Nitrate interference with potassium-selective microelectrodes

Initial attempts to measure K+ activity (a(k)) in vacuoles of barley leaf epidermal cells using triple-barrelled K+-Selective microelectrodes gave values that were only about one-third of those expected. This was due to high (c. 200 mM) NO3- concentrations in the vacuoles interfering with the K+-sensor, The effect of NO3- was on 1,2,-dimethyl-3-nitrobenzene (DNB) used as a plasticizer in the K+-sensor. Replacing DNB with dibutyl sebacate, but not with 2-nitrophenyl octyl ether, overcame this problem and the modified sensor gave acceptable calibration curves with no interference from physiological concentrations of other ions, These electrodes were used successfully to measure a mean a(k) of 235 mM in vacuoles of epidermal cells of K+-replete barley leaves.