Limit test for Iron

• The limit test for iron is a qualitative test that checks whether the iron ion (Fe³⁺) concentration in a sample is below a specified limit.
• It is based on a colorimetric reaction between iron ions (Fe³⁺) and potassium thiocyanate (KSCN), forming a red-colored complex.

Chemical Reaction:

Fe3+(aq) + 6SCN−(aq) → [Fe(SCN)6]3−(aq)

Procedure of Limit test for Iron:

Prepare Reference Standard Solution:

• Make a reference solution with a known concentration of iron ions, typically from ferric ammonium sulfate.
• This solution represents the acceptable iron limit.

Prepare Test Solution:

• Prepare a test solution from the sample, ensuring that any iron ions present are in a soluble form.

Add Potassium Thiocyanate (KSCN):

• To both the test and reference solutions, add equal volumes of potassium thiocyanate solution to provide the thiocyanate ions for the reaction.

Allow Formation of Red Complex:

• Let both solutions stand for a few minutes to allow the red-colored iron-thiocyanate complex to form.

Observe Color Formation:

• Observe the intensity of the red color in both the test and reference solutions.
• The presence of a red color in the test solution indicates iron ions.

Compare Color Intensity:

• Compare the intensity of the red color in the test solution with the reference standard.
• If the test solution shows less intense color, the iron ion concentration is below the acceptable limit.

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