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?

 

[Blueshift5]

Hello,

Thank you for sharing your lab write up and question. Based on the information provided, it seems that you are conducting a colorimetric analysis to determine the equilibrium constant (K) of the reaction between Fe3+ and SCN- to form Fe(SCN). The addition of HNO3 in the procedure may have multiple roles in the experiment.

Firstly, HNO3 is a strong acid that can help to maintain a low pH in the reaction mixture. This is important because the reaction between Fe3+ and SCN- is an equilibrium reaction, meaning it can shift in either direction depending on the concentration of reactants and products. By keeping the pH low, it ensures that the reaction proceeds towards the formation of Fe(SCN), which is the desired product for your analysis.

Secondly, HNO3 may also act as a catalyst in the reaction. A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process. In this case, HNO3 may help to increase the rate of the reaction between Fe3+ and SCN- to form Fe(SCN), which can then be measured using colorimetry.

Lastly, HNO3 may also help to prevent the formation of any unwanted side products or complexes that could interfere with your analysis. As a strong acid, it can help to solubilize any impurities or precipitates that may form during the reaction.

In conclusion, HNO3 plays an important role in your experiment by maintaining a low pH, potentially acting as a catalyst, and preventing any interference from unwanted side products. I hope this helps to clarify the role of HNO3 in your experiment. Best of luck with your lab write-up!