a method and apparatus are disclosed for an analyzer which measures concentration of a liquid specimen by monitoring the coloration of test paper which is either dipped into the specimen or is otherwise painted with it . in the method , inner stray rays produced by a light source which do not reach the test paper are experimentally monitored , so that the effect of such inner stray rays upon the accuracy of the test paper analysis is eliminated . additionally , a method of relating relative reflectivity of the test paper to the concentration of the specimen is disclosed , in which method the calibration curve is modeled as a section of a hyperbola . in the apparatus , a microcomputer performs the evaluation and the detection of the coloration of the test paper and the compensation for various sources of instrumental error takes place electronically .