Maybe, you have got too concentrate a plant extract solution or it isn`t clear. Try dissolve and  filter through the filter paper a plant extract.

Hello Lucy, I also think that your samples could be very concentrate. In my lab, we use another proportions: 15 µl of extract, 240 µl destilled water, 15 µd folin and 30 µl ·         Na2CO3 4N, all these in a 96 well plate just like you and read at 725 nm. We base our metodology in Swain, T. y Hillis, W.E. 1959. and Singleton et al., 1999. Good luck!

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Article Hepatoprotective Activity of Guazoma Tomentosa Leaf Extracts...

Thank you so much, i will try to dilute my samples and see the reaction.

Dulce, whats the concentration of your Na2co3.

 My leaf samples have been freeze dried, then i add 3ml of 80% methanol to 50mg sample, put in a roller then centrifuge. i use the methanol solution for analysis of  phenolics. 

try this one

Thank you

Hi Lucy i use Na2CO3 4N, and disolve 100 mg of dried sample in 1 mL of methanol 80%.

Consider a reaction with Folin-Ciocalteu reagent (FCR). You can easily find the protocol in the web.

GOOD LUCK.