PH papers normally are supplied with a PH color chart which shows different colors corresponding to different PH levels. To use the paper, you tear off a strip and immerse it in the fountain solution. The color developed in the paper is then compared with the PH color chart to determine the PH level of the solution.
Please note that the conventional fountain solution requires a certain level of acidity. This acidity together with the gum arabic desensitizes the non image areas of the plate. This prevents scumming in the non image areas. Too much acidity, however, will reduce the ability of image areas to hold ink, resulting in ghosting. It is important, therefore, to maintain the right PH level.
A PH of 7 is neutral. Values higher indicates alkalinity. Values lower indicates acidity. A PH range of 3.5 to 4.5 is generally required. However this may vary depending on the ink, paper and fountain concentrate used.
Hope this info helps.