The term "pH" is the abbreviation for "puissance de hydrogen." It refers to the alkalinity or acidity of a substance, be it water, potting mix or any other compound. Every plant has a pH range in which it thrives.
The center point on the pH value range is 7.0. Above 7.0 is alkaline and below 7.0 is acidic. African violets prefer a slightly acidic pH environment. Violets are most comfortable in the 5.8 to 6.5 range.
The potting soil pH is crucial to the health of African violets. A pH level outside the optimal range blocks the plant from getting necessary nutrients. The pH in the water affects the soil pH, making it a secondary factor. Water that has been processed through a softener system or distilled water should not be used to water houseplants. It throws off pH levels quickly.
If the potting mix has inhospitable pH for the plant, the plant starves for nutrients, and the leaves show it. The most common symptoms are leaf tips, edges, or veins that turn yellow. The quickest treatment is to remove all the old soil from the plant and repot it into fresh African violet mix. If you have a water softener system, use water that has not been treated to water your plants.