The accuracy of QAC tests strips does not depend on the pH value of solutions to be tested in the range of 1.86 – 9.2 pH, but rather it depends on buffering capacity. When the pH of a solution to be tested is a result of acidic or alkaline compounds without buffering capacity, then the accuracy of the test is not influenced. In the case that the pH of a solution to be tested is a result of a buffer, then the measure of influence depends on the buffering capacity of the solution.

Test:
Sets of control solutions with QAC concentrations 0 – 50 – 100 – 200 – 400 ppm were prepared in following solutions:

  • Hydrochloric acid /0.1 M/, pH 1.1
  • Tartaric acid /0.1M/ , pH 1.86
  • McIlvaine buffers (0.1M citric acid, 0.2 M  disodium hydrogen  phosphate), pH 2.2 – 3.0 – 4.0 – 5.0 – 6.0 – 7.0 – 8.0 – 9.0
  • Sodium tetraborate /0.01M/ , 9.20

The table below contains readings according to the regular color chart:

    pH buffer  /b/  0 ppm QAC   50 ppm QAC  100 ppm QAC  200 ppm QAC  400ppm QAC
    1.1       0       0       0     ≤50      50
    1.86       0      50     100     200     400
    2.2  /b/       0    0-50       50     100     200
    3.0  /b/       0      50     100     200     400
    4.0  /b/       0      50     100     200     400
    5.0  /b/     ≤ 50     100     200     400    ≥400
    6.0  /b/      50     200     400   higher  higher
    7.0  /b/     100     400     ≥400   higher  higher
    8.0  /b/     200     400     higher   higher  higher
    9.0  /b/     400    higher     higher   higher  higher
    9.2       0       50      100     200     400