1,10-PHENANTHROLINE METHOD FOR Fe(II) AND TOTAL Fe

by

Joseph W. Stucki and Fabienne Favre

University of Illinois

 

A.     REFERENCES

B.     REAGENTS

 

1. 10 % (w/w) 1,10-phenanthroline monohydrate in 95 % ethanol (denoted hereafter as phen) (Sigma Chemical, melting point 90-100 ^oC, 6-9 % H₂O; other vendors’ products have proven unsatisfactory) and placed in a 100-250 mL dispensing bottle set at 2 mL
2. 3.6 N sulfuric acid (H₂SO₄) in 250-500 mL dispensing bottle set at 12 mL
3. 48 % hydrofluoric acid (HF), kept in hood in plastic dispensing bottle (HF dissolves glass)
4. 5 % (w/w) boric acid (H₃BO₄) in water in a 1-2 L dispensing bottle
5. 1 % (w/w) sodium citrate dihydrate (Na₃C₆H₅O₇.2H₂O) in water
6. Ferrous ammonium sulfate (Fe(NH₄)₂(SO₄)₂.6H₂O) salt for standards
7. High-purity water (preferably with a resistance of 18 megohm or more) for all dilutions and solutions

 

C.    EQUIPMENT

 

1. Brinkmann Dosimat dilutor
2. Varian Cary 5 UV-VIS spectrophotometer.
3. High-intensity mercury vapor lamp.
4. Hot plate.
5. Pan for boiling water on hot plate and rust-proof holding rack for digestion tubes.
6. Digestion tubes: either 100-mL polypropylene centrifuge tubes or 50-mL polycarbonate Oak Ridge-type tubes
7. 125-mL Pyrex Erlenmeyer flasks
8. Balance with at least 0.0001 g resolution
9. Automatic solution dispenser set at 12 mL with 250-500 mL bottle filled with 3.6 M H₂SO₄
10. Automatic solution dispenser set at 2 mL with 100-250 mL bottle filled with 10% phen solution
11. 1-mL plastic pipet for transfer of 48% HF
12. 10-mL pipet for H₃BO₄

 

D.    PROCEDURE

 

1. Turn on hot plate in hood; add water to about 3-4 inches depth.
2. Use at least five digestion tubes for standards, including one blank and four containing appropriate amounts of Fe(NH₄)₂(SO₄)₂*.6H₂O (approximately 7 mg/ppm Fe in final dilution if using 100 mL digestion tubes; or 3.08 mg/ppm Fe desired in final dilution if using 40 mL tubes; valid standard curve range is 0-10 ppm); and one tube for each sample. Place tubes in rack.
3. Record the weight of each empty tube.
4. Add standard or sample quantitatively to each tube, and record the combined weight of tube + sample (or standard).
5. Check all reagents for proper amounts. Be sure that all volume adjustments are proper. Pump each at least once into waste beaker.
6. Turn off room lights. Turn on red lamps at both counters and at balance.
7. To each tube add 12 mL H₂SO₄, being sure to wash down any sample that may be clinging to the tube wall; followed immediately by 2 mL 10% phen. A precision of ± 1-5 % in these transfers is sufficient.
8. Place rack with tubes in hood and add 1 mL 48 % HF.
9. Place rack with tubes in boiling water bath for 30 min.
10. While boiling, set up on a moveable cart two Erlenmeyer flasks for each digestion tube, and cut parafilm for flasks and tubes.
11. Remove samples from boiling water bath and allow to cool for 15 min.
12. While cooling, prepare the UV-visible spectrophotometer.
13. Add 10 mL H₃BO₄ to each tube.
14. Dilute all tubes to approximately equal volumes with water, to within about ½ inch of the rim.
15. Weigh each tube on the balance and record the weight of the tube + sample + solution. Be sure to correct for the weight of the tube holder on the balance pan.
16. Cover each tube with parafilm and invert 3 times, being sure the solution is well mixed.
17. Using the Dosimat automatic dilutor, transfer one aliquot <update calibration> of each sample to each of two flasks and dilute to final volume (21.88648 g).

a.      Fill the Dosimat bottle with 1 % Na-citrate solution.

b.      On the Dosimat keyboard, press <recall> 1, then press <Go>. Dil 2 2.000 mL should appear on the Dosimar display.

c.      Clean the pipet tip with water and wipe dry with a Chimwipe.

a.      Fill a 50-mL beaker with water for rising and cleaning the pipet tip between samples.

b.      Insert the pipet tip into the water, press <Go>. When Dil 2 appears on the Dosimat display, move the pipet tip over a discard beaker and press <Go> once more. The entire solution (about 22 mL) should then be expelled into the beaker.

c.      Rinse and wipe the tip.

d.      Be sure Dil 1 2.0000 mL is displayed, then place pipet tip into the first digestion tube. Press <Go>. When Dil 2 appears on the display, move the pipet tip to the first flask and press <Go> again. Cover the flask with parafilm. Repeat this process, transferring another aliquot from the first digestion tube to the second flask. Rinse pipet tip and dry with Chimwipe.

e.      Repeat step g for each digestion tube.

1. Cover sample tubes and set aside as a precaution in the event a flask is ruined.
2. Turn off lights in room where the UV-visible spectrophotometer is located. Turn on red lamp over the instrument. Transfer samples to this room.
3. Invert each flask 3 times, sip a portion into the spectrophotometer flow cell, and record the absorbance as per UV-Visible Data Collection procedures.
4. Replace the parafilm covers on each flask, then place all flasks on the glass shelf above the Hg vapor lamp in room W-316. Turn on the lamp and leave for 2 hrs. [Comment: in the light reduction step, one should be aware that the UV lamp efficiency may change with time. After many hours the power of the light declines and one should be able either to change the lamp or to adapt the exposure time. One way to control the lamp efficiency is, from time to time, to prepare a set of 5 samples of known total Fe(III) content and expose them to the lamp for different times and see if the total Fe(III) is recovered.]
5. Re-measure the absorbance at 510 nm.
6. Calculate results as per UV-Visible Data Collection procedures.