Introduction

Most analytical techniques used for rocks and minerals cannot distinguish between the oxidation states of different chemical elements, including iron. Because of its abundance, the redox state of iron can be particularly important. The iron in common rocks is in the Fe2+ and Fe3+ states, usually present in minerals as the nominal chemical components FeO and Fe2O3, respectively. The analytical procedure described here permits the analysis of FeO in a sample by titration.

How it works

  1. The powdered sample is dissolved with HF at room temperature in a plastic container. Dissolution occurs in the presence of excess V5+. V5+ oxidizes the Fe2+ to Fe3+ as it is released from the minerals, while reducing the vanadium to V4+. Oxidation of Fe2+ by air is rendered unimportant, and fortunately the reoxidation of V4+ back to V5+ by air is very slow.
  2. V5+ remaining after dissolution is titrated using a standardized Fe2+ solution.
  3. Initial moles V5+ - final moles V5+ = moles Fe2+ in sample.

This procedure can easily yield accuracy and precision of ~1%, but it is subject to interference from other reduced species such as Mn2+ and sulfide. This method will not dissolve some Fe-bearing minerals such as chromite. Consult a geochemical methods text for a detailed discussion of analytical concerns. Though not especially complicated, this procedure involves the usual calculations involving volumes, weights, molecular weights, and moles. All of the calculations are given on this spreadsheet.

 

One warning is that the titration is done using live HF. You absolutely must take all reasonable precautions including wearing safety clothing, goggles or a face mask, and you must remember to keep the hood fan turned on. This method is designed for approximately 40-50 titrations.

 

Table 1. Atomic and formula weights.
Compound Molecular weight

Notes
H 1.00794 Atomic weight
N 14.0067 Atomic weight
O 15.9994 Atomic weight
S 32.066 Atomic weight
K 39.0983 Atomic weight
V 50.9415 Atomic weight
Cr 51.996 Atomic weight
Fe 55.847 Atomic weight
FeO 71.8464 Nominal Fe2+ component in minerals
NH4VO3 116.97816 V5+ oxidant during sample dissolution
Fe(NH4)2(SO4)2•6H2O 392.1428 Fe2+ titrating solution
K2Cr2O7 294.1844 Standard for calibrating the Fe2+ titrating solution

 

The following table summarizes the different solutions and materials needed.

 

Table 2. The solutions needed.
What's needed Material Amount Units Notes

Fe2+ in rock

 Rock sample 0.200 g Sample weight around which these calculations are made
FeO maximum 15 % FeO in rocks is rarely higher than this. Adjust sample weight, on the line above, as necessary for your own samples.
FeO present 0.0300 g Typical maximum 15%
Fe2+ present 0.0233 g -
Moles Fe to oxidize 0.000418 moles -
V5+ solution Moles V5+ needed 0.00060 moles ~50% excess over highest expected Fe2+ concentration
Solution volume 0.002 liters Add 2 ml of solution to the sample
Solution concentration 0.3 molar Required V5+ concentration
NH4VO3 35.1 g/liter -
Fe2+ solution Ferrous solution 25 ml Titrating solution
Fe2+ needed 0.00065 moles With a burette volume of 25 ml, this gives a slight excess of Fe2+ titrant over the maximum possible amount of V5+ that must be titrated.
Solution molarity 0.026 moles/liter -
Fe(NH4)2(SO4)2•6H2O 10.20 g/liter -
Dichromate solution Fe2+ in burette (Fe2+ solution from above) 0.00065 moles Titrating solution
Dichromate needed 0.0000975 moles Moles dichromate is 1/6 that of moles Fe2+, because Fe2+ = Fe3+ + 1e-, compared to 2Cr6+ + 6e- = 2Cr3+ (note that the 2 in 2Cr is the di- in dichromate). The amount of dichromate is needed is actually 10% less, to assure that the 25 ml of Fe2+ solution in the burette can reach the end point.
Dichromate needed 0.0287 g -

Analytical procedure

Analysis takes two or three days. On the first day sample dissolution is started. On the second or third day the titration is done. Later on, calculations can be done. The following three tables give the complete procedure. In general, it is best to process all samples through each step before proceeding to the next step.

Day 1

Table 3. Procedures that can be done on the first day.
Step Do these things Notes
H2SO4 solution Fill a 2 liter plastic bottle 3/4 full with deionized water. Put it in a cold water bath (e.g., the sink). To help keep transition metals in solution and not as solid fluorides.
Add to the bottle 535 ml (~980 grams) of concentrated H2SO4 (~5 M) Exact amount not critical.
Let the bottle cool, then mix, cool again, and dilute with DI water to fill the bottle. Exact concentration not critical. Do not use a volumetric flask.
V5+ solution Fill a 100 ml volumetric flask half full with the H2SO4 solution. -
Add 3.51 g of NH4VO3 powder. -
Swirl till dissolved, then dilute to volume with the H2SO4 solution. -
Digesting sample Weigh 0.200 +/-0.001 g sample into a clean polystyrene vial Record weight to the full precision of the balance. In addition to the samples, make at least 3 blanks (no sample in the vials).
Add 2 ml of the V5+ solution, swirl to mix Powder must be dispersed before HF dissolution begins, otherwise fluoride salts cement the mass into a lump.
Add 4 ml of HF, swirl to mix Dissolution now begins. Replace the vial cap. Let stand for 1-2 days in the dark (under a box, for example; we don't want any photo-oxidation now, do we?).
Redox indicator solution In 100 ml of water add 0.2 g of Ba-diphenylamine sulfonate. Swirl to dissolve. May take awhile to dissolve. Volumetric flask not necessary.

Day 2 or 3 (depending on how long digestion takes)

Table 4. These procedures should be done on the day of the analysis, which should not follow the start of dissolution by more than 2 days.
Step Do these things Notes
Dichromate solution Dry the K2Cr2O7 powder at 150°C To remove traces of adsorbed water.
Weigh 0.0287 +/-0.001 g of potassium dichromate into a polystyrene vial Record the weight of the powder to the precision limit of the balance. This is Value A used below.
Add 2 ml of the H2SO4 solution. -
Fe2+ solution Fill a 1000 ml volumetric flask half full with the H2SO4 solution. -
Add 10.2 g of Fe(NH4)2(SO4)2•6H2O powder, swirl to dissolve. -
Fill to volume with the H2SO4 solution. -
Calibrate the Fe2+ solution Fill the burette with Fe2+ solution. Be sure beforehand that the burette is clean and well-rinsed. The burette tip should be covered with a plastic tube to avoid etching the glass.
Add 0.5 ml of the indicator solution to the dichromate solution -
Add a stirring bar and start the stirring Stirring bar should be clean.
Titrate the chromate standard solution to the end point The orange dichromate solution gradually turns brownish, then to greenish as Cr3+ replaces Cr6+. As it becomes green, suddenly the dark blue color of the indicator starts to show. The end point is a light blue color, probably appearing blue-green. The volume of titrant used is Value D, used below.
Titrate the blanks and samples Fill the burette with ferrous sulfate solution Be sure beforehand that the burette is clean and well-rinsed. The burette tip should be covered with a plastic tube.
Add 0.5 ml of the indicator solution to the sample or blank solution -
Add a stirring bar and start the stirring Stirring bar should be clean.
Titrate the solution to the end point Light blue end point. The volumes of titrant used is recorded for each sample, are are the Values F used below.

Calculations

Table 5. Calculations after analyses are complete. Again, this spreadsheet has the calculations built in.
Calculation Item Values Units Notes
Calibration calculations Weight dichromate A g From Day 2 or 3 tasks
Moles dichromate weighed B = A/294.1844 moles Calculation
moles Fe2+ in titrant iron used C = B*6 moles 6 e- released for each dichromate reduced
Volume Fe2+ titrant solution used to titrate dichromate solution D ml From Day 2 tasks
moles Fe2+ per ml of solution E = C/D moles/ml Calculation
Initial blank and sample calculations Volume of Fe2+ titrant solution used, different for each blank and sample F ml From Day 2 tasks, value different for each sample and blank
moles Fe2+ in titrant used G = F*E moles Each sample and blank has its own G result
Calculate the average blank Average of moles Fe2+ in titrant for all blanks H moles Calculation using all blank values of G above
Calculate the amount of FeO in the samples moles Fe2+ in titrant used G moles Fe2+ in titrant consumed for a sample From above for each sample (not blanks).
moles Fe2+ in sample J = G-H moles Amount of Fe2+ actually in the sample (= V5+ reduced), blank corrected.
Grams iron oxidized K = J*55.847 g moles Fe2+ to grams Fe2+
Grams FeO oxidized L = K*1.2865 g Grams Fe2+ to grams FeO
% FeO in sample M = 100*L/sample weight weight % Weight % FeO in sample

 

Kurt Hollocher

Geology Department

Union College

Schenectady, NY  12308

U.S.A.