Determining K of Fe3+ + SCN- <-> Fe(SCN) Reaction

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SUMMARY

The discussion focuses on determining the equilibrium constant (K) for the reaction of Fe3+ and SCN- to form Fe(SCN), a red pigment. The procedure involves mixing Fe(NO3)3, KSCN, and HNO3. HNO3 serves as an acid to maintain the acidic environment necessary for the stability of Fe3+ ions in solution. The pKa of thiocyanic acid is approximately 5.4, indicating its weak acidity, which may influence the availability of SCN- ions in the reaction.

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Hey so I am doing a lab write up right now for determining the value of K of the reaction of Fe3+ + SCN- <---> Fe(SCN).
In the procedure for creating dilute solutions for analysis (by colourimetry) we mixed Fe(NO3)3 KSCN and HNO3. I know that the first two will ionize and react to form Fe(SCN) (which is a red pigment) I am however confused with the role that HNO3 plays in the emperiment.
Thanks so much.
 
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The pKa of thiocyanic acid is ~5.4. What is its pKb? How might that affect iron +3 in solution?
 

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