In the chemical industry, understanding the pH of solutions is essential for process control and safety. The pH scale ranges from 0 to 14, where values below 7 denote acidity, and values above 7 indicate alkalinity. pH is mathematically defined as the negative logarithm of the hydrogen ion concentration: pH = –log[H⁺].

While a pH test strip confirms that sodium hydroxide (NaOH) is a strong base, determining its precise pH requires calculating its molarity first. Below is a detailed, expert‑approved method.

Step 1: Compute the Molarity of Your NaOH Solution

Molarity (M) represents the number of moles of solute per liter of solution: M = moles solute ÷ liters solution. For example, if 1 g of NaOH is dissolved in water to a final volume of 250 mL:

• Calculate moles of NaOH: 1 g ÷ 40 g mol⁻¹ = 0.025 mol (molecular mass of NaOH = 40 g mol⁻¹).
• Convert volume to liters: 250 mL ÷ 1000 = 0.25 L.
• Determine molarity: 0.025 mol ÷ 0.25 L = 0.1 M.

Step 2: Understand NaOH Ionization in Aqueous Solution

NaOH fully dissociates in water, producing sodium (Na⁺) and hydroxide (OH⁻) ions: NaOH → Na⁺ + OH⁻. For a 0.1 M solution, this yields 0.1 mol L⁻¹ of OH⁻ ions.

Step 3: Calculate the pOH and Convert to pH

Use the relationship between hydroxide ion concentration and pOH: pOH = –log[OH⁻]. With [OH⁻] = 0.1 M, we get:

• pOH = –log(0.1) = 1.0
• Apply the complementary relation: pH + pOH = 14.
• Thus, pH = 14 – 1 = 13.

So the NaOH solution in this example has a pH of 13, confirming its strong alkaline nature.

For more complex concentrations or temperature variations, use a calibrated pH meter or consult the relevant solubility tables.