1. A method of determining pH in blood invasively, in vivo, and within the physiological ranges observed in said blood utilizing blood spectra which spectra contains histidine information, said method comprising steps of: a) generating light at three or more different wavelengths, said wavelengths being in the range of 1000 nm to 2500 nm; b) invasively irradiating said blood with said wavelengths so that there is differential attenuation of at least some intensities of wavelengths, said wavelengths dependent differential attenuation being a function of said blood, including histidine in said blood; c) measuring at least a portion of said intensities of said wavelengths emerging from said blood to obtain a set of at least three spectral intensities v. wavelengths; and d) estimating said value of said pH from said measured intensities by utilizing wavelength dependent differential attenuation derived from said histidine, said value being within said physiological ranges.