The origin and importance of iron

Origin and Importance of Iron

Iron (Fe) is the first element in group VIII of the periodic table. It has an atomic number of 26, an atomic weight of 55.85, and valencies of 2 and 3 (and sometimes 1, 4, and 6). The average abundance of Fe in the Earth's crust is 6.22%; in soil Fe ranges from 0.5 to 4.3%. In rivers it is about 0.7 mg/L. And in groundwater it is 0.1 to 10 mg/L. Iron is found in hematite, magnetite, taconite, and pyrite ores. It is widely used in the production of steel and other alloys.

The solubility of iron ions (Fe2+) is controlled by the concentration of carbonate. Since groundwater is often oxygen-deficient, soluble iron in groundwater is usually in the form of iron salts. When exposed to air or the addition of oxidants, iron is oxidized to the ferrous state (Fe3+) and can be hydrolyzed to produce the insoluble red hydrated iron oxide. In the absence of complexing ions, iron is not significantly soluble unless the pH is very low. High levels of iron in water can cause stains on plumbing, laundry, and cookware, and can impart off-flavors and colors to foods. The Food and Agriculture Organization of the United Nations recommends a maximum level of 5 mg/L for iron in water used for irrigation of agricultural crops. The U.S. EPA drinking water standard MCL is 0.3 mg/L.

Method Selection

The sensitivity and detection levels for the atomic absorption spectrometer methods (Sections 3111B and C), the inductively coupled plasma method (Section 3120), and the phenanthroline colorimetric method described here (3500-FeB) are similar and are commonly used for the analysis of natural or treated waters. When an appropriate matrix modifier is used, lower detection levels can be achieved with electrothermal atomic absorption spectrometry (Section 3113B). The complex reactions used in the colorimetric methods are specific to organic iron, but not for the atomic absorption methods. However, due to the instability of iron, which readily changes to the ferric form in solutions upon contact with air, iron determination requires special precautions and may need to be performed at the same location as the samples are collected.

The iron determination process using 1,10-phenanthroline (3500-Fe.B.4c) has somewhat limited applicability; avoid prolonged storage or exposure of samples to light. The Bathophenanthroline method, which is an accurate method, can distinguish between ferrous and ferric iron. Spectrophotometric methods using bathophenanthroline 1-6 and other complex organic reactions such as ferrizin 7 or TPTZ8 can determine iron concentrations down to 1 μg/L. One method is the chemiluminescence method, which has a detection limit of 5 ng/L. Further methods are described elsewhere.

Sampling and Storage

Prepare methods for collecting, storing, and pre-treating samples. Wash the sample mixture with acid and water. On-site membrane filtration equipment may be required to detect iron in solution (dissolved iron). Dissolved iron is considered to be ferrous iron that passes through a 0.45-μm membrane filter, which may include colloidal iron. The quality of the measurement depends to a considerable extent on the care taken to obtain the sample. The concentration and form of iron in well or tap water samples may vary depending on the duration and degree of discharge, before and during sampling. When taking a portion of the sample for the determination of suspended iron, shake the sample bottle regularly and vigorously to obtain a uniform suspension of iron. Exercise special care when colloidal iron is poured into the sample bottle. This problem can be more acute with plastic bottles.

For accurate determination of total iron, use a separate container to collect the sample. Acidify the medium at the time of sample collection to keep the iron in solution and to prevent iron from adsorbing or precipitating on the walls of the sample container. Calculate the amount of acid added for analysis. Adding acid to the sample may eliminate the need to add acid before digestion (3500-Fe.B.4a).

Phenanthroline 3500 Fe B Method

General Discussion

Principles:Iron metal, dissolved in solution, is reduced to the divalent iron state by boiling with acid and hydroxylamine and prepared at pH 3.2 to 3.3 by 1,10-phenanthroline. Three molecules of phenanthroline chelate with each iron atom to form an orange-red complex. The colored solution obeys Beer's law; its color intensity is independent at pH 3 to 9. A pH between 2.9 and 3.5 causes a rapid color change in the presence of a large amount of phenanthroline. Standard colors are stable for at least 6 months.

Interferences:Among the interfering compounds, strong oxidizing agents, cyanide, nitrite and phosphates (polyphosphates more than orophosphates), chromium, zinc in concentrations greater than 10 times that of iron, cobalt and copper in concentrations greater than 5 mg/l and nickel in concentrations greater than 2 mg/l. Bismuth, cadmium, mercury, molybdate, and silver precipitates of phenanthroline. Initial boiling with acid converts the polyphosphate to orthophosphate and removes cyanide and nitrite, which would otherwise interfere. Addition of excess hydroxylamine eliminates errors due to excessive concentrations of strong oxidants. In the presence of interfering metal ions, use a large excess of phenanthroline to replace the interfering metals. Where excessive concentrations of interfering metal ions are present, the extraction method may be used.

If appreciable amounts of dye or organic matter are present, it may be necessary to evaporate the sample, ashing the residue gently, and redissolving in acid. Ashing may be carried out on silica, porcelain, or platinum ores which have been boiled in 6N HCl for several hours. In the presence of large amounts of organic matter, digestion may be necessary before using the extraction method.

Minimum detectable concentration:The concentration of dissolved iron or total iron with a minimum concentration of 10 μg/L can be determined using a spectrophotometer using cells with a light path of 5 cm or more. To control the accuracy of the method, use a blank solution during the measurement.

Apparatus

Colorimetric equipment: One of the following is required:

1. Spectrophotometer, providing a beam width of 1 cm or more for use at a wavelength of 510 nm.
2. Filter photometer, providing a beam width of 1 cm or more and equipped with a green filter with a maximum transmittance near 510 nm.
3. Nessler tubes, identical, 100 ml, long form.

Acid-washed containers:Wash all containers with concentrated hydrochloric acid (HCl) and rinse with water before use to remove iron oxide deposits.

Separating funnels:125 ml, Squibb shape, with glass or TFE sectioned storage containers.

Reagents

Use a small amount of indicator in the measurement of iron. Use of reagent water (see Section 1080 and Section 3111B.3c) is used in the preparation of standards and reagent solutions as well as in the process. Store reagents in glass bottles. Stable indefinitely if tightly closed in the container of HCl and ammonium acetate solution. Hydroxylamine, phenanthroline, and iron solutions contained therein are stable for several months. Iron standard solutions are not stable; prepare daily and as needed by diluting the stock solution. These standards are stable in Nessel tubes for several months if protected from light.

1. Hydrochloric acid, HCl, concentrated, containing less than 0.5 ppm iron.
2.  Hydroxylamine solution: Dissolve 10 g NH2OH·HCl in 100 mL of water.
3. Ammonium acetate buffer solution: Dissolve 250 g NH4C2H3O2 in 150 mL of water. Add 700 ml of glacial acetic acid. Since even a good grade of NH4C2H3O2 contains a significant amount of iron, prepare new reference standards with each buffer preparation.
4. Sodium acetate solution: Dissolve 200 g of NaC2H3O2·3H2O in 800 ml of water.
5. Phenanthroline solution: Dissolve 100 mg of 1,10-phenanthroline monohydrate,        C12H8N2·H2O, in 100 ml of water, stirring and heating to 80 °C. The solution should not boil. If the solution becomes dark, discard it. Heating is unnecessary if 2 drops of concentrated HCl are added to the water. (Note: One ml of this reagent is suitable for amounts less than 100 µg of iron.)
6. Potassium permanganate, 0.1 M: Dissolve 0.316 KMnO4 in reagent water and make up to 100 ml.
7. Iron solution: Use metal (1) or salt (2) to prepare the base solution. (1) To prepare the solution, use an electrical iron wire or an iron wire for standardization. If necessary, prepare the wire with fine emery paper to remove the oxide coating and produce a bright surface. Place 200 mg of the wire in a 1000 ml volumetric flask. Dissolve in 20 ml of 6N sulfuric acid (H2SO4) and make up to volume with water. 1.00 ml = 200 µg of iron. (2) If ammonium iron sulfate is preferred, slowly add 20 mL of concentrated H2SO4 to 50 mL of water and dissolve 1.404 g of Fe(NH4)2(SO4)2·6H2O. Add 0.1M potassium permanganate (KMnO4) dropwise until a faint pink color develops. Make up to 1000 mL with water. 1.00 mL = 200 μg of iron.
8. Iron Standard Solutions: Prepare daily for use. (1) Pipette 50.00 mL of stock solution into a 1000 mL volumetric flask and make up to volume with water. 1.00 mL = 10.0 μg iron. (2) Pipette 5.00 mL of stock solution into a 1000 mL volumetric flask and make up to volume with water. 1.00 mL = 1.00 μg iron.
9. Diisopropyl or isopropyl ether. Warning: Ethers may form explosive peroxides; test before use.

Procedure

Total iron: Mix the sample thoroughly and transfer 50 mL to a 125 mL Erlenmeyer flask. If this sample volume contains more than 200 μg iron, use a smaller volume of sample and dilute to 50.0 mL. Add 2 mL of concentrated HCl and 1 mL of NH2OH HCl solution. Add a little glassware and heat to boiling. Continue boiling until the volume is reduced to 15 to 20 mL to ensure that all the iron has dissolved. (If the sample has been ashed, boil the residue in 2 mL of concentrated HCl and 5 mL of water.) Cool to room temperature and transfer to a 50 or 100 mL volumetric flask or Nesseler tube. Add 10 mL of NH4C2H3O2 buffer solution and 4 mL of phenanthroline solution and dilute with water. Mix thoroughly and allow at least 10 minutes for maximum color development.

Dissolved Iron: Filter immediately after collection through a 0.45 μm membrane filter into a vacuum flask containing 1 mL of concentrated HCl/100 mL sample. Analyze the filtered sample to calculate total dissolved iron and/or dissolved ferrous iron (4A). ((4C) This method can also be used in the laboratory if it is considered that iron precipitation in the air may occur in normal samples during transport.)

Calculate suspended iron by subtracting the dissolved iron present from the total iron.

Organic iron: Determine ferrous iron because of the possibility of changes in the ferrous-ferric ratio in the acid solution over time at the sampling site. To determine ferrous iron, acidify a separate sample with 2 mL concentrated HCl/100 mL sample at the time of sampling. Fill the bottle directly from the sampling source and probe. Immediately remove 50 mL of the acidified sample and add 20 mL of phenanthroline solution and 10 mL of NH4C2H3O2 solution while stirring vigorously.

Dilute to 100 ml and measure the intensity of the color within 5 to 10 minutes. Do not expose to sunlight (color development is rapid in the presence of excess phenanthroline. The volume of phenanthroline added is appropriate for total iron levels of less than 50 µg; if higher levels are present, use larger amounts of phenanthroline or a more concentrated reagent.)

Calculate ferric iron by subtracting ferrous iron from total iron.

Colorimetry: Prepare a series of standards by accurately and continuously measuring calculated volumes of iron standard solutions (use the solutions described in 3h2 to measure 1 to 10 µg amounts) into a 125 ml Erlenmeyer flask, dilute to 50 ml by adding measured volumes of water, and perform steps 4a, beginning with transfer to a 100 ml volumetric flask or Nesseler tube.

For visual comparison, prepare a set of at least 10 standards, ranging from 1 to 100 μg in a final volume of 100 mL. Compare the colors in 100 mL long-necked Nessel tubes.

For photometric measurements, use Table 3500-Fe:I as a rough guide to selecting the correct light path at 510 nm. Read the standard values ​​for water at zero absorbance and plot a calibration curve, including a blank sample (see Section 3c and General Introduction).

If the samples are colored or dirty, run a second set of samples following all steps of this procedure without adding phenanthroline. Instead of water, use the blank sample prepared to set the photometer at zero absorbance and read each sample with phenanthroline against the corresponding blank without phenanthroline. Interpret the readings on the photometer relative to the iron content using the calibration curve. This method does not compensate for the effect of interfering ions.

Samples containing organic interferences: Samples containing significant amounts of organic matter should be digested according to the instructions in Section 3030G or Section 3030H.

1. If a digested sample was prepared according to the instructions in Section 3030G or Section 3030H, pipette 0.10 mL or other appropriate aliquot containing 20 to 500 μg of iron into a 125 mL separating funnel. If the volume removed is less than 10 mL, add water to make up to 10 mL. Add 15 mL of concentrated HCl per 10 mL of water to the separating funnel. Or, if the volume used is greater than 10 mL, add 1.5 mL of concentrated HCl/sample. Mix, cool, and continue the process.
2. To prepare a sample for iron determination only, measure out a suitable volume containing 20 to 500 μg of iron and prepare it by the digestion procedure described in Section 3030G or Section 3030H. In either case, use only 5 mL of H2SO4 or HClO4 and eliminate the H2O2. When the digestion is complete, cool, dilute with 10 mL of water, heat to boiling to dissolve dissolved salts slowly, and if the sample is still cloudy, filter through a glass fiber, muslin, or porcelain filter. Rinse with 2 to 3 mL of water. Transfer the filtered sample or clear solution aliquot into a 25 mL volumetric flask and make up to 25 mL. Transfer the contents of the flask to a 125 mL separatory funnel, rinse the flask with 5 mL of concentrated HCl and add to the funnel. Measure 25 mL of concentrated HCl into the same flask. Mix and cool to room temperature.
3. Extract the iron from the HCl solution in the separatory funnel with 25 mL of isopropyl ether by stirring vigorously for 30 min. Separate the less acidic layer in a second separatory funnel. Extract the acidic solution again with 25 mL of isopropyl ether, drain the acidic layer into a suitable clean container, and add the ether layer to the ether in the first funnel. Pour the acidic layer into the second separatory funnel and extract again with 25 mL of isopropyl ether. Separate and discard the acidic layer, and add the ether layer to the first funnel. The persistence of the yellow color in the HCl solution after three extractions does not indicate incomplete separation of the iron, since the unextracted copper gives a similar yellow color.

Shake the combined ether extract with 25 mL of water to transfer the iron to the aqueous phase, and transfer the lower aqueous layer to a 100 mL volumetric flask. Repeat the extraction with a second 25 ml of water, adding this sample to the first aqueous extraction product. Discard the ether layer.

Add 4 ml of NH2OH·HCl solution, 10 ml of phenanthroline solution and 10 ml of NaC2H3O2 solution. Make up to 100 ml with water, mix thoroughly and leave for at least 10 minutes. Measure the absorbance at 510 nm using a 5 cm absorption cell for iron values ​​less than 100 µg, or a 1 cm cell for values ​​between 100 and 500 µg. As a reference, use water or a blank prepared with the specified amount of acid during the analysis. If water is used as a blank, correct the sample absorbance by subtracting the blank absorbance.

Determine the micrograms of iron in the sample in terms of absorbance (corrected, if necessary) from a calibration curve prepared using a range of iron standards containing similar amounts of phenanthroline, hydroxylamine, and sodium acetate as the sample.

Calculations

When the sample is prepared according to 4a, 4b, 4c, or 4e2:

When the sample is prepared according to 4e1:

Report details of sampling, storage, and sample preparation if relevant to interpretation of results.

Accuracy and precision

Accuracy and precision depend on the method of sample collection and storage, the method of color measurement, the concentration of iron and the presence of interfering color, turbidity and foreign ions. In general, the optimal reliability of visual comparison in Nelswell tubes is not better than 5% and often only 10%, while under optimal conditions, photometric measurements may be reliable to 3% or 3 μg (whichever is greater). The sensitivity limit for visual observation in Nelswell tubes is about 1 μg of iron. Sample variability and instability may affect the accuracy and precision of this measurement more than analytical errors. Serious discrepancies have been observed in the reports of different laboratories due to variations in sample collection and preparation methods.

A synthetic sample containing 300 μg iron/L, 500 μg aluminum/L, 50 μg cadmium/L, 110 μg chromium/L, 470 μg copper/L, 70 μg lead/L, 120 μg manganese/L, 150 μg silver/L, and 650 μg zinc/L in distilled water was analyzed in 44 laboratories by the phenanthroline method, with a relative standard deviation of 25.5% and a relative error of 13.3%.

7. Bibliography

CHRONHEIM, G. & W. WINK. 1942. Determination of divalent iron (by o-nitrosophenol). Ind. Eng. Chem., Anal. Ed. 14:447.

MEHLIG, R.P. & R.H. HULETT. 1942. Spectrophotometric determination of iron with o-phenanthroline and with nitro-o-phenanthroline. Ind. Eng. Chem., Anal. Ed. 14:869.

CALDWELL, D.H. & R.B. ADAMS. 1946. Colorimetric determination of iron in water with o-phenanthroline. J. Amer. Water Works Assoc. 38: 727.

WELCHER, F.J. 1947. Organic Analytical Reagents. D. Van Nostrand Co., Princeton, N.J., Vol. 3, pp. 85–93.

KOLTHOFF, I.M., T.S. LEE & D.L. LEUSSING. 1948. Equilibrium and kinetic studies on the formation and dissociation of ferroin and ferrin. Anal. Chem. 20:985.

RYAN, J.A. & G.H. BOTHAM. 1949. Iron in aluminum alloys: Colorimetric determination using 1,10-phenanthroline. Anal. Chem. 21:1521.

REITZ, L.K., A.S. O’BRIEN & T.L. DAVIS. 1950. Evaluation of three iron methods using a factorial experiment. Anal. Chem. 22:1470.

SANDELL, E.B. 1959. Chapter 22 in Colorimetric Determination of Traces of Metals, 3rd ed. Interscience Publishers, New York, N.Y.

SKOUGSTAD, M.W., M.J. FISHMAN, L.C. FRIEDMAN, D.E. ERDMANN & S.S. DUNCAN. 1979. Methods for Determination of Inorganic Substances in Water and Fluvial Sediment. Chapter A1 in Book 5, Techniques of Water Resources Investigations of the United States Geological Survey. U.S. Geological Surv., Washington, D.C.

Reference: Standard Methods for the Examination of Water and Wastewater

Translated by Maryam Soltani Sarvestani