Urinary excretion of titratable acid and its relationship to urinary pH and inorganic phosphorus excretion.

Abstract

SummaryThe relationship between urinary pH, titratable acid (T.A.) and inorganic phosphorus (I.P.) excretion was examined in a group of 81 hospital out‐patients and some normal volunteers, who presented for one‐day renal function testing. Although the patients had a variety of renal diseases, with glomerular filtration rates ranging from 7 to 146 ml. per minute per 1·73 sq. m., a strictly linear relationship was found between the “ratio of urinary T.A. over urinary I.P.” and urinary pH, expressed by the following equation: urinary T.A./urinary I.P. = 4·2409‐0·5678 × urinary pH. From this relationship it was deduced that abnormalities of urinary T.A. excretion in these patients were mainly dependent on abnormalities of either urinary I.P. excretion, or urinary pH, or both. For any given level of urinary pH, variations in urinary T.A. excretion were caused mainly by variations in urinary I.P. excretion. The theoretically derived sigmoid titration curve of a buffer with a pK′ of 7·20 most closely fitted the experimentally determined regression line, especially between urinary pH values of 6·0 and 8·0 units ; this was consistent with the behaviour of phosphoric acid at 25° C., the temperature of titration. The standard error of the method for deriving urinary I.P. excretion from urinary pH and urinary T.A. excretion measured titrimetrically was 5% ; and this was considered sufficient to allow its use in clinical biochemistry as a screening procedure for urinary I.P. excretion, as part of a one‐day renal function testing system.