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Spectrophotometry - determination of arginine

  1. Apr 8, 2014 #1
    Hi everyone!

    I'm making a research and I want to figure out how much L-arginine is in a capsule, which is said to contain 0,5 grams of L-arginine. Basically I want to check if the nutrition values on the lebel of the product are right.

    I decided to try with spectrophotometric determination (there is a short abstract of this method - http://www.sciencedirect.com/science/article/pii/0308814684901109).

    In the description it writes that I need thymol (0,02 % in 1N alkali), sodium hypobromite (2%: 0,68 ml of bromine in 100 ml of 5 % sodium hydroxide) and arginine solution (0,1%). After I have everything I have to put into a 5 ml standard volumetric flask an aliquot of arginine solution (0,2-2,6 ml), thymol (2 ml) and sodium hypobromite (0,1 ml) and then I mix them and diluted to the mark with distilled water. The absorbance should be measured at 440 nm. And than the amount of arginine is deduced from a standard calibration curve.

    That is what is in the description.
    - I put 0,02 g of tymol in 100 ml 1 M NaOH
    - I put 0,1 g of chemical arginine in 100 ml distilled water
    - I put 5 grams of NaOH in 100 ml distilled water and added 0,68 ml of bromine

    Than I tried to make samples for calibration curve. I made five samples, I followed the instructions. In one sample I gave 0,2 ml of arginine solution, in one 0,8 ml, in one 1,2 ml, in one 2 ml and in the last I gave 2,6 ml of arginine solution.
    I'm worried because colours in the volumetic flasks at the end didn't go from light to dark. The darkest one was the one which contained 0,8 ml od arginine. Well that makes no sense to me... And results on the spectrophotometer were disastrous. There was no line.

    Have you have any idea what could have gone wrong?
    I am a high school student so I really need some help there.
    Sorry for my English..

    Thank you!
  2. jcsd
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