[pickaxes]

I am trying to determine the concentration of iron in a sample I prepared. Using ferrous ammonium sulfate (FW 392.16g, 0.01% ferric iron), I dissolved 1g of the ferrous ammonium sulfate into 100mL of RO water. Then I adjusted the pH to be below 2 with nitric acid. This was then my prepared sample.

For testing iron content in my prepared sample I followed the following procedure:

• Make standard curves with values of 0, .25, .5, 1, 2, and 5ppm.
  Dilute 1mL of 1000ppm stock Fe solution to 100mL using DI water in a volumetric flask to make a 10ppm Fe solution.
  The curve value of 0 is just 50mL of DI water measured using a volumetric flask.  Transfer to a 100mL beaker
  For curve value of .25, dilute 1.25mL of the 10ppm Fe solution to 50mL using DI water in a volumetric flask and transfer to a 100mL beaker.
  For curve value of .5, dilute 2.5mL of the 10ppm Fe solution to 50mL using DI water in a volumetric flask and so on for the rest of the values transferring all into 100mL beakers.
  For water samples, dilute 25mL of sample to 50mL using DI water in a volumetric flask and transfer to a 100mL beaker.  For soil samples, dilute 5mL of sample (set up in steps 7.1-7.4 above) to 50mL using DI water in a volumetric flask and transfer to a 100mL beaker.
  Add 1mL NH2OH-HCl.
  Add 2mLconcentrated HCl. (DO NOT use a pipette to add concentrated acid.  Use a plastic dropper instead.)
  Boil solutions on hot plate until only 15-20mL are left in each beaker.  Remove and let cool.
    Add 10mL of NH4C2H3O2.
    Add 4mL of phenanthroline.
    Dilute to 50mL using DI water in a volumetric flask.
    Allow color to develop for 15 minutes.
    Read in spectrometer at 510nm.

To calculate my expected concentration from this sample I simply multiplied the formula weight or ferrous ammonium by the content of ferrous iron (392.16*0.01%). Then I determined my sample concentration by plugging in the absorbance value into the regression equation from the prepared curve.

Is this correct? What should the concentration of iron in my prepared sample have been?

[Borek]

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