I would like to feed my garden a solution of water and Trisodium Phosphate (TSP), but I am uncertain of the correct amount of TSP to water ratio. Too acidic or too basic of a solution can harm my plants. Since TSP is very basic, it should be more conductive, or inversely, has less resistance than pure water which has a high resistance.
I figure I might be able to use a multimeter to measure the pH, but how I would calibrate and interpret the readings. A resistance of zero would mean it's very basic, but it could also mean it's very acidic. Since regular tap water is already fairly conductive, the readings would require some scaling using a resistor in series with the probes.
Alternatively, would supplying a small voltage (3V) in series with the probes while measuring of the solution to determine its polarity and resistance and thereby its pH? A negative reading, implies it's acidic whereas a positive reading implies its basic.