CALCULATION OF PH IN FRESH AND SEA-WATER AQUACULTURE SYSTEMS

A procedure for the calculation of pH in fresh and salt waters has bee n developed. The method is based on a fourth-order polynomial relation ship between hydrogen ion concentration and other (conservative) water quality parameters. The method avoids trial and error estimations and results in a direct calculation procedure that can be implemented in models developed in various modeling environments, such as spreadsheet s, conventional programming languages (BASIC, C, FORTRAN, PASCAL, etc. ) or specialized modeling languages (ExtendTM, StellaTM). The method d eveloped is based on the solution of the full alkalinity-pH equation. Because of the need for simplification of the equations to yield expli citly solvable polynomial equations, the accuracy of the solutions dep ends on the simplification made and varies with water properties. Thre e simplifications are tested based on a second-, a third- and a fourth -order polynomial equation for hydrogen ion concentrations. The equati ons have been tested for salinities ranging from 0 to 35 parts per tho usand (fresh to sea water), for temperatures ranging from 0 to 35 degr ees C, for total carbonate carbon concentrations of 0.1 and 5.0 mmol/l iter, and for total ammonia nitrogen concentrations of 0 and 10 mg/lit er. Approximations are most accurate in waters of high total carbonate carbon and low ammonia concentrations, where the fourth-order approxi mation yields results that are within 0.05 pH units for the full range of pH values tested (5-10).