[herminatorr7]

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!

[Arkcon]

Many things could be happening, but it will be hard to get to the bottom of it.  Briefly, you will have to restart your samples.  Some insights:

Prepare your samples with care.  Build all of them at once, and consider making them by serial dilution:  Back the biggest one first, and dilute it to make the next, and then again a needed.  You will not gain accuracy, but your precision may improve and you'll get a better line.

Prepare your other reagents carefully.  Make enough for all samples, and even more, so they'll be more easy to prepare.

In general terms, you might not be able to see colors.  If they're visible colored, they may be too dark for the instrument to detect.

[herminatorr7]

Thank you for your advice.

I hope that the problem is that I prepared the sodium hypobromite the day before I did spectrophotometry ...
Otherwise did I prepare the reagents right?

Do you know what kind of reaction should happen when I mix them together?