate one mole of hydrogen ions to base; one gram-equivalent of the base is that amount which can accept one mole of hydrogen ions. For a monoprotic acid such as acetic acid, which yields one mole of hydrogen ions upon complete dissociation, the gram-equivalent weight is equal to the molecular weight.
Fumaric acid, a diprotic acid, yields two moles of protons when reacted completely with a base.
The gram-equivalent weight of Fumaric acid is half the molecular weight.
The gram-equivalent weight of an oxidizing or a reducing agent in an oxidation-reduction (redox) reaction is the amount of compound that will transfer one mole of electrons. In the reduction of nicotinamide adenine dinuceotide (NAD+), two moles of electrons are transferred per mole of NAD–. Therefore, gram-equivalent weight of NAD+ is half its molecular weight.
Expression of concentration as percentage of solute is frequently for liquids and solids of undetermined molecular weights. Three types of percentage of solute designations are used: volume per volume (v/v), weight per volume (w/v), and weight per weight (w/w). For example, a solution with 5.5 ml of liquid solute in 100 ml of solution would be 5.5% (v/v) solution, and 5.5 g of solid solute in 100 g of solution is 5.5% (w/w) solution. The most commonly used is weight per volume, in which 5.5 g of solid solute is dissolved in 100 ml of solvent, is a 5.5% (w/v) solution.
Making Dilutions:
Experimental conditions frequently require the dilution of an extract or a standard solution. There are several conventions for indicating the method of dilution. For example, “1-to-5 dilution” can indicate two cases.
First, one part of the original solution could be diluted with four part of solvent to give a final diluted volume of five parts. This dilution is 1/5, since the concentration of the diluted solution is 1/5 of the concentration of the original solution.
However, “1-to-5 dilution” could also indicate that one part of the original solution is diluted with five parts of the solvent to give a solution with 1 /6 of the original concentration. The former convention appears to be more suitable and is more frequently used because the dilution factor is immediately apparent. In this case it is best to read “dilute 1:5” as “dilute one part solution with solvent to give five parts of total volume”.
Serial dilution involves the systemic dilution of an original solution in fixed steps, such as 1:2, 1:4, 1:8, 1:16 or 1:10, 1:100, 1:1000. This type of dilution is used in immunology and microbiology. Serial dilutions require careful attention and accuracy of measurement because errors made in an early part of the process will be carried out to all subsequent dilutions.
Standard Solutions:
Many experiments require the use of a standard solution containing a precisely measured amount of a pure substance. For example, in the quantitative determination of amino acids in a protein hydrolyzate, a solution of each pure amino acid is reacted with ninhydrin and the amount of colour developed is measured and used for comparison. The standard solutions are prepared by accurately weighing each anhydrous amino acid and adding an accurately measured volume of solvent.
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Care must be taken to control all variables, such as temperature, rate of hydration of solute, and pH. The accuracy of the prepared standard should be 1%. Sometimes standard solutions of single amino acid or single carbohydrate are required at different concentrations. These can be prepared by making a stock solution from which dilutions are made.
