Dear Luis,

The Gold Mine I work for is using Bacterial oxidation. We use the same titration method that you have suggested and find it to be very effective for Fe(II). We start with 5mL of sample at pH from 1 to 2 and add 10mL of H3PO4:H2SO4 (1:1). We use 0.439 g/L Potassium Dichromate so that each mL of dichromate is equivalent to 0.1 g/L Fe(II). Diphenylamine sulfonate is the indicator.

We use a slightly different method for total Fe after reduiction of Fe(III) with Stannous Chloride. We cannot use Zinc metal as there is arsenic present from oxidation of arsenopyrite. Zinc would react with this and produce toxic arsine gas. If you want that method I can attach it also.

Determination of the concentration of Fe2+ and Fe3+ in a sample (by titration with potassium dichromate in acidic medium and indicator sodium diphenylaminesulphonate)

Potassium dichromate can oxidise Fe2+ to Fe3+ in acidic conditions. Zinc can reduce Fe3+ to Fe2+. If a mixture of Fe2+ and Fe3+ is treated with excess zincFe3+ is reduced to Fe2+. After the excess zinc is removed by filtration, all the Fe2+ can be oxidised back to Fe3+ by titration with potassium dichromate. This can be used to calculate the total amount of iron in a sample.It can therefore be used to determine the amount of Fe2+ in a sample.

The total amount of iron in the sample is supposed to be about 0.15moldm-3.

Procedure

1.         Prepare a 25.0 cm3 sample of the iron (II) salt for titration. Add 1 cm3 of indicator and 25 cm3 of 1 moldm-3 sulphuric acid.

2.         Fill the burette with 0.02 moldm-3 potassium dichromate.

3.Titrate until three concordant results have been obtained.

4. Pour about 100 cm3 of the iron solution into a beaker and add an excess of zinc powder.

       Stir the mixture vigorously for five minutes, and filter the solution to remove the excess zinc.

 Use the filtrate to prepare a 25.0 cm3 sample of the reduced iron (II) salt for titration. Add 1 cm3 of indicator and 25 cm3 of 1 moldm-3 sulphuric acid.

Repeat steps 2 and 3.

Analysis

1.Use the first titration to determine the concentration of Fe2+ in the sample.

2.Use the second titration to determine the total iron concentration in the sample.

3.Hence deduce the concentration of Fe3+ in the sample

4.         Estimate the apparatus error in the experiment.

I would be a bit concerned about the selectivity and sensitivity of such a method. Is it not likely that the Cr(VI) would also oxidise other reducing matter such as organic matter (similar to to COD analysis).

I like to suggest a colorimetric analysis based on Ferrozine. It is both very specific and sensitive.

Article The ferrozine method revisited: Fe(II)/Fe(III) determination...

Generally I agree that the spectroscopic analysis has to be more sensible than the titration in case, taken into account that S.D.(standard deviation) of experiment correlate with the aleatory errors: subjective determination of end point, personal level of working technique, etc, but the elevated number of good trials tend to diminish it.

I attached the doc. file with detailed procedure of titration (using HCl SnCl2 solution, HgCl2 solution as redactors).

You can choose to perform the determinations in our case by two ways, work out the errors and get more definite data.

I send you some references to check:

A kinetic spectrophotometric method that requires no prior measurement of rate constants is developed for the simultaneous determination of iron(II) and iron(III). The method is based on the aerial oxidation of iron(II) in the presence of tiron and acetate ions. The iron(III) formed is subsequently complexed with tiron and the absorbance/time relation is evaluated.

http://dx.doi.org/10.1016/S0003-2670(00)85235-6

Anal. Chim Acta, vol.181, 1986, Pages 203–209

R. Kuroda, T. Nara and K. Oguma

Analyst, 1988, vol. 113, 1557

Simultaneous Determination of Iron(ll1) and Total Iron by FlowInjection Analysis Using Kinetic Spectrophotometry With Tiron

http://pubs.rsc.org/en/content/articlepdf/1988/an/an9881301557

J.F. van Staden, L.G. Kluever, Fres. J. Anal. Chem. 362 (1998) 319.

http://download.springer.com/static/pdf/111/art%253A10.1007%252Fs002160051081.pdf?auth66=1415977730_908e2195520fef04af7f231fb3fe3e7d&ext=.pdf

Dear Luis,

The Gold Mine I work for is using Bacterial oxidation. We use the same titration method that you have suggested and find it to be very effective for Fe(II). We start with 5mL of sample at pH from 1 to 2 and add 10mL of H3PO4:H2SO4 (1:1). We use 0.439 g/L Potassium Dichromate so that each mL of dichromate is equivalent to 0.1 g/L Fe(II). Diphenylamine sulfonate is the indicator.

We use a slightly different method for total Fe after reduiction of Fe(III) with Stannous Chloride. We cannot use Zinc metal as there is arsenic present from oxidation of arsenopyrite. Zinc would react with this and produce toxic arsine gas. If you want that method I can attach it also.

Krassimira, I agree with your method, quite a straightforward reaction but not sure why you will reduce the Fe3+ to Fe2+ before the titration when the point is to calculate the concentration of Fe2+ in sample. You can also use potassium manganate (VII) which does not require an indicator. 

Hello Aaron

The method with potassium dichromate (note the oxidized form of the iron does not react with K2Cr2O7) and reductor comprises more issues than simple Fe2+ determination, and validate total unknown Fe concentration in ore sample. The method permits monitoring the potential Fe2+ oxidation by air during the experiment comparing as well the Fe3+ concentration that was determined in each trial titration. (The reduction step must be done one trial at the time and the titration of that sample has to be performed immediately after the reduction).

By this method can obtain more widespread data for the iron content of ore sample and diminish the possible experimental errors on Fe2+/Fe3+ quantification.

The reductant’ selection will depend of expected ore sample composition.

Hello 

I suggest the complexation with Ferrozine.

If you are interest I can sen you the article.

sincerely yours.

Henrik is right. The problem is not with titration of Fe(II) with dichromate - especially in the presence of indicators - but the selectivity.  If you use a bioleachate, other reducing agents will be reacted with dichromate in the acidic medium - it is true, slower than Fe(II).   The presence of Fe(II) will not disturb the reaction of Fe(II) and dichromate, especially in the presence of phosphoric acid.

Hello Luis

Your question was correlated with potassium dichromate method of determination of fe2+/fe3+ in sample with unknown composition (chemical or biochemical?) based on unidentified bioleaching process that you performed. For example, the last could be conducted in two reactors (chemical and bio-reactor) separately. However I attached 2 articles that disputed the pro- and contra- of different methods for iron determination and think they are pertinent and attractive for your research.

1"Comparative study of alternative methods for the simultaneous determination of Fe+3 and Fe+2 in leaching solutions and in acid mine drainages"

Minerals Engineering 18 (2005) 1116–1119

C. Paipa, M. Mateo, I. Godoy, E. Poblete, M.I. Toral, T. Vargas

2 "Limitations of the ferrozine method for quantitative assay of mineral systems for ferrous and total iron"

Geochimica et Cosmochimica Acta 72 (2008) 5001–5008

Alexandre S. Anastacio, Brittany Harris, Hae-In Yoo, Jose´ Domingos Fabris,

Joseph W. Stucki

Thanks you for all us.

Hi all

I would like to know is this methode effective for determination of fe2+ and fe3+ in iron ore? if yes please let me know about the process

thanks

sorry, help me please!

I don't understand how to determine the amount of Fe2+ by use potassium dichromate

I want to method, please. 

Thanks you