Preparation of Solution: Formulae

Preparation of solution: Some Important Formulae

S. No.	Formulae
1	Mass= molarity X molecular weight X volume of solution in litre
2	Mass= normality X equivalent weight X volume of solution in litre
3	Volume of desired solution in ml = molarity X molecular weight X volume of solution in ml X 100 / [1000 X specific gravity X assay(purity) ]
4	Volume of desired solution in ml = normality X equivalent weight X volume of solution in ml X 100 / [1000 X specific gravity X assay (purity) ]
5	Dilution factor = concentration of stock solution/ concentration of working solution
6	A working solution (lower concentration) can be made from a stock solution (high concentration) by using the following formula: Volume of stock solution (in ml) = (c X v ) / S where; c = concentration of working solution v = volume of working solution S = strength of stock solution
7	Conversion of a normal solution in a molar (mol/L) solution To change a normal solution into mol/L, use the formula: Mol/L = Normality of solution / valency of substance ( no of electrons gained or lost)
8	Formula to convert a percentage solution to a mol/L solution (molar solution): Mol/L = g% (w/v) solution X 10 / [molecular mass of the substance]
9	Weight of antibiotic stock solution for Minimum Inhibitory Concentration (MIC): Weight in milligram (mg) = [Volume of antibiotic stock solution in mL (V) X concentration of stock in µg / mL (C) ] [Potency (µg / mg)] i.e. W = [V X C] / P Trouble shooting: In some circumstances, the potency of the antibiotics may not be given. In such case, potency is needed to be calculated from the manufacturer's details wherein the active fraction, percentage purity (assay) and water content of the antibiotics needs to be known. After preparing the stock, series of two fold dilution can be made or the dilution can be calculated as per the required concentration of antibiotics to be poured in media. And if same amount of solution is added to the diluent, it reduces the solution of stock to its half. Note: It is very feasible to have 10X concentration solution before pouring in the media. So that when we add 18ml of media and 2 ml of antibiotics, we can get our 1X soultion. For e.g We are going to prepare 32 µ / mL of antibiotic plate then we have to make 320 µg/ mL of solution from the stock solution. In short, prepare 10X solution of antibiotics to make it 1X for MIC (2 ml antibiotics + 18 ml media). Most importantly, we will be using Mueller Hinton's Agar (MHA) for antimicrobial activity. However, you can change your ratio as per your requirement. Practical: Prepare 16µ/mL of antibiotics to be incorporated in the MHA for MIC. Hint: for making 16 µ/mL i.e 1X so prepare 10X i.e. 160 µg/ mL working from the stock solution of 320 µg/mL. Take a Finally, mix 10X solution (2mL) and 18mL media.

References

S. No.	List of References
1	Cheesbrough M., 2009, "District laoratory Practice in Tropical Countries", Cambridge University Press, (Page 40-44)