I am now at the stage where I get +/- about 3 mv of the calculated value with a lot of noise due to the low signal (mv range). Sometimes, due to small amounts of contamination or the phase of the moon, I get a constant offset of up to about -30 mv in my measurements. Use of a potentiometer and a bias voltage steadies the signal and I can eliminate the constant bias. So, I am making progress.
I have built a KNO3 bridge that works and is stable. I hope to publish a complete set of instructions.