[Danandada]

Method:
Take approximately 50 mg extract and add 50 mg sodium hydrogencarbonate to the precisely weighed sample and dissolve in 30 ml water. If required gently heat. Place 10 ml of the solution in 100 ml RB flask add 20 m lof ironchloride solution and reflux for 2o minutes in the water bath. Add 1.0ml HCL36% and heat for another 2o minutes with frequent shaking. After cooling place the mixture into separating funnel and sahke three times with 25ml ether each time. Wash the combined ether extract two times with 15ml water each time then filter through cotton into measuring flask and dilute with ether 10 100 ml.

Carefully evaporate 1oml of the solution on the water bath to dryness and take up the residue in 10 ml of 0.5% solution (m/v)of magnesium acetate in methanol. Measure the absorption of the solution immediately at 515nm against methanol as compensating liquid. At the same time and under the same condition produce a reference solution by dissolving 100 mg Danthronin ether to 250 ml diluting 5 ml of the solution with ether to 100ml and carefully evaporating 5ml of the dilution on the water bath to dryness. Take up the residue in 10 ml of 0.5 percent solution (m/v) of magnesium acetate in methanol and continue treatment as described before.

Calculation: The calculation of the hydroxyanthracene derivative calculated as Rhein relating to the dried extract is carried out by the following formula:

355   x  A1  x  EW2
------------------------
A2  x  EW1  x (100 – TV)

How do I get this 355 factor? Could you help me?

[Arkcon]

Once in a while, people come to the Analytical Chemistry sub-forum, and ask, why this factor?  Sometimes we can figure it out from the steps, sometimes we can't, and its just a fudge factor to relate something hidden to real life.  If you have to follow the method, that you just follow it.  If you're making your own, then you could possibly make sense of it all.