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Alkalinity In Boiler Water

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Why monitoring alkalinity in boiler water matters

Monitoring alkalinity in boiler water protects efficiency, safety, and asset life. When alkalinity drifts, corrosion, embrittlement, foaming, and carryover start to show up. Cooling systems feel it too, as scale and corrosion treatments lose bite outside the right window. Stay inside the control band and the plant runs cleaner, steadier, and with lower fuel spend.

The three basic forms of alkalinity

Alkalinity appears as three species, with the balance driven by pH.

  • Carbonate (CO₃²⁻).
  • Bicarbonate (HCO₃⁻).
  • Hydroxide (OH⁻).

Total alkalinity is the sum of these contributors. You measure it by titrating a sample with standard acid and reading defined endpoints. Results are expressed as P, M, or T alkalinity and reported as mg/L as CaCO₃.

P, M, and T alkalinity at a glance

  • P Alkalinity: Titrate to pH 8.3 using phenolphthalein.
  • M Alkalinity: Titrate to pH 4.6 using methyl orange.
  • T Alkalinity: Titrate to pH 4.5 using a total alkalinity indicator.

You can use standard relationship tables to interpret carbonate/bicarbonate/hydroxide splits and to confirm treatment impact.
These tables help verify whether caustic addition, deaeration changes, or softening steps are delivering the expected shift.

How control is set and held

Treatment specialists target M (or T) alkalinity below the boiler maker’s limit for the pressure class. Higher pressures call for higher purity and tighter control. Enough OH⁻ alkalinity is still required to protect steel and keep hardness from plating out; a common target sits above 150 mg/L as CaCO₃ within the manufacturer’s guidance. That balance supports passivation, stabilises pH, and limits scale growth on hot surfaces.

Cooling water notes

Low alkalinity invites corrosion. High alkalinity invites calcium carbonate scale. A steady mid-band supports inhibitor packages and keeps heat transfer stable.

Helpful tool: pH treatment solutions such as pHREADY can steady pH and alkalinity when load or water quality moves around.

Field method: step-by-step titrations

  1. Collect a fresh, representative grab sample.
  2. Measure temperature and initial pH.
  3. Run P alkalinity to pH 8.3.
  4. Continue to M alkalinity at pH 4.6.
  5. If required, verify T alkalinity at pH 4.5 with the total indicator.
  6. Record mg/L as CaCO₃, time, location, and operator initials, then trend.

Consistent technique beats sporadic testing. Small, frequent checks catch drift before it becomes downtime.

Alkalinity testing interferences and how to handle them

  • Highly turbid or coloured samples can hide the endpoint.
  • Free chlorine alters the titration result.
  • Suspended solids and colloids skew readings.
  • Some polymers read as M and T alkalinity.
  • CO₂ absorption during handling depresses alkalinity.
  • Acid dosing upstream shifts the P/T relationship.

Practical fixes that work on site:

Dechlorination: Add sodium thiosulphate before titration to neutralize chlorine.

Clarification: Remove suspended solids with paper filtration; use syringe filters for colloidal color.

Polymer systems: Expect polymers to titrate as M and T; for OH alkalinity, titrate directly using the barium chloride method.

Avoid 2P–M bias: The 2P–M shortcut under-reads OH in polymer-treated waters; use the direct OH method instead.

Limit CO₂ uptake: Analyze at the sample point where possible.

Off-site samples: Fill bottles to overflowing and cap tightly; run tests promptly on arrival.

Process acid: If acid is being added for pH control, revisit the P/T interpretation, as the relation shifts under acid feed.

Odour present during testing.
Tackle the source and stabilise chemistry with pHREADY for Odour Control, which can help while you hold the required alkalinity band.

Control actions that keep systems reliable

  1. Set monitoring frequency by pressure, load, and history.
  2. Tie caustic and conditioning feeds to alkalinity trend, not guesswork.
  3. Match blowdown to solids load and conductivity, then recheck alkalinity.
  4. Review deaerator performance and makeup quality whenever trend lines wobble.
  5. Document setpoints, endpoints, and corrective actions in the shift log.

Quick reference ranges and notes

  1. Target ranges vary by pressure class and supplier guidance.
  2. Low-pressure packages often allow a wider band for M/T values.
  3. High-pressure units require tighter limits and cleaner makeup to protect turbines and superheaters.
  4. Keep OH⁻ in the protective zone and keep total alkalinity inside the maker’s stated limits for the duty.
Having Alkalinity Issues?

About The Author

Nick Piskura is the Marketing and Web Development Specialist at ChemREADY who utilizes expertise in digital marketing strategies to provide knowledgeable insights in each segment of our business. Nick provides insights through web development and multimedia resources that support ChemREADY’s full range of services, including Legionella management, ANSI/AAMI ST108 compliance, boiler and cooling tower treatment, wastewater processing, and industrial water quality solutions.

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