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.