hydroproline assay citrate buffer - (Dec/19/2004 )

I am trying to prepare a acetate-citrate buffer for the proline assay and after mixing all solutions ph = 13.00
They say to bring it down to 6.5; I tried to use HCl, 10ml at 800ml solution without any result, ph still 13.00.
Final volume should be 1000ml and the buffer contains citric acid, acetic acid, sodium hydroxide and sodium acetate trihydrate.
Do you have any sugestions?

Thanks,
cosmin

- Try with concentrated HCl...
- Check pH-meter