We have leached ashes of some plant samples to determine the Ni concentration in the leachate using sulfuric acid. In our method, we have used two concentrations of sulfuric acid (1M and 5M) and three pulp densities (10g/L, 100 g/L and 200 g/L). However, I observed a white precipitate in the filtered samples of the with leachate 5M sulfuric acid, after few days. I tried adding few drops of conc. HCl to dissolve it, but it did not disappear.
What could have caused the formation of this precipitate? Is there any method to prevent it?
I really appreciate your valuable suggestions to improve this.
Thank you
It could be a number of things depending on the type of plants the ash came from.
Some plants accumulate a significant amount of silicon which will precipitate as silica from sulphuric acid.
It could also be from calcium which precipitates as calcium sulphate. This is the more likely cause.
I share Richard Symes' opinion. Plants incorporate silica as a supporting structure. When incinerated, the SiO2 remains as an insoluble residue. You can test if it is silica by adding some 5M NaOH solution. When the precipitate dissolves, soluble sodium silicate, also called water glass, is formed. You can do a micro test. Take some crumbs in a petri dish and add a drop of NaOH solution. Best viewed under stereo microscope.
If the precipitate is calcium sulfate, i.e. gypsum or anhydrite, it will slowly dissolve in the water. You can also assess this test under the stereo microscope if you take a crumb and add a drop of deionized water. At the edges of the crumb the precipitate first dissolves if it is gypsum or anhydrite.
Dear Pubudi Dilshara Please do recommend my answer if helpful
When plant ash is leached with sulfuric acid, a common white-colored precipitate that can form is likely to be insoluble sulfate salts. One possible compound is calcium sulfate (CaSO₄), which is only sparingly soluble in water. If calcium ions (Ca²⁺) are present in the plant ash, they can react with sulfate ions (SO₄²⁻) from the sulfuric acid to form calcium sulfate. The precipitation of calcium sulfate would result in a white solid.
The balanced chemical equation for the reaction between calcium ions and sulfate ions is:
{Ca}^{2+} + SO}_4^{2-} rightarrow {CaSO}_4
Apart from calcium sulfate, other white precipitates may form depending on the specific composition of the plant ash, including other metal sulfates or insoluble compounds containing sulfate ions. The identification of the precise compound would require a detailed chemical analysis of the plant ash and the resulting precipitate.
Richard Symes, Frank Haubrich and Tobias Makuochukwu Onyia thank you very much for the answers. It is really supportive. I will develop my experiments considering your comments.
Thank you.
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