Hi Frank, the pH of a mature sourdough starter can vary quite a bit, depending on when it was last fed and how fermented it is, but generally you're aiming for something in the 3.5-5.0 range, so it does sound like the pH of your starter is a little too high. Once a starter develops this level of acidity, it's better able to ward off unfriendly bacteria and mold. If your starter is still in the creation phase it may not have developed the requisite acidity yet. However, if your starter is fully developed it may be that your feeding routine is shutting down fermentation too rapidly. This could happen if you generally store your starter in the refrigerator and don't allow it any room temperature fermentation time after a feeding and before refrigeration. Once your starter goes in the refrigerator, fermentation slows down significantly, and if your starter is rarely allowed time to rise and ferment at room temperature it may not be getting enough time and opportunity to ferment properly and develop the proper level of acidity. Try  giving your starter a few days of twice a day feedings at room temperature (about 12 hours apart) and see if your starter improves. When you plan to store your starter in the refrigerator, be sure to give it a few hours at room temperature to start bubbling and fermenting before you stick it in the fridge. 

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