The sulfur dioxide present in must and wine is not all useful as a preservative. Some is bound to other chemical components, reducing its effectiveness as a preservative by 30 - 70%. Also, the degree of binding can change over time, affecting the ability of an adequate dose to continue providing its preservative properties. The effectiveness of sulfur dioxide as a preservative is also pH-dependent, with low pH wines requiring lower levels, and high pH wines requiring significantly greater concentrations to achieve the same effectiveness as in a typical wine with a pH of 3.3 - 3.4. The Accuvin Free SO2 Kit is divided into a Low Range (0 - 40 ppm) and a High Range (40 - 130 ppm). 10 Tests per kit.