Dear Abass,

In BP or in another Pharmacopoeia, you will not get any equation or linear equation. Because it depends upon the range you selected and that range based upon your expected lower concentration and higher concentration in your samples or formulations.

As per my opinion, you should construct a calibration curve and from that you can find out your linear equation i.e., y = mx + c, where y will be you absorbance, c is constant, m is slope and x is concentration.

Otherwise, you can try this in some articles, but in pharmacopoeia you will not get it.

I hope it will help you.

Thank you

Dear Abass,

In addition to what was mentioned by Lalit,, please note the following:

1-The analysis of Gentamycin by BP is microbiological assay:

http://www.uspbpep.com/bp2008/data/6886.asp

2-The analysis of Gentamycin by USP is by HPLC method:

https://www.myantec.com/downloads/notes/pharma/217_030_03%20-%20Gentamicin%20according%20EP%20and%20USP.pdf

3-Recent studies have demonstrated an efficient method for the analysis of Gentamycin by complexing the antibacterial agent with ninhydrin:

http://www.japsonline.com/admin/php/uploads/1741_pdf.pdf

4-It should be emphasized that Gentamycin potency is declared in international units and this potency might changed depending on the source of the antibacterial agent. Therefore, when performing spectroscopic determination a calibration curve is a necessary step.

Hoping this will be helpful,

Rafik

Dear  Abass

I am agree with Lalit purpose. You need to calibrate your experiment . I am going t purpose you 2 approaches.

1-You can prepare increasing concentration of gentamicin and determine respective Optical density in the objective to establish a standard curve and mathematic expression. 

2-The solution  can be set up using the equation of Beer 's Laws, which simply says that the ratio of the concentrations is proportional to the ratio of absorbances. 

C1 / C2 = A1 / A2. Beer's law is expressed as Absorbance = e L c. 

Example 1:  A1 = 0.37; A2= 0.43 & C2=0.14M  you can determine that C1 is 0.12M

Example2: To find the concentration for a solution that has an absorbance of 0.60, you will first need to find the slope of the BEST-FIT line. From the slope of the best-fit line together with the absorbance, you can now calculate the concentration for that solution (i.e. Concentration = Absorbance / Slope)

Cordialy

Marius