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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.

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    $\begingroup$ Trisodium phosphate may be very bad idea unless very diluted. It is too basic for ordinary life, similarly as sodium carbonate. $\endgroup$
    – Poutnik
    Nov 25, 2023 at 17:24
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    $\begingroup$ The answer to the question in the title is "no." A multimeter cannot measure pH. Dissolving any ionic compound, whether acidic, basic, or neutral, will lower the resistance. There is no way to tell the difference. If you are asking whether a multimeter can determine the concentration of TSP in your water, then yeah, but why not just weigh it when you make the solution? Besides, the pH of your fertilizer is not especially useful for determining what happens when it is applied to soil. Soils generally have some buffering capacity. $\endgroup$
    – anon
    Nov 25, 2023 at 19:49

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A multimeter measures resistance, i.e., the blocking of electron (or ion) flow. A pH meter measures the voltage difference between glass-encased electrodes.

As you might realize, silica-based glasses are not very conductive, so a meter with very high input impedance must be attached to those probes.

Your proposed circuit might measure something, and perhaps what the circuit measures might have some relationship to ion concentration and ion mobility, but it would not measure pH, per se.

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  • $\begingroup$ Confused. Why would the meter need to have a high impedance? Wouldn't it need a lower impedance if the electrodes are not conductive? $\endgroup$
    – user148298
    Nov 27, 2023 at 0:25
  • $\begingroup$ See Ohm's law for resistance in series. $\endgroup$ Nov 27, 2023 at 0:28
  • $\begingroup$ @user148298 An ideal voltmeter has infinite impedance. The next best is having the impedance several orders higher than the impedance of the measured circuit. $\endgroup$
    – Poutnik
    Nov 27, 2023 at 7:21
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You cannot use a multimeter to measure the solution pH. What you could do is to measure it's conductance, measuring AC current going through the solution from AC low voltage ($\le \ \pu{12 V}$) source in defined geometry.

Carefully add small amounts of the trisodium salt until reaching maximal acceptable pH, measured by pH kits or paper strips. Note it's conductance (or AC current)

Then you can indirectly control pH of future prepared solutions by measuring it's conductance or AC current. This way you can avoid expenses for regularly spent pH strips or kits. OTOH, pH kits may last long.

But there remains factual objection - why trisodium phosphate? It is too alkalic and brings sodium to soil, what may not be desired.

I heard TSP was soft banned as an ingredient in household cleaners precisely because it's a powerful fertilizer, leading to the overgrowth of algae which leads to the death of fish in streams and lakes due to asphyxiation.

It is valid for all phosphates, But they were mostly used in solid formulas for wash machine. The question remains why trisodium phosphate? Other phosphates are more suitable as fertilizer. High pH would lead to phosphate precipitation by soil cations.

Why AC and not DC current?

AC is used to measure conductance to avoid deposition of chemicals on electrodes, what affects measurement. Solutions do not have simple resistance/conductance values like solids.

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  • $\begingroup$ I heard TSP was soft banned as an ingredient in household cleaners precisely because it's a powerful fertilizer, leading to the overgrowth of algae which leads to the death of fish in streams and lakes due to asphyxiation. $\endgroup$
    – user148298
    Nov 27, 2023 at 0:21
  • $\begingroup$ Why AC and not DC current? $\endgroup$
    – user148298
    Nov 27, 2023 at 0:22

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