The other electrode responds to the acidity of the water sample.
A voltmeter in the probe measures the difference between the voltages of the two electrodes.
Red cabbage juice, for example, changes color in response to p H all the way from red (p H = 2) through blue at neutral p H to greenish-yellow at p H = 12, plus you are more likely to find cabbage at the local grocery store than lichen.
The dyes orcein and azolitmin yield results comparable to those of litmus paper.
Inside the thin glass bulb at the end of the probe there are two electrodes that measure voltage.
One electrode is contained in a liquid that has a fixed acidity, or p H.
The litmus test is a quick method of determining whether a liquid or gaseous solution is acidic or basic (alkaline.) The test can be performed using litmus paper or an aqueous solution containing litmus dye. The blue paper changes to red, indicating acidity somewhere between the p H range of 4.5 to 8.3.
(Note, however, that 8.3 is alkaline.) Red litmus paper can indicate alkalinity with a change to blue.
Litmus paper may be dampened with distilled water to give a color change for a gaseous sample.
Gases change the color of the entire litmus strip since the whole surface is exposed.