1.1 pH Sensor

Since the aquarium will house live fish, the quality of the water is extremely important. The Atlas Scientific Consumer Grade pH (potential of hydrogen) probe is a sensor that measures the hydrogen ion activity in a liquid. The pH scale (ranging from 1-14) is used to determine the acidity of a substance. When a substance has a pH value of “1”, the substance is considered highly acidic and has the highest amount of hydrogen ions. A pH value of “7” is considered neutral, while a value of “14” is considered highly alkaline and has fewer hydrogen ions. Figure 1-1 shows common substances and their associated pH values.

Figure 1-1. pH Scale

When water has a pH value of “1”, there are one million times more hydrogen ions than if the water had a neutral value of “7”, which is considered “pure” water or water that does not contain dissolved substances. This is due to the fact that a pH value of “1” represents 10¹ hydrogen ions, while the neutral value of “7” represents 10⁷ hydrogen ions. The difference between a pH of “1” and a pH of “7” is 10⁶, or one million. A pH value of “1” will have ten trillion (10¹³) times more hydrogen ions than a substance with a pH value of “14”.

The Atlas Scientific probe uses a silver chloride electrode and a reference wire to detect changes in ion activity. The electrode consists of a small, glass membrane constructed of silica glass containing metal salts. Inside of the glass membrane is a silver chloride solution and a silver wire. The reference wire is suspended in a neutral solution. Figure 1-2 shows the view of the glass electrode suspended in a test liquid.

Figure 1-2. pH Electrode

When the probe is placed into a test liquid, an ion exchange occurs between the metal salts on the inside of the glass membrane and the chloride solution. This creates a very small current in the chloride solution. Simultaneously, the test liquid also exchanges ions with the outer surface of the electrode, creating a current in the test liquid. The output of the probe represents the potential difference as an analog voltage signal.

The output of the probe is connected to the Atlas Scientific Gravity pH meter, which has hardware built in to provide any necessary analog signal conditioning, such as signal amplification. This allows the probe’s output voltage to be large enough to be read by an ADC.

Important: The analog output voltage of the probe is too small of a signal to detect with a voltmeter or through an ADC. Proper signal conditioning is required to obtain accurate results.