I am a student in my last year of high school and currently making a research on how to find sulfites in dried fruits. I've collected data and different methods online and combined few of them into one. In my method I use hydrogen peroxide and barium chloride to create a white precipitate (result should be barium sulfite).

But the problem is that I never get any precipitate. During my research it mentioned that I was to leave the dried fruit in distilled water over night (I used dried apricots). I had 26g, 30g, 40g of dried apricot in 100mL of distilled water. Once done, I filter out all of the liquids into a different beaker. Then filter it using filter paper. Once done I separate them into different beakers to test. And this is where I find the problem, I've done 6 trials so far, and all 6 have failed. Because I don't know what amount of H2O2 or what amount of the concentration I need to put into the apricot liquid for it to properly react, or for how long I should leave it in for it to react (I've waited 3 and 5 minutes). I'm then supposed to drop a single drop of barium chloride into the solution to produce the white precipitate. But so far I haven't gotten any sign of any precipitate. I've tried different amounts of H2O2 (1mL, 5mL, 10mL, 15mL, 20mL & 25mL with a 35% concentration in the first trials but rest of the trials were with a 3% concentration). Each with a drop of barium chloride. Seeing that they did not react to a drop, I tried to use 5 drops each on the first few trials. Yet I still got no reaction.

Could I get some help or advice please?

Thank you

  • $\begingroup$ Try a spiking experiment: add a small amount of sodium sulfite to a typical apricot and distilled water sample. Process as per your protocol. See if you get a precipitate with the barium addition. If so, try it again, adding an even smaller amount of sodium sulfite. Eventually, you get to the point where no precipitate can be seen to occur. That tells you the approximate “limit of detection”. It also tells you the sulfited dried apricots do not have much sulfite content. That may be fine: the sulfite is supposed to get oxidized in order to keep the apricot from getting oxidized/stale. $\endgroup$
    – Ed V
    Nov 26 at 13:19

The colorimetric method using rosaniline and formaldehyde is more sensitive and it is well documented.

A somewhat safer version of the test (avoiding use of highly dangerous $\ce{HgCl2}$) is documented at MDPI - Open Access Publishing:

"A concentrated pararosaniline stock solution (3.088 × 10 −3 mol/L) was made by dissolving pararosaniline hydrochloride in a 10% aqueous ethanol solution. The concentrated stock solution was used to make... [a] working solution contain[ing] pararosaniline hydrochloride, hydrochloric acid (HCl) and formaldehyde in the molar ratio of 1:1255:125 respectively. For example, a working solution with PRA concentration of 4.68 × 10 −4 mol/L in 25 mL contained 3,790 μL of concentrated PRA stock (3.088 × 10 −3 mol/L), 1,439 μL of 32% HCl, 189 μL of 36% formaldehyde solution and 25 mL with water."

Pararosaniline is available from Sigma-Aldrich, though it seems a bit pricey at ~$US82/25 g; perhaps your school would obtain some for use by yourself and by other students, as you would need perhaps a gram for your experiments.


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