The understanding of the interaction of such parameters as pH, temperature and salinity of water is fundamental to many applications, environmental monitoring and health studies.
Solution pH is a key variable used to describe the equilibrium and kinetics of chemical processes in oceanic and fresh waters, and deep reservoirs (B. Yang et al., 2014). The influence of acidity or alkalinity of the surrounding aqueous medium measured by pH on the aquatic life is significant because it controls the extent of ionization, enzyme function and oxidation potential. The average pH for sea water is. 8.2 but can range between 7.5 and 8.5. However, the determination of the pH of seawater has always been a rather difficult problem because it contains various ions (Gieskes, 1969).
Measurements of pH and salinity of formation water have a wide range of applications including determination of reservoir compartmentalization and contamination of water with drilling mud, characterization of transition zone and delineating of oil/water contact, prediction of corrosion and scaling, and for injection of polymers and gels. In Arabian Gulf region, salinity of aquifers reaches 20 g/L while brine salinity varies from 50 to 150 g/L (Alsharhan et al., 2001). Salinity of formation water can be as high as 350 g/L. For example, salinity of Shuaiba and Natih formations in North Oman shows above 200 g/L (Al Lamki and Terken, 1996).
Water studies are usually performed using synthetic brines prepared using tap water in the distilled or deionised form. The distilled and deionised water are preferable to tap water because the latter contains ions and elements that can be corrosive or cause other complications and unpredictable results during investigations. The distilled water obtained by the steam condensing is free of impurities. High purity deionised water, that is believed generally similar to distilled water, is produced by deionization from tap water, and it is more available and cheap.
The optimum pH of tap water varies in different supplies according to the composition of the water and the nature of the construction materials used in the distribution system, but it is often in the range 6.5–9.5 (Eaton et al., 2017). The pH of most raw water sources lies within the range 6.5–8.5. The pH of distilled water is 4.5–6.5 (Youmans, 1972).
Temperature is one of the most important parameters for many chemical and microbiological processes controlling the quality of tap water due to its effects on copper solubility, the rate of corrosion, lead leaching from brass fixtures, bulk chlorine decay rate and formation of disinfection by-products (Zlatanovich et al.2017, Ong et al., 2007). In pure water, a decrease in pH of about 0.45 occurs as the temperature is raised by 25 oC .
Temperature decreases the pH due to the change of water dissociation constant. The pH measurements are sensitive to temperature and differ when measured by different devices or computed using different models (Marcus, 1989). The pH of water when heated in a closed environment changed from 7 at 25 oC to 5.8 at 150 oC. Higher temperature can increase biological activity twofold when temperature increases by 10°C (V. Kooij, 2003).
The solubility of the CO2 and H2S gases has a significant impact on the acidity of water. It generally decreases with increased water salinity and temperature and increases with pressure. An increased carbon dioxide concentration lowers pH, whereas a decrease causes it to rise.
Thus, the obtained results can in a great extent depend on the properties and quality of applied laboratory water.
Electrical conductivity is widely used for monitoring the mixing of fresh water and saline water, separating stream hydrographs, and geophysical mapping of contaminated ground water (Covington and Whitfield, 1988). Uncertainties, however, still exist regarding predictions of salinity using conductivity computed by various models. The generally used formulae for temperature correction give results that deviate considerably from the values determined by actual measurements.
(Jones et al., 2016) conducted measurements of pH, conductivity and dissolved CO2 to asses carbon removal from drinking water, particularly from a carbon footprint/GHG perspective.
However, most of the publications available on the subject of pH dependency on salinity cover low salinity range, study each parameter differently, or did not study the relationships that existed between pH and temperature over the broad range of salinities. Sodium chloride solutions are mostly neutral solutions and might be expected to vary little in terms of their pH value. However, there is no clarity regarding whether the salt presence has increasing, decreasing or neutral effect on the pH.
The objective of this investigation is to study the relationship between pH, conductivity, temperature in aqueous high salinity solutions of sodium chloride (0-140 g/L) and different range of temperature (13–43 oC). The synthetic brines are prepared using distilled, deionised and tap water, boiled and unboiled, to investigate pH changes with the salinity and temperature and find out whether the pH differ for these types of water.