text: ph measurements? describe the princibles of ph measurements the ph scale measures the acidity of a substance.  ph 7 is neutral. de-ionised water has a ph of 7.  a ph less than 7 (i.e 0 to 6.9) indicates an acidic substance. (such as vinegar)  a ph greater than 7 (i.e 7.1 to 14 (the scale ends at 14)) indicates an alkali substance (think of alkali as opposite to acid). alkaline is also know as basic.  hope this helps