PURPOSE: To reduce the effect of an obstacle substance without using a toxious heavy metal complex by adding a borie acid compd. and an alkalinity control compd. to a buffering compsn. used in the assay of a test sample by an ion selective electrode.

CONSTITUTION: This compsn. contains about 0.3-5wt.%, pref. about 1-4wt.% of a boric acid compd. and an alkaline or actidic alkalinity control compd. of which the amt. is sufficient to impart pH proper to identification, for example, tetramethylammonium hydroxide. This heavy metal free compsn. is added in a ratio of about 3-25 pts. per 1 pts. of a test sample in a selective electrode method and used in order to dilute and buffer the test sample. By this constitution, an ion selective electrode can be reused and sensitive certification is rapidly performed and, since the ion selective electrodes can be continuously arranged in the assay of a single sample with respect to various analytes, this buffering compsn. is especially useful for an automatic analyser.