Problem 8.10: Using alkalinity from Question 8 compute pHeq

From Chapter 8 Problem Set. Level 3: System Integration.

Question

Using alkalinity from Question 8, compute pHeq and PSI. Interpret.

Answer

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pHeq = 1.465 × log₁₀(220) + 4.54 = 1.465 × 2.34 + 4.54 = 3.43 + 4.54 = 7.97. PSI = 2 × 6.83 − 7.97 = 13.66 − 7.97 = 5.69. PSI < 6 indicates scale-forming tendency, consistent with both LSI and RSI. PSI’s contribution here is nuance rather than contradiction. By substituting the equilibrium pH for the measured pH, it accounts for the buffering capacity of the alkalinity during precipitation. The result is still scale-forming, but the PSI value (5.69) sits closer to the borderline than RSI (5.06), reflecting that this water’s alkalinity provides some resistance to runaway precipitation compared to a low-alkalinity water at the same LSI.