Sodium bicarbonate gives a white precipitate with a difference of atmospheric pressure. This effect is sometimes used in barometers.

Sodium bicarbonate gives a white precipitate with a difference of atmospheric pressure. This effect is sometimes used in barometers.

The sodium carbonate solution generally used in this assay is quite saturated; I have had substantial crystal formation if the solution has sat around for a while. I also have noticed a white precipitate from time to time in my spec tubes. Did you filter the sodium carbonate? The AOAC method suggests filtering the solution after it sits overnight, and then adding a few grains of sodium carbonate to prevent crystallization. 

I also advice to filtrate tyebsolution in order to separate the formed white precipitate.

Good luck

thank's every one !!!

@erin davis: No!!  i did not filter the sodium cabonate this time, but i will do it in the next tests ....

so this precipitate does not affect my results ???

Tell me which extract have you used for assay.

As for the precipitate affecting your results, I don't know, but I would redo that sample set to be on the safe side... 

This is what the method says to do for the sodium carbonate solution: 

Dissolve 200 g anhydrous sodium carbonate in 800 ml water and bring to a boil. After cooling, add a few crystals of sodium carbonate and let sit 24 hr at room temperature. Filter through Whatman no. 1 filter paper and add water to 1 liter. Store indefinitely at room temperature.

@ Sonla S Patel

i used hexane-chloroform- ethyl acetate-butanol  and water

strangely the ethyl acetate is the only extract that does not show the precipitate, and it's the extract that contain the highest rate of Polyphenols ????

@Erin Davis

thank's alot for the advice !!!

In my assays I have used 7.5% anhydrous sodium carbonate and never had a problem with any kind of precipitations.

i have also used 7.5% anhydrous sodium carbonate !!!!

If you prepare all dilution for reagents in methanol and avoid aqueous phase for dilution and reagents preparation for all fractions except water .

Dilution of phenol reagent in methanol for all except water 

Same problem appeared to me especially with alchol extracts when sodium carbonate 7% was prepared a day before the test and stored over night at 4c. I think this problem can be avoided when sodium carbonate is freshly prepared and well solubilized. Best of luck. 

Generally, we use solution freshly prepared and well dissolved by vortex and we'd never got this problem.

order of addition of the reagents to the extract samples makes a great difference, u should add folin ciocalteu reagent first to the sample extract, stay around 2 mins then add sod. bicarbonate.

u'll avoid precipitaion.

@yorsa, I did it the same way as you described i.e., I have added FC before Na2Co3 stayed for around 2 min then add the 35% Na2Co3 but still I'm getting a lots of precipitation.

I have used 10 microgram/ml concentration of Gallic acid and 1:1 dilution of FC, I don't know where I'm going wrong, would you please suggest me a right method