Larson-Skold Index (Water Corrosivity)
Computes the Larson-Skold index, the ratio between the aggressive and the protective anions in a water: chloride plus sulphate divided by alkalinity, all converted to milliequivalents per litre with the equivalent weights 35.45 for chloride, 48.03 for sulphate and 50.04 for alkalinity expressed as CaCO₃. The reading is direct: below 0.8 alkalinity dominates and the carbonate film protects carbon steel; between 0.8 and 1.2 corrosion stops being negligible; above 1.2 chloride and sulphate break the film and the localised corrosion rate takes off, the typical scenario of cooling tower makeup water running at many cycles of concentration. Unlike the Langelier index, this one does not say whether the water will scale — it measures only the corrosive power of the anions, which is why the two readings complement each other rather than compete. Total alkalinity was adopted as the input, instead of separate bicarbonate and carbonate, because that is what a routine laboratory reports, and converting it through the CaCO₃ equivalent returns exactly the sum of the two in milliequivalents per litre. Enter the chloride, the sulphate and the total alkalinity.
Result
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Larson-Skold index: when water eats carbon steel
The tower moved up to six cycles of concentration, the Langelier index turned positive, nobody saw a trace of scale, and the heat exchanger still started leaking. That is the scenario the Larson-Skold index explains: concentrating the water multiplies chloride and sulphate, while alkalinity fails to follow, since carbonate precipitates and leaves with the blowdown. The protective carbonate film over carbon steel stops forming, and the attack turns into localised pitting, the kind that punches through a tube with the surrounding wall still at full thickness.
The index gives the ratio between aggressive and protective anions, all in milliequivalents per litre: LS = (Cl⁻/35.45 + SO₄²⁻/48.03) ÷ (alkalinity/50.04), with concentrations in mg/L and alkalinity expressed as CaCO₃. Each divisor equals the equivalent weight of that ion. The reading runs straight: below 0.8 alkalinity dominates and the carbonate film protects; between 0.8 and 1.2 corrosion stops being negligible and mass-loss coupons deserve watching; above 1.2 chloride and sulphate break the film and the localised rate takes off. With the values on screen — 60 mg/L chloride, 80 sulphate, 120 alkalinity — the index reads 1.400, plainly aggressive water.
Larson and Skold calibrated the index on Midwestern waters running through carbon steel pipe, and its domain ends there. It says nothing about stainless, where chloride pitting carries criteria of its own, nor about copper, and it ignores temperature, velocity, dissolved oxygen and biofilm. It reports nothing about scaling either: Langelier covers that, and the two readings add up. Total alkalinity was adopted as the input, not bicarbonate and carbonate in separate fields, because routine labs report it that way and because dividing by 50.04 returns exactly the sum of both in milliequivalents. Feeding bicarbonate in mg/L of HCO₃⁻ instead drops the index by 18% and flips the conclusion.
Frequently asked questions
Why do the default values give 1.400, and what should I do with it?
Does the Larson-Skold index replace the Langelier index?
Which unit does the alkalinity field expect?
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.