I am working on biosorption of arsenic. I made arsenic stock solution by dissolving 1.320g arsenic trioxide with 4g NaOH and making up the volume to 1000 mL with distilled water. 2% nitric acid was added in the distilled water to preserve the stock solution. The dilutions were further made with 2% nitric acid. But when the biosorbent was added and the solution was agitated, it turned to yellow colour. Could this be due to a possible reaction of biosorbent functional group (mainly proteins) with nitric acid?
Ive used a similar stock solution about 8 months after i created it and i acted as it should. It was kept refrigerated for most of that time.
The yellow colour is either NO2 or xanthoprotein resulting from the reaction of nitric acid with proteins. Stock solutions are generally fine in the lab fridge for several months if they are preserved with 2% nitric acid.
preparation of fresh standard solutions is a good practice. Because accuracy of your data mostly calibrated with standards. preparation of standard solution depends upon the concentration of arsenic. Arsenic is a very sensitive issue in drinking water and it should be given more response.
IF THE STANDARDS are refrigerated then also it is better that before use it should be brought to the room temperature and then again standardized to see the variation in the concentration.
If you used 4 grams of NaOH in your preparation then you didn't come anywhere close to ending up in 2% nitric. Check your pH. It is important that you keep the As as the trioxide since the bioavailability of the trioxide and the pentoxide are radically different.
We normally put in our stock solution 6 months in ambient temperature (25 C)
Extra note for my answer: We prepared intermediate std, 10 ppm and 1 ppm, we put that solution for 6 months, but working standards for As (5-25 ppb) are prepared daily
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