Problem 8.8: Compute pH and LSI for pH 86 T 40C TDS 2200
From Chapter 8 Problem Set. Level 3: System Integration.
Question
Compute pHₛ and LSI for: pH = 8.6, T = 40°C, TDS = 2,200 mg/L, Ca hardness = 320 mg/L as CaCO₃, M-alkalinity = 220 mg/L as CaCO₃. Round logs to 2 decimals.
Answer
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A = (log₁₀(2,200) − 1) / 10 = (3.34 − 1.00) / 10 = 0.23. B = −13.12 × log₁₀(313) + 34.55 = −13.12 × 2.50 + 34.55 = 1.75. C = log₁₀(320) − 0.4 = 2.51 − 0.40 = 2.11. D = log₁₀(220) = 2.34. pHₛ = (9.30 + 0.23 + 1.75) − (2.11 + 2.34) = 11.28 − 4.45 = 6.83. LSI = 8.60 − 6.83 = +1.77. Strongly scale-forming. (values are rounding-sensitive; full-precision yields LSI ≈ 1.70 – same interpretation)