This is not particularly surprising, although I have no first-hand experience with this system. Barium citrate is very insoluble, and the stability constant of the barium-citrate complex is quite low. Under acidic conditions you would expect to have an acid salt rather than Ba3(citrate)2, and unfortunately I can't find anything relevant about the acid barium citrates; however, it is possible that the acid salt has a low solubility as well (like BaHPO4 vs. Ba3(PO4)2 - see for example solubility data tabulated in Lange's handbook).

So it's likely that what's left at the bottom of your flask is either barium citrate (or barium hydrogen citrate) or barium nitrate grains coated with a precipitated barium citrate.

To understand better what is happening, you could try to make a saturated aqueous barium nitrate solution and add concentrated lemon juice to it. It's likely that you'll see precipitation. (For a "cleaner" experiment add a concentrated citric acid solution instead of lemon juice.)

Thank you for your response But I m working on bio fuel using lemon juice what I am getting in the bottom of flask is very vry small amount of white powder my confusion is to continue the spl gel synthesis or wht to do m confused

Dear Sushant S. Kakati

It looks like you'll have to figure out what the precipitate is. If it's a "very very small amount" (compared to the added barium nitrate, I assume), maybe it's due not to citric acid but to a different lower-concentration component of lemon juice, such as malic acid or ascorbic acid.

You can filter your precipitate, rinse it with a little deionized water, and send it out for analysis. Elemental analysis is quick and inexpensive and will give you at least a partial answer.

But maybe you can just "live" with formation of the precipitate, as long as you figure out the right conditions for filtering it out.