Understanding Concrete pH and pH Testing

By Ray Thompson

 

 

 

 

To understand pH and pH testing, let’s review some important definitions. pH is the measurement of acidity or alkalinity of a substance It is expressed as a number from 0-14. pH is the concentration of hydrogen ions commonly expressed in terms of the pH scale. Low (pH 0-7) corresponds to high hydrogen ion concentration. A substance that when added to water increases the concentration of hydrogen ions (lowers the pH) is an acid. A substance that reduces the concentration of hydrogen ions, raises the pH, (pH 7-14) is an alkali base.

The relative strengths of various materials increase as the values move away from pH of 7.0 neutral. Examples are as follows:

Concentration of Hydrogen Ions pH Examples of solutions at that pH

10,000,000 – pH 0 = Sulfuric acid (battery acid)

1,000,000 – pH 1 = Hydrochloric acid

100,000 – pH 2 = Vinegar

10,000 – pH 3 = Grapefruit juice (Citric acid)

1,000 – pH 4 = Tomato juice

100 – pH 5 = Black coffee

10 – pH 6 = Urine

1 – pH 7 = Distilled water

1/10 – pH 8 = Sea water

1/100 – pH 9 = Soda

1/1,000 – pH 10 = Milk of magnesia

1/10,000 – pH 11 = Ammonia

1/100,000 – pH 12 = Soapy water

1/1,000,000 – pH 13 = Oven cleaner

1/10,000,000 – pH 14 = Drain cleaner

Taking a look at the mathematical definition. The key word is logarithmic. This means that as the values change from neutral (pH 7) the value increases 10 times the previous value. For example, a pH of 6 is ten times more acidic than neutral and a pH of 8 is ten times more alkaline than neutral.

Most flooring adhesives are designed to handle a pH of 9. A pH of 9 is 100 times greater than neutral and a pH of 10 is 1000 times greater than neutral. Most flooring adhesives are designed to be alkaline resistant, yet there are few that are acid resistant. Therefore, it is important that when evaluating a concrete slab it be checked for either acidity or alkalinity. While rare, most adhesives will fail when the pH drops below a pH of 5.

pH Paper Strip Testing

There are several methods of pH testing. A spray solution that is applied to the surface of the concrete will turn color and then will be matched to the color chart to determine the pH.

  1. A pH pencil (not shown) which is used to place an “X” on the concrete then covered with distilled water will turn color to be matched up with the color chart.
  2. pH paper which is then placed into a small puddle of distilled or de-ionized water will then turn color to determine the pH.
  3. pH meters range from inexpensive in the foreground an expensive meter (in case). The meters have a probe that is placed into a puddle of distilled or de-ionized water and will give you a digital value of the pH.

The most common is the qualitative pH paper test shown here with a 100 pack of pH strips and a small bottle of distilled or de-ionized water. The pH paper strips are used to test the surface as follows:

  1. Be sure the surface of the concrete is free from any residual. If a residual exists it must be removed this is done by lightly sanding the surface of the concrete with a fine 320 grit wet and dry sandpaper (dry). One must be careful not to go too deep into the concrete. If you were to penetrate below the carbonation layer you will get high alkalinity readings usually in the 12-13 brackets.
  2. Pour a small quantity of distilled or de-ionized water onto the surface of the concrete approximately 1-inch in diameter.
  3. Allow the water to set on the surface of the concrete for approximately one minute before placement of the pH strip into the water.
  4. Allow the pH strip to remain in the water for about 10-seconds to change color.
  5. Remove the color strip and compare the strip with the color chart to determine the pH of the concrete surface. Check with the flooring adhesive manufacturer’s guidelines to determine the acceptable pH levels.

The pH paper test is a qualitative test and requires a visual color match to give you a pH reading.

pH Meter Testing

The pH meter is the most accurate of the pH test methods and will give you a quantitative value. Along with a meter you will find three units of buffer solution, a pH probe and thermometer. To do this testing requires a little more time but the accuracy is worth the effort. To do the test you need to do the following:

  1. Be sure the surface of the concrete is free from any residual. If a residual exists it must be removed this is done by lightly sanding the surface of the concrete with a fine 320 grit wet and dry sandpaper (dry). One must be careful not to go too deep into the concrete. Carbonation layers are approximately 1mm thick.
  2. (Photo #4 – P4250010.jpg) Next, take an accurate ambient temperature reading to aid in the calibration of the unit.
  3. Once the thermometer has stabilized, attach the probe lead to the meter (This is confusing here) and insert the probe into the pH 7.0 buffer solution. Adjust the dial to the proper ambient temperature, then place the probe into the buffer solution and adjust the calibration dial to the pH of the buffer solution. When the calibration is complete you are ready to take pH reading of the slab.
  4. Pour a small amount of distilled or de-ionized water on the surface of the concrete and carefully place the tip of the probe into the water. The tips of these probes are very delicate and must be used carefully. Be sure to leave the probe in the solutions enough time for the meter to stabilize.

pH testing is a necessary part of determining if the concrete at a level acceptable for installation of flooring materials. In situations where you are getting high pH readings you will find higher than normal moisture migration.

Study the pH chart given below carefully. Note that each decrease in pH by one pH unit means a tenfold increase in the concentration of hydrogen ions.

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