(A–D) Potential of the IL-based reference electrode in Ca2+, Na+, K+, Mg2+, solutions respectively. (E,F) Potential of the IL-based reference electrode after successive dilution of calcium in artificial serum.(G) Potential of IL-based reference electrodes after spiking the serum with Ca2+. (H,I) Calibration curves of the calcium sensor versus IL-based reference electrode in deionized water and artificial serum respectively. Green bars highlight the physiological relevant Ca2+ concentration range in milk fever.