Antibiotic sensitivity test for clinical pathogen << Back

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Introduction:

In any biotechnology, biochemistry and life science laboratory it is essential to maintain standard and stocks of solution of various reagents. The correct storage of such solution is vital for accurate results.

Solute: It is any substances, which dissolves in liquid.

Solvent: It is any liquid in which a solute dissolves.

Solution: Any solvent becomes a solution after dissolving a solute.

Sodium chloride +distilled water = sodium chloride solution

Reagent:

According to British standards solution are classified as

1.    Reagent Solution: which have approximate concentration

2.    Standard solution: which have accurately known concentration of some chemical

Methods for expressing the concentration of a solution

Concentration of solution refers to the amount of solute present in the given quantity of solution or solvent. The concentration of the solution may be expressed in the any of following ways.

Percent solution: It may be defined as amount of solute dissolved in 100 units of total solution. There are 3 types of percent solution.

1.    Weight per volume percent

2.    Mass percentage

3.    Volume percentage

1. Weight per volume percent (w/v): It is defined as mass of solute (in grams) dissolved per 100 ml of total solution

Example: 10% NaOH means 10g of sodium hydroxide pellet in water and make final volume 100 ml.

2. Volume percentage: It may be defined as number of parts by volume of solute per hundred parts by volume of solution.

Example: 25% ethyl alcohol solution will be prepared by taking 25 ml of ethyl alcohol and 75 ml water.

3. Mass percentage: It may be defined as the number of parts by mass of solute per hundred parts by mass of solution.

                  Mass percentage= Mass of solute x100

                                                    Total mass of solution

Example: 5% sugar solution means that 100 g of solution contain 5g of sugar.

Molarity (M): It is defined as the number of gram moles solute dissolved per liter of the solution.

Example: 1M NaOH means dissolve 40g (molecular weight) of NaOH in 1000 ml D/W.

Molarity of liquid compound is calculated using following                   

Molarity = Molecular weightx100

                 Specific gravity of liquid x %purity

Normality (N): It is defined as the number of gram equivalents of the solute dissolved per liter of the solution.

N=Mass of solute in grams per liter

   Gram equivalent mass of solute

i.e. such solution contain one gram equivalent of solutes per liter of solution 

Normality of solution is always found in reference to acid or base

Example: 1M NaOH means dissolve 40g (molecular weight) of NaOH in 1000 ml D/W.

Both normality and molarity of solution are temperature dependent

Molality (m): Molality of solution may be defined as the number of grams moles of the solute dissolved in 1000g of solvent.

It is expressed in units of moles per kilogram (mol-Kg-1). The molality of a solution does not change with temperature.

Parts per million (ppm): It may be defined as the number of parts by mass of solute per million parts by mass of the solution.

                   ppm= Mass of solute x106

                               Mass of solution

This mode of concentration is generally used to express hardness of water or concentration of Cl2 in public supply of potable water

Stock solution: Stock of a substance is the one having a concentration many fold higher than actually required in the experiment. Stock solutions are prepared for frequently used solutions and which are stable at higher concentration for several days and can be used after appropriate dilution just before use.

Diluted solution can be prepared by stock solution using formula

    N1V1=N2V2

Where N1 = Concentration of the stock solution

            V1 = Volume of the stock solution

            N2 = Concentration of solution to be prepared

            V2 =Volume of the required solution

 

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