Table salt, known chemically as sodium chloride (NaCl), is a common compound. A solid substance does not have a pH; the measurement is only relevant when the compound is dissolved in water. When pure sodium chloride is dissolved in pure water, the resulting solution is theoretically neutral, registering a pH of 7. This neutrality is a direct consequence of the chemical origins of sodium chloride.
Why Table Salt is Chemically Neutral
Sodium chloride is classified as a neutral salt because of the strength of the acid and base used to form it. The compound is the product of a neutralization reaction between hydrochloric acid (HCl), a strong acid, and sodium hydroxide (NaOH), a strong base.
When the salt dissolves in water, it dissociates completely into its constituent ions: the sodium cation (Na+) and the chloride anion (Cl-). The neutral nature of the solution hinges on the behavior of these ions in the presence of water molecules. Neither the sodium ion nor the chloride ion reacts significantly with water in a process called hydrolysis.
Since the chloride ion is derived from a strong acid, it has no tendency to accept a proton (H+) from water. Similarly, the sodium ion, derived from a strong base, does not readily donate a proton to water molecules. This lack of reaction means the equilibrium between hydrogen ions (H+) and hydroxide ions (OH-) naturally present in water is undisturbed.
Because the concentrations of H+ and OH- remain equal, the solution maintains a pH of 7. The ions simply exist as dissolved entities surrounded by water molecules, without altering the fundamental balance that defines water’s neutrality. This theoretical neutrality is a benchmark for laboratory-grade sodium chloride dissolved in pure water.
Measuring pH in a Salt Solution
To accurately determine the pH of a salt, the solid must first be dissolved in a solvent, typically distilled or deionized water. Using purified water is important because tap water contains dissolved minerals and gasses like carbon dioxide that can already make the starting solution slightly acidic or basic. The most reliable method for measuring the resulting aqueous solution is the use of a calibrated laboratory pH meter.
These meters employ a glass electrode that measures the concentration of hydrogen ions. However, measuring the pH of concentrated salt solutions presents a practical challenge due to their high ionic strength. The dense concentration of sodium and chloride ions can affect the electrode’s response, sometimes leading to a measurement error called the liquid junction potential.
This effect can cause the pH meter to register a reading that is slightly lower than the true chemical pH, sometimes by as much as 0.2 pH units. Therefore, the results for highly concentrated salt water must be interpreted with caution, even though a pH meter is the most precise tool available. Specialized electrodes or calibration with high ionic strength buffers are sometimes employed to minimize this measurement bias.
Factors That Influence Salt pH
Real-world table salt often contains minor additives and impurities that cause the pH of its solution to deviate slightly from the ideal 7. One of the most common reasons for a variation above 7 is the fortification of salt with iodine. To prevent the iodine compound, such as potassium iodide, from oxidizing and being lost, manufacturers often add stabilizing agents.
These stabilizers are frequently alkaline compounds, such as sodium carbonate or calcium carbonate. Their purpose is to maintain a slightly basic environment, often resulting in a pH near 8, which slows down the degradation of the iodine. The small amount of these basic additives is what shifts the pH of iodized salt solution away from perfect neutrality.
Conversely, some specialty salts, like certain sea salts, may register a pH slightly below 7 due to trace mineral content. These salts naturally contain small amounts of other chloride compounds, such as magnesium chloride (MgCl2) and calcium chloride (CaCl2). While seemingly similar to sodium chloride, the metal ions in these compounds are derived from weaker bases.
When dissolved, the magnesium and calcium ions can slightly influence the water’s H+ concentration, leading to a minimally acidic solution, typically registering a pH between 6.5 and 7.0. The final pH of a table salt solution is therefore a balance between the inherently neutral sodium chloride and the minor acidic or basic components added or naturally present.