Fast and Sufficiently Accurate Methods for Determining Iron Concentration
Spectrometric and Visual Colorimetric Determination of Iron Concentration
Certified and accredited water testing laboratories determine the iron concentration in water using the spectrometric method with 1,10-phenanthroline (ISO 6332). I too used this method for quite a long time. It is a fairly reliable and accurate method. However, this method is time-consuming and can essentially only be used in a laboratory.
17 years ago I became acquainted with another spectrometric method for determining iron, which struck me as considerably more attractive than the one mentioned above. The essence of this method is as follows: Fe2+ ions react in an acidic medium with the disodium salt of 3-(2-pyridyl)-5,6-bis(4-phenylsulfonic acid)-1,2,4-triazine, forming a violet-colored complex compound, which is measured photometrically at a light wavelength of 565 nm. The method is highly selective and sensitive; the molar extinction coefficient is ε = 27,570. For comparison, I will note that the molar extinction coefficient of the colored iron compound with 1,10-phenanthroline is ε = 8,000 (the higher the ε value, the more sensitive the reaction, and the lower the iron concentrations that can be determined without additional concentration). This method determines the total iron concentration. All the reagents needed (an organic dye, a solvent for colloidal and coarse-dispersed forms of iron, a reducing agent, a buffer system, etc.) are combined in a single bottle. The resulting colored complex compound remains stable for a sufficiently long time (about 20 hours), and the reaction can be carried out over a fairly wide temperature range (5–40 °C). It is worth repeating that all the reagents needed to carry out the analysis are combined in a single bottle, and that the manufacturer of this reagent is the German company "Merck" (catalog No. 14761). Instructions for using this reagent are provided in the test packaging in various languages. Here is a translation of these instructions into Lithuanian:
1. Pour 5–10 mL of the test solution (water) into a beaker.
2. Add 3–4 drops of Fe-AN reagent to the beaker and mix.
3. Wait 3–5 min (reaction time).
4. Pour the colored solution into a cuvette of the appropriate length and measure the absorbance at a light wavelength of 565 nm. The reference solution is distilled water or the test water (if it is not turbid).
Calculating the result: the measured absorbance (A) value of the solution must be multiplied by the appropriate factor (F):
For a 1 cm cuvette, F = 2.08;
For a 2 cm cuvette, F = 1.08;
For a 5 cm cuvette, F = 0.416.
For example, if A = 0.150 is measured with a 1 cm cuvette, then the iron concentration will be:
C = A · F = 0.150 · 2.08 = 0.312 mg/L.
Brief Commentary on the Method
This method has quite a few advantages compared with other spectrophotometric methods for determining iron. I had previously worked considerably with sulfosalicylic acid, rhodanide, and o-phenanthroline. The rhodanide method is decent, but very insensitive, and using it without concentration allows analysis only of solutions with a fairly high iron content. The sulfosalicylic acid and o-phenanthroline methods are considerably more sensitive, but they are still about 3 times less sensitive than this method. For this reason, this method is very convenient when it is necessary to analyze solutions with very low iron content, for example drinking water after iron-removal equipment, or feed water for steam boilers. In this case, without any additional concentration, using a 5 cm cuvette, iron can be determined fairly accurately even when its concentration in the water reaches only thousandths of a milligram per liter.
The second advantage is the very short analysis time; a result can be obtained after just 5–6 minutes.
The third advantage is that there is no need to boil the test solutions (natural waters) with HCl. The reagent mixture used converts not only colloidal but also coarse-dispersed forms of iron into ionic form. In this case the reaction time is somewhat longer, so it is advisable to measure the absorbance after 5–6 min and, say, after 30 min, and to calculate the result only once the absorbance is no longer increasing. Incidentally, when using modern microprocessor-based spectrometers with a liquid crystal display, you can simply see on the screen whether the absorbance (concentration) has already stabilized or is still increasing.
Comparative measurements carried out using this method and the o-phenanthroline method produced very good agreement between the results.
This method also has one drawback. Unlike the o-phenanthroline method, where slightly modifying the procedure allows determination of both divalent and total iron, this method determines only the total iron concentration.
Determining Iron Concentration Using Visual Colorimetry
These same reagents, combined in a single bottle, can be successfully used not only for spectrometric but also for visual colorimetric determination of iron concentration.
The German company "Merck" Microquant® Fe test (0 – 0.1 – 0.2 – 0.3 – 0.5 – 0.8 – 1.2 – 2 – 3 – 5 mg/L Fe), catalog No. 14759 (Fig. 1).
Fig. 1. Visual colorimetric determination of iron concentration using the Microquant® test.
This test is convenient for determining iron both in groundwater and after water treatment equipment. On the rotating colored disc (comparator) there are colored patches corresponding to the following iron concentrations: 0 – 0.1 – 0.2 – 0.3 – 0.5 – 0.8 – 1.2 – 2 – 3 – 5 mg/L. The accuracy of this test is not very high, but it is sufficient if the iron concentration in the tested water does not exceed 0.5 mg/L.
Very accurate is the Aquaquant® test (Fig. 2) from the same German company "Merck". Catalog No. 14403. On the color palette there are the following Fe concentration values: 0.00-0.01-0.02-0.03-0.04-0.06-0.08-0.1-0.15-0.2 mg/L. This test is very accurate. It would be worth acquiring for drinking water supply companies that do not have their own in-house water testing laboratories, and especially for those that operate the water sources of small settlements. With the help of either test, the Fe concentration after water treatment equipment or at the consumer can already be determined after just 3-5 min., allowing a prompt response in cases where the Fe concentration after water treatment equipment or at the consumer's point of entry fails to meet the requirements of drinking water hygiene standard HN 24:2003.
Fig. 2. Visual colorimetric determination of iron concentration using the Aquaquant® test.
The same German company "Merck" produces a very wide range of the most varied tests. I myself have tried many of them, including the Microquant® and Aquaquant® manganese, nitrite, and ammonium tests. These latter tests would, again, be very useful for small water supply companies. With the help of these tests, they could independently and promptly monitor drinking water quality in small settlements where water quality improvement facilities have been built in recent years.
It remains to note that the official distributor of the aforementioned tests in Lithuania is UAB "Bioeksma".
P.S. The iron concentration test reagent Fe-An, combined in a single bottle, is also excellent for removing rust stains from white and colored fabrics. I have tried this myself. The effect is fantastic. The procedure is very simple: add a few drops of Fe-An reagent to 1-2 mL of tap water and pour the resulting solution onto the rust stain. This reagent converts rust particles into a violet-colored, water-soluble iron complex compound. After 5-10 min (the time depends on the amount of rust), rinse the treated spot with tap water. All that remains is to dry the fabric. Not a trace of the stain remains.