as detailed below. The difference between p[H] and pH is quite small. It has been stated that pH = p[H] + 0.04. It is common practice to use the term "pH" for both types of measurement. pH indicators Indicators may be used to measure pH, by making use of the fact that their color changes with pH. Visual comparison of the color of a test solution with a standard color chart provides a means to measure pH accurate to the nearest whole number. More precise measurements are possible if the color is measured spectrophotometrically, using a colorimeter or spectrophotometer. Universal indicator consists of a mixture of indicators such that there is a continuous color change from about pH 2 to pH 10. Universal indicator paper is made from absorbent paper that has been impregnated with universal indicator. Another method of measuring pH is using an electronic pH meter. pOH pOH is sometimes used as a measure of the concentration of hydroxide ions, . pOH values are derived from pH measurements. The concentration of hydroxide ions in water is related to the concentration of hydrogen ions by where KW is the self-ionization constant of water. Taking logarithms So, at room temperature, pOH ≈ 14 − pH. However this relationship is not strictly valid in other circumstances, such as in measurements of soil alkalinity. Extremes of pH Measurement of pH below about 2.5 (ca. 0.003 mol/dm3 acid) and above about 10.5 (ca. 0.0003 mol/dm3 alkaline) requires special procedures because, when using the glass electrode, the Nernst law breaks down under those conditions. Various factors contribute to this. It cannot be assumed that liquid junction potentials are independent of pH. Also, extreme pH implies that the solution is concentrated, so electrode potentials are affected by ionic strength variation. At high pH the glass electrode may be affected by "alkaline error", because the electrode becomes sensitive to the concentration of cations such as and in the solution. Specially constructed electrodes are available which partly overcome these problems. Runoff from mines or mine tailings can produce some very low pH values. Non-aqueous solutions Hydrogen ion concentrations (activities) can be measured in non-aqueous solvents. pH values based on these measurements belong to a different scale from aqueous pH values, because activities relate to different standard states. Hydrogen ion activity, aH+, can be defined as: where μH+ is the chemical potential of the hydrogen ion, is its