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

[cemar.]

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.

 

[chemisttree]

The pKa of thiocyanic acid is ~5.4. What is its pKb? How might that affect iron +3 in solution?