Measuring Total Dissolved Solids

The conductance of an aqueous solution is a measure of its Total Dissolved Solids.

Conductance ranges

Using a cell constant of K = 0.0922 cm^-1, we expect the following values of conductance and conductivity for some common substances:

Solution	Conductivity (µS/cm)	Conductance	Resistance
Pure deionized water	0.055	0.6 µS	1.67 MΩ
Distilled water	0.5	5.4 µS	184 KΩ
Power plant boiler water	1	10.9 µS	92 kΩ
Soft city tap water	50 (range: 5-100)	543 µS	1.84 kΩ
Hard city tap water	500 (range: 100-700)	5.43 mS	184 Ω
Blood	6700	0.073 S	13.7 Ω
Ocean water	53,000	0.575 S	1.74 Ω
31% HNO3	865,000	9.39 S	0.1 Ω

Conductivity standard solutions range from 1µS to 200 mS in value. The conductivities of some common substances are as follows:

Conductivity values at 25°C
Concentration		Conductivity, µS/cm
wt-%	ppm (mg/L)	NaCl	NaOH	HCl	Acetic acid
0.0001	1	2.2	6.2	11.7	4.2
0.001	10	21.4	61.1	116	15.5
0.01	100	210	603	1140	63
0.1	1000	1990	5820	11,100	209
1.0	10,000	17,600	53,200	103,000	640
10.0	—	140,000	358,000	709,000	1530

Measuring total dissolved solids (TDS)

A common use for conductivity sensors is to measure the concentration of total dissolved solids (TDS) in water samples. Usually there's a roughly linear relationship between conductivity and the concentration of ions in a solution, at least until very great ion concentrations are attained. In particular, for salts, an average of 2 μS/cm is produced for each ppm (by weight, or mg/L) of dissolved solids. Most acids and bases are much more conductive than their salts because of the vastly greater mobilities of the H₊ and OH_- ions.

Because the proportionality constant varies with the type of ion, TDS should be computed from the conductivity with the solution composition in mind. Pre-formulated ppm TDS standard calibrated solutions are available to match the solutions you are analyzing. Calibrated solutions are formulated with calcium carbonate (CaCO₃), sodium chloride (NaCl), potassium chloride (KCl), or the 442 (40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride) natural water formulation. If your analyte's major dissolved solids components are similar to these, you can choose the formulation that best matches your solution. Generally, CaCO₃ is used for boiler waters, NaCl is used for brines, and the 442 formulation is used for lakes, streams, wells, and boilers.

Temperature compensation

Conductivity depends on both ion concentration and ion mobility. A mobility generally increases with temperature, conductivity measurements are temperature dependent, increasing about 2% for each °C increase in temperature. The degree to which temperature affects conductivity depends on the types of ions involved. The conductivity can be temperature compensated with an equation of the following form:

Eqn. 1

σ(T) = σ(T_o) ( 1 + α (T - T_o))

where,

σ(T) is the solution conductivity at any temperature T;
σ(T_o) is the solution conductivity at a reference temperature T_o, usually 25°C; and,
α is the temperature coefficient in °C^-1.

Common alphas (α) are listed in the table below.

Temperature coefficients of various electrolytes at 25°C
Substance	Concentration wt%	Conductivity mS/cm	Temperature coefficient, α %/°C
Ultrapure deionized water	–	0.000055	5.5
Fresh or tap water	–	0.005-0.1	∼2.0
NaOH	5	–	2.01
HF	1.5	–	7.20
HCl	5	395	1.58
	10	630	1.56
	20	762	1.54
	30	–	1.52
HNO₃	6.2	312	1.47
	12.4	542	1.42
	31	782	1.39
H₂SO₄	5	209	1.21
	10	392	1.28
	20	653	1.45
	50	–	1.93
CH₃COOH	10	1.53	1.69
	15	1.62	1.74
	20	1.61	1.79
KCl	5	69	2.01
	10	136	1.88
	15	202	1.79
	20	–	1.68
NaCl	5	67	2.17
	10	121	2.14
	15	164	2.12

Because dissolved solids are usually dominated by sodium chloride, a value of α=2%/°C is often used.