Reasons for Measuring Iron in Water
Many businesses need to measure the concentration of iron in water. Beverage, drinking water, textile, and laundry industries measure iron to prevent discoloration and off-flavors that it can cause. Oil, gas, mining, boiler, and cooling water applications measure iron as it can lead to harmful scaling or indicate corrosion of metal components. Aquaculture businesses will measure iron as high concentrations can be toxic to health and growth of their crop. If a business isn’t measuring iron in their processes, it is likely being measured in the wastewater it generates. While the need for reliable iron test kits is obvious, deciding which test kit is right for you can be difficult due to the chemistry of iron.
Forms of Iron
Iron naturally exists in three different chemical oxidation states, Fe0 for metallic elemental iron, ferrous (Fe2+) and ferric (Fe3+). Depending on conditions such as the levels of oxygen in the water and pH, most natural waters contain some ratio of ferrous and ferric iron, including insoluble forms (particulate and colloidal) that are predominantly ferric — all of which adds up to the Total Iron.
Total Iron = Ferrous Iron + Ferric Iron (Includes ALL insoluble forms)
CHEMetrics offers two methods for the analysis of iron: 1,10-phenanthroline and ferric thiocyanate. Each is referring to the name of the complexing agent that detects dissolved iron by forming a colored compound. The 1,10-phenanthroline method is most commonly used, while the ferric thiocyanate test is most often used for oil & gas applications where iron is measured in brine solutions.
The 1,10-phenanthroline method can provide you with 4 different iron measurements.
- Total Iron
- Ferrous only
- Ferric only (estimate)
- Soluble iron
Each type of iron is a different measurement that can be useful to a water treatment professional.
Measuring Total Iron: K-6010 / K-6003 / K-6210 / K-6203
All phenanthroline-based iron test kits (K-6010, K-6003, K-6210, K-6203) offered by CHEMetrics include an A-6000 Activator Solution containing the reducing agent thioglycolic acid which is effective at dissolving and converting insoluble forms of iron to the ferrous state. A well-ventilated work area is strongly advised due to the odor.
There is one key stipulation to this procedure. The A-6000 Activator solution is incompatible with samples containing molybdates (MoO42-). Also, samples containing nitrites (NO2–) must first be fully treated with A-9600 (Sulfamic acid) BEFORE performing the A-6000 procedure.
The Total Iron test procedure is designed to provide a reasonable estimate of the total iron profile of the water sample by pre-treating it with A-6000 Activator Solution for a 4-minute reaction time. This promotes the dissolution of particulate iron and its conversion to the ferrous form.
In environmental monitoring applications, USEPA guidelines require digesting the sample in strong acid which provide more aggressive conditions prior to analysis that will ensure a more complete total iron recovery. For high frequency testing, analysts can evaluate the disparity of results obtained with and without acid digestion to decide if the additional time expense is justified for future analyses.
Total iron measurements are often made for boiler and cooling water system monitoring, general water quality, safety, and environmental regulatory compliance.
Measuring Ferrous Iron Only: K-6210 / K-6203
If the goal is to only measure the concentration of ferrous iron (Fe2+) in your water sample, use the K-6210 or K-6203 test kits. The 1,10-phenanthroline readily forms an intense reddish orange complex with ferrous iron. This reagent is sealed inside CHEMetrics K-6210/6203 CHEMets® and Vacu-vials® ampoules.
Ferrous iron is the most soluble form of iron. Measuring ferrous iron is common when you only want to measure the dissolved iron in your sample. This is particularly important in water treatment or applications where you are anticipating the iron to precipitate out of the solution and want to know how much there will be. Measuring ferrous iron may also be necessary for determining the concentration of ferric iron (Fe3+). The general procedure to indirectly measure ferric iron is to subtract the ferrous iron test result from the measured Total Iron / Soluble Iron test result.
Estimating Ferric Iron: K-6210 / K-6203
Estimating Ferric Iron requires two test results and then a calculation. The two tests can be performed with either K-6210 or K-6203.
To measure ferric iron by the 1,10-phenanthroline method, the Total Iron must first be determined, then Ferrous Iron Only, and finally you need to subtract Ferrous Iron Only from Total Iron.
Estimate of Ferric Iron = Total Iron – Ferrous Only
To calculate Total Iron, follow the same procedure listed under the Measuring Total Iron section. As previously stated, the Total Iron test procedure involves converting ferric iron to the ferrous state using a suitable reducing agent. Analysts should be aware, however, there are insoluble ferric iron forms that have refractory characteristics. This means they resist conversion to the ferrous iron form. Examples of unreactive iron minerals are magnetite and ferrite. In that situation samples must be digested (refluxed or boiled) in strong acids to bring those forms of iron into solution.
Once you have your Total Iron measurement, you need to measure Ferrous Iron using the procedure detailed under Measuring Ferrous Iron Only. Finally, you subtract the ferrous iron measurement from the total iron measurement to calculate your Ferric Iron content.
Ferric iron is most frequently measured to assess corrosion. Since it is insoluble it is most likely to clog filters and pipes. Routine monitoring can help prevent damage to industrial infrastructure. It is also commonly measured in environmental applications as the iron can react with pollutants and is an indicator of biological activity.
Measuring soluble iron: K-6010 / K-6003
The K-6010/K-6003 ampoules contain another component in addition to 1,10 phenanthroline. This chemical component is called hydroxylamine hydrochloride which is a mild reducing agent, making it suitable for measuring all iron species that are readily solubilized. These tests are best for water samples that are not expected to have significant levels of unreactive insoluble iron.
At first glance you may think soluble iron and ferrous iron are the same measurement, but there is a key difference. The Ferrous Iron test does not contain hydroxylamine hydrochloride, so it will not be able to solubilize compounds with refractory characteristics and will not measure any type of insoluble iron.
Examples of suitable sample matrices that are applicable to K-6001/6003 test kits:
- Acidified iron standard solutions available commercially
- Recently prepared solutions from soluble iron salts (e.g. nitrates, chlorides, sulfates)
- Natural water that exhibits conditions which discourage ferric iron formation (e.g. low oxygen and low pH).
If you obtain a value that is significantly lower than expected (e.g. based on historical data), it implies the presence of other interfering oxidizing compounds or that a more rigorous procedure is needed: Total Iron Procedure with / without acid digestion.
This test is best conducted by experienced operators who understand their testing conditions well.
Further Information and Where to Purchase Iron Test Kits
All of the CHEMetrics iron test kits are available on the AquaPhoenix estore along with other testing supplies. If you need further assistance or have a question, please reach out to our Technical Services team.
–Written by: Harris Handoko and Zachary Waszczak, July 2024