The pH of water plays an important role in determining the water quality status since the normal range of pH for surface water systems is 6.5 to 8.5. Any value outside the range must be examined. It is also important to note that the relationship between pH and WQI varies from place to place. This is because pH of water can be affected by natural changes or man-made (eg: acid rain, anthropogenic activity, wastewater discharged). If you use NSFWQI, for example, in your calculation, it is found that the value of Quality index of pH is highest when it reaches 7.5 and decreases through going to 1 or 14 values (See pH curves in NSFWQI method)

pH indicate acidity and alkalinity. water quality depend upon various chemical ,physical and bacteriological parameters.

The Water Quality Index (WQI) considers pH as one of its parameters, but it has a negative correlation. As the pH value decreases, water tends to become more acidic, making it less suitable for consumption or aquatic life. On the other hand, parameters like dissolved oxygen and total dissolved solids have a positive correlation with the WQI as they indicate healthier water conditions.