pH & Alkalinity

Purpose

Measure the hydrogen-ion concentration (pH) and the alkalinity of the mud and filtrate to assess lime content, bicarbonate/carbonate balance, and overall system buffering capacity. These values guide treatment decisions for water-based fluids.

Equipment

• pH meter with temperature compensation, or pH paper (wide range, then narrow range)
• Buffer solutions for pH calibration (pH 7 and pH 10 typical)
• Phenolphthalein indicator solution
• Methyl-orange indicator solution (for Mf)
• Titration burette or syringe (0.01 N H₂SO₄)
• Filtrate sample (from API filtration test)
• Mud sample
• White porcelain spot plate

Procedure

pH (meter method):

1. Calibrate the pH meter with buffer solutions according to the manufacturer’s instructions.
2. Rinse the electrode with distilled water and blot dry.
3. Immerse the electrode in the mud sample (or filtrate) and allow the reading to stabilise.
4. Record the pH value and sample temperature.

Pm — Mud Alkalinity (phenolphthalein):

1. Place 1 mL of mud on the spot plate or in a small beaker.
2. Add 2–3 drops of phenolphthalein indicator.
3. Titrate with 0.01 N H₂SO₄ until the pink colour disappears.
4. Record the volume of acid used (mL). Pm = mL of acid used.

Pf — Filtrate Phenolphthalein Alkalinity:

1. Place 1 mL of filtrate in a small beaker; add 2–3 drops of phenolphthalein.
2. Titrate with 0.01 N H₂SO₄ to colour change. Record Pf = mL acid.

Mf — Filtrate Methyl-Orange Alkalinity:

1. Continue titrating the Pf sample, or use a fresh 1 mL filtrate aliquot.
2. Add 2–3 drops of methyl-orange indicator.
3. Titrate with 0.01 N H₂SO₄ from blue/green to orange endpoint. Record Mf = total mL acid.

Notes

• Report pH, Pm, Pf, and Mf on the daily mud report.
• pH paper may be used for quick field checks but the meter method is preferred for accuracy.
• Calibrate the pH meter with fresh buffer solutions at the start of each shift.
• High Pm relative to Pf indicates significant excess lime in the mud phase.