**2.4 Effect of pH, Ca2+, and Al3+ on molar absorptivity of DOM**

In three sampling campaigns, the effects of pH, Ca2+ and Al3+ concentrations on the molar absorptivity of DOM was checked. The effect of pH was studied by using *Dissolved Organic Matter and Its Ultraviolet Absorbance at 254 Nm in Different Compartments… DOI: http://dx.doi.org/10.5772/intechopen.98861*

a titration system. To each one 25 ml of filtered sample 0.025 *M* HCl or NaOH was added until a final pH of 3, 4, 5, 6, 7, and 8 (± 0.1) was established. Addition of acid or base was performed with the titration system 725 Dosimat and pH-meter 691 (Metrohm). Dilution of the samples by adding acid or base was taken into account when calculating final DOM concentration. The influence of Ca2+ and Al3+ concentrations was studied by adding 1 ml salt solution (blank: 1 ml de-ionized water) with known amounts of CaCl2 or AlCl3 to 25 ml sample to give final concentrations of 1, 10, and 100 mmolc l−1. Ultraviolet absorbance was measured and the molar absorptivity was calculated as described above. All experiments were performed in triplicate. By variance analysis it was tested whether differences among the treatments were statistically significant (*F*-test, *p* < 0.05).

#### **2.5 Statistical evaluation**

All statistical calculations were performed with SPSS 10.0 (*Program SPSS* Inc., Chicago, U.S.A.).
