Question

How do you measure concentration of salt in water?

Answer 1

One way to measure the concentration of salt in water is to titrate a sample with a silver nitrate solution.

Silver nitrate reacts with sodium chloride to produce insoluble silver chloride.

AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaCl(aq)

You titrate a sample of the salt solution with silver nitrate solution. The endpoint occurs when no more of the cloudy white silver chloride precipitates. You often add dichlorofluorescein dye to make the endpoint more visible. The colour changes from greenish-yellow to pink at the endpoint.

Example:

What is the concentration of NaCl in aquarium water if a 3.00 mL sample requires 30.0 mL of 0.0250 mol/L AgNO₃?

Solution:

0.0300 L AgNO₃ × `(0.0250 mol AgNO₃)/(1 L AgNO₃) × (1 mol NaCl)/(1 mol AgNO₃)` = 7.50 × 10⁻⁴ mol NaCl

[NaCl] = moles/litres = `(7.50 × 10⁻⁴ mol)/(0.00300 L)` = 0.250 mol/L

The salinity of seawater is about 0.60 mol/L. Our aquarium isn't salty enough for marine fish.