Limit test for Iron -

Limit test for Iron

Principle:

The limit test for iron is based on the formation of the purple color through interaction of iron with thioglycolic acid within a solution buffered with ammonium citrate. This resultant produced is then compared with a reference standard with a predetermined iron content (0.14 mg of Fe).

Thioglycolic acid facilitates the reduction of ferric iron to ferrous iron, which produce ferrous thioglycollate. Citric acid forms complex with iron, rendering it solubility and preventing its precipitation as ferrous hydroxide when exposed to ammonia. In neutral or acidic environments, ferrous thioglycollate remains colorless, however, presence of alkali triggers color development. The compound maintains stability in the absence of air but fades upon exposure to air due to oxidation to the ferric form. Hence, color comparisons should be immediately done as the completion of color development.

Apparatus Required in the limit for iron:

1. Nessler cylinders
2. Glass rod
3. Pipettes
4. Stand

Chemicals Required:

1. Standard Iron solution Ferric ammonium sulphate (1.726 g) dissolved in 10 ml of 0.1 N H₂SO4 and sufficient water to produce 1000 ml.
2. Sulphuric acid (0. 1 N): 10.0 ml.
3. Iron-free citric acid solution (20% w/v): 2.0 ml.
4. Thioglycolic acid: 0.1 ml.
5. Iron-free ammonia solution: 20 ml.

Chemicals Required:

Procedure for limit test for iron:

Observation:

The intensity purple color produced in the sample solution should not exceeds that of the standard solution. Intensity of the colour less than the standard solution indicates pass the limit test and intensity of the colour more than the standard solution indicates fail the limit test for iron.

Reasons:

Citric acid aids in the precipitation of iron by ammonia through the formation of a complex with it.
Thioglycolic acid facilitates the oxidation of iron (II) to iron (III).
Ammonia to make solution alkaline.

Different methods employed for limit test for Iron:

Following are the methods that can be used to perform a limit test for iron. These are-

Titration:

This procedure involves addition a specified volume of a reagent, which interacts with iron, to a measured volume of the sample. Subsequently, the volume of reagent needed to react with all iron in the sample is determined. This method is frequently employed when the initial iron concentration in the sample is uncertain or when the sample contains additional impurities that could affect the analysis.

Spectrophotometry:

In this approach, absorption or transmission of light by the sample at a designated wavelength is measured. This measurement is directly proportional to the iron concentration within the sample. By using a standard curve or calibration curve, this method allows for precise determination of the iron concentration.

Inductively coupled plasma mass spectrometry (ICP-MS):

In this technique, the sample undergoes vaporization followed by ionization of the resulting atoms or molecules via inductively coupled plasma. Subsequently, the ions are segregated according to their mass-to-charge ratio and identified using a mass spectrometer, facilitating accurate determination of the iron concentration in the sample.

Atomic absorption spectroscopy (AAS):

In this method, the light absorbance by the samples at a specific wavelength is measured. This absorbance is directly linked to the concentration of iron in the sample, enabling determination of iron concentration via a standard curve or calibration curve.

Atomic emission spectroscopy (AES):

This technique involves assessing the sample’s light emission at a specific wavelength. The emitted light is directly correlated with the iron concentration in the sample, allowing for determination of iron concentration via a standard curve or calibration curve.

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