Title: Ph Resistance

pH Resistance

Geomembranes can accomodate a wide range of pH. This technote lets you convert pH values to concentrations so that you can compare chemical resistance with pH.

The pH of a solution is a measure of how acidic (or basic) a solution is. A neutral solution has a pH of 7.0 which is neither acidic or basic. An acidic solution will have a pH of between 0 (strongly acidic) and 7.0 (neutral). A basic solution will have a pH between 14 (strongly basic or caustic) and 7.0 (neutral). pH is easily measured by using hand-held measuring devices or litmus paper tests. pH can show how acidic a solution is (or how basic it is) when more than one acid or base are mixed together. This is commonly measured when adding a base to neutralize an acidic solution.

The definition of pH is the negative logarithm of the actual number of hydrogen ions (for acids) or hydroxy ions (for bases) in one litre of solution. Since the scale is logarithmic, a single step in pH represents a change of ten times the number of ions available.

We can relate the two ends of the pH scale to common solutions. A pH of 0 is defined as one mole of Hydrygen ions (H+) in one litre of solution; a 36.5% solution of HCl (hydrochloric acid). A pH of 14 is defined as one mole of hydroxy ions (OH-) in one litre of solution; a 31% solution of NaOH (sodium hydroxide). Any solution that has one mole of ions (there are 6.02 x 1023 ions in a mole) per litre would be either a 0 pH for an acid or a 14 pH for a base.

Geomembranes are made with highly chemically resistant plastics. Most geomembranes have outstanding resistance to acids and bases. Normally geomembrane chemical resistance is stated in terms of the percentage of a solution. What is needed is a conversion between the concentrations given in a chemical chart and the pH of acidic and basic solutions.

Each acid or base produces a slightly different pH depending on the molecular structure of the compound. Table 1 presents a conversion table for chemical concentration to pH. In this table the range of pH is compared to the concentration of typical acids and bases at 20°C. The concentrations in Table 1 are the theoretical concentrations that these compounds would produce. Some concentrations listed may be greater than what a saturated solution would produce. It may not be possible to dissolve a mole of some compounds to produce a 0 or a 14 pH (due to solubility limits).


Table 1: pH vs. Concentration of Selected Compounds
pH 0* 1 2 3 4 5 6 7 8 9 10 11 12 13 14*
Acetic Acid 60.1% 6.01 0.601 0.060 0.006 0.0006 0.00006 0.000006              
Barium Hydroxide               0.000009 0.00009 0.0009 0.009 0.086 0.857 8.57 85.7%
Calcium Hydroxide               0.000004 0.00004 0.0004 0.004 0.037 0.370 3.70 37.0%
Citric Acid 70.0% 7.00 0.700 0.070 0.007 0.0007 0.00007 0.000007              
Hydrochloric Acid 36.5% 3.65 0.365 0.037 0.004 0.0004 0.00004 0.000004              
Lactic Acid 90.1% 9.01 0.900 0.090 0.009 0.0009 0.00009 0.000009              
Nitric Acid 63.0% 6.30 0.630 0.063 0.006 0.0006 0.00006 0.000006              
Phosphoric Acid 32.7% 3.27 0.327 0.033 0.003 0.0003 0.00003 0.000003              
Potassium Hydroxide               0.000006 0.00006 0.0006 0.006 0.056 0.561 5.61 56.1%
Sodium Hydroxide               0.000003 0.00003 0.0003 0.003 0.031 0.310 3.10 31.0%
Sulphuric Acid 49.0% 4.90 0.490 0.049 0.005 0.0005 0.00005 0.000005              
* The concentrations shown may not be practical due to the limits of solubility of certain chemicals. The concentrations shown are theoretical. All pH values are at 20°C.

Geomembranes are made from some of the most chemically resistant polymers available. Even a geomembrane considered to have "poorer" chemical resistance will likely have excellent resistance to acids and bases.

Geomembranes do not usually have difficulty with pH in the range of 1 to 13. Some geomembranes will be resistant to acids and bases outside the 1 to 13 pH range but at that point an individual chemical test should be performed with the chemical and geomembrane combination.

A listing of geomembrane resistance to different concentrations of acids and bases is available in the Layfield Plastics Tech Note on Chemical Resistance.

Temperature has a profound effect on pH. All the calculations in Table 1 pH vs Concentration of Selected Compounds refer to a temperature of 20°C. If containment will take place at temperatures significantly different from 20°C then we recommend that an immersion test be carried out with the chemical and geomembrane combination proposed for your project.

Table 2 Variation of pH with Temperature shows the change in pH for a saturated solution of Calcium Hydroxide (a 1.48% solution of CaOH). Note that the solution becomes more basic at lower temperatures. At higher temperatures the pH actually becomes less. This is not how you would expect temperature to affect a solution. Since the pH of solutions may react to temperature in different ways we caution you to test any solution that is at a temperature other than 20°C.


Table 2: Variation of pH with Temperature
Temp 0°C 10°C 20°C 30°C 40°C 50°C 60°C 70°C 80°C
Calcium Hydroxide 1.48% 13.360 pH 12.695 pH 12.602 pH 12.431 pH 11.959 pH 11.678 pH 11.423 pH 11.192 pH 10.984 pH

Normally when a pH range is specified for a geomembrane containment, the concentration of the acid (or base) is quite reasonable. It is common for a concentrated solution to be refered to by a percentage concentration, and a dilute solution of an acidic or basic solution is not chemically challenging for a geomembrane.

Please make sure that you take the information on this Tech Note and compare it with the concentrations listed in our Chemical Resistance Tech Note before choosing your geomembrane material. In many cases a pH between 1 and 13 will mean that almost all geomembrane materials may be suitable in a secondary containment application. In long term primary containment applications we will warrant our geomembranes for pH between 4 and 9. Please contact your Layfield representative.