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I couldnt find a method for Ni in Cacao products in the AOAC.
but ther is for Tea which could be applied but of no use for a school assigment since it requires AA.
basicly wetash(wet oxidation using HNO3(69-72%) then HClO4(70-72%), caution not for the inexperianced:eek: ) a well mixed sample(either by freezeing and graiting or heating to 50C and mixing then take test sample) then intial dilution then filter and dilute to known volume then detect with AA at (232.00 nm for Ni) with matrix matched stadard solutions to avoid interferance from Na and K.
Now the APHA stadard methods for water and waste water has two chemical methods for determination of Ni these could be applied to the wet ash diltuion.
The Heptoxime Method and the Dimethylglyoxime Method.
in these methods the (water) sample is HNO3-H2SO4 digested with H2O2, so i dont know if wet ashing with HNO3 & HClO4 would be classed as the same or you would to do the extra digestion step after or just the Nitric Sulphuric digetsion instead :confused:
these are both colourmetric(heptosime and Dimethylglyoxime used for colour development, heptoxime method of choice) so you would need a spectrophotometer capable of 445 nm or you could do crude comparison by eye.
It involves treating a portion of the sample
(containg 50-250 ug of Ni) to seperate Cu and Fe then separation of Ni followed by the colour devolping step. then compared to standard curve.
If you want exact details I can post them tomorrow.