Addition of KNO3 to HF(aq)

[lanrete]

Hi,

 

in my lab i have a procedure by which excess KNO3 is added to an unknown HF sample and then titrated with NaOH with phenolphthalein as indicator.

 

After this first titration, the sample is then placed in an 80oC waterbath for 1 hour before conducting a 2nd titration with NaOH again with phenolphthalein as indicator.

 

From my manual, it states that this procedure is to determine the HF and H2SiF6 concentration in the sample.

 

So my question is what is the purpose of KNO3 in the procedure? Also can someone deduce the formula of the reaction(s) taking place with KNO3 in this procedure?

 

Thanks