Methylene blue is a synthetic organic compound with the chemical formula C₁₆H₁₈N₃SCl. It is a cationic dye, meaning that it carries a positive charge and is attracted to negatively charged surfaces. It is soluble in water and alcohol.

A rapid estimate of the amount of montmorillonite present in a mud or clay can be obtained by means of the methylene blue test. This test measures the amount of methylene blue dye adsorbed by clays, which is a function of their base exchange capacity. Since montmorillonite has a much larger base exchange capacity than other clay minerals, the test has come to be regarded as measure of the amount of montmorillonite present, reported as estimated bentonite content.

• Methylene blue solution:1 ml = 0.01 milliequivalents.
  • (This solution has a concentration where 1 milliliter (ml) equals 0.01 illiequivalents. It’s prepared by dissolving 3.74 grams of USP-grade methylene blue (C₁₆H₁₈N₃SCl•3H₂O) per liter)
• Hydrogen peroxide, 3% solution.
• 5 N sulfuric acid solution.

• Measure 2 ml of drilling mud (or a volume that requires 2–10 ml of methylene blue for titration).
• Add the mud to 10 ml of water in a 150-ml Erlenmeyer flask.
• Add 15 ml of 3% hydrogen peroxide and 0.5 ml of 5 N sulfuric acid.
• Swirl the contents thoroughly and then gently boil the mixture for 10 minutes.
• After boiling, dilute the mixture to about 50 ml with water.

Note: Hydrogen peroxide treatment is used to remove interference from organic substances such as CMC, polyacrylates, lignosulfonates, and lignites, which can also absorb methylene blue.

• Add methylene blue solution in 0.5 ml increments from a burette or pipette.
• After each addition, stopper the flask and shake vigorously for about 30 seconds.
• While the solids are still suspended, withdraw a drop with a glass rod and place it on filter paper.
• Look for a greenish-blue ring around the stained solids — this indicates that the clay can no longer absorb more dye.

• Once a greenish-blue halo is observed, shake the flask for an additional 2 minutes and test another drop.
• If the ring is still present, the endpoint has been confirmed.
• If not, continue adding methylene blue in 0.5 ml increments, shaking and testing after each addition, until the ring consistently appears.
• Record the ml of methylene blue solution used.

• Accurately weigh approximately 1 gram of dry, ground shale.
• Transfer to a 150-ml Erlenmeyer flask
• Add 50 ml of deionized water and 0.5 ml of 5 N sulfuric acid.
• Gently boil the slurry for 10 minutes.
• Allow the sample to cool before titration.

• Add 0.01 N methylene blue solution in 0.5 ml increments.
• Follow the same endpoint detection procedure as above using filter paper.

MBC (lb/bbl) = CEC x 5

MBC (kg/m³) = CECx14

The above formula used to calculate the Cation Exchange Capacity (CEC) of shale is based on the assumption that the methylene blue (MB) solution has a known concentration — typically:

1 mL of MB = 0.01 milliequivalents (meq)

This value comes from the normality (or strength) of the MB solution prepared in the lab. The full formula is:

This formula is only valid if the methylene blue solution you’re using contains 0.01 meq of dye per ml..

What If the Methylene Blue Solution Has a Different Concentration?

So, for example, If you’re using 0.005 meq/ml MB solution, the formula becomes:

Suppose you used 3 ml of methylene blue for a sample that weighs 1 gram:

This tells you that 1—grams of this shale would have the capacity to hold 3 milliequivalents of positively charged ion (cations).

This means the shale contains reactive clay minerals (such as smectite or montmorillonite) with negatively charged surfaces that can adsorb 3 milliequivalents of cations per 100 grams of rock. A higher CEC value generally indicates higher clay activity, swelling potential, and greater impact on mud properties — such as viscosity, stability, and fluid loss control. This is critical information when evaluating how shales will interact with water-based drilling fluids.