The relationship between alkalinity and pH in a demineralized water system

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In demineralized water systems, the relationship between alkalinity and pH is completely different from that in conventional water bodies. The demineralized water system (ion exchange, reverse osmosis + mixed-bed/EDI) aims to remove almost all ions from the water, and alkalinity, as an anion (such as bicarbonate and carbonate), is also removed as a result. 1. The alkalinity is physically separated; however, acidic gases remain in the cation exchanger (softener). The existing bicarbonate alkalinity in the water reacts with the H⁺ ions on the resin, thereby converting into carbon dioxide (CO₂). At this point: Alkalinity drops to almost zero (bicarbonate is retained by the resin, while H⁺ is exchanged out), and the pH drops to around 4.3–5.5. The water discharged at this point is acidic and contains CO₂; however, its acidity originates from dissolved gases, not from strong acid anions. The decarbonation tower (decarbonator) is precisely designed to strip away this CO₂. After removing CO₂, the pH quickly rises back above 6, but the alkalinity remains zero. This shows that in a demineralized water system, without any alkalinity, the pH value is entirely determined by CO₂. 2. The pH of ideal pure water is affected by the atmosphere; after passing through mixed-bed or EDI systems, the electrical conductivity of the water is extremely low (< 0.1µS/cm), and theoretically the pH is 7.00. But if you take an open beaker and measure it, you’ll often find that the pH falls between 5.6 and 6.8; it becomes acidic for no apparent reason. It’s not due to substandard water quality; rather, it’s because the alkalinity is zero. Without alkaline buffering, pure water will immediately absorb the trace amounts of CO₂ in the air to form carbonic acid, causing the pH to drop sharply; in practical applications, measures such as nitrogen sealing are employed. Such pH fluctuations require only an extremely small amount of CO2 (a few milligrams per liter), and they have almost no effect on the ionic load of the entire system; yet the pH meter still shows acidity. In this scenario, there is no classical carbonate equilibrium relationship between pH and alkalinity; pH merely reflects how much CO2 the water has absorbed from the air. 3. Measuring alkalinity is meaningless; it is difficult to measure pH. In conventional water treatment, these two parameters serve as cross-verifications for each other. In demineralized water: Alkalinity changes from a control parameter to an “impurity parameter”. The detection of significant bicarbonate alkalinity indicates that the anion exchanger has failed, there is leakage in the mixed bed, or the performance of EDI has deteriorated; these are signs of a problem in the system. In normally operating demineralized water, the total alkalinity should be close to 0. pH measurement is a huge technical challenge. In pure water with low conductivity, ordinary pH meters exhibit extremely slow electrode response, unstable junction potentials, and readings that drift in ways that are simply unbelievable. The pH value read at this time may not represent the actual hydrogen ion activity, and since there is no alkalinity to maintain balance, it cannot be used to guide dosing. In industry, the control of the pH of demineralized water often does not rely on the absolute readings of offline or online pH meters, but is achieved by adjusting the ratio of specific conductivity to flow rate along with the addition of ammonia. For boiler make-up water or high-purity water used for chip cleaning, the pH must be adjusted to 8.8–9.3 in order to prevent steel corrosion. Since pure water has no alkalinity, adding just one drop of alkali causes the pH to skyrocket to 10 ； Blowing in some CO₂ brings the pH back down to 6. Therefore, a volatile alkalizing agent (such as ammonia, ma? Alkylamines are used to create a very low level of alkalinity: when ammonia is added, it combines with water to form ammonium hydroxide, providing a slight amount of hydroxide alkalinity and keeping the pH at a slightly alkaline level. At this point, the alkalinity is not carbonate alkalinity, but rather a weak alkalinity contributed by hydroxides. Its function is to provide a tiny buffer center, so that the pH no longer fluctuates wildly with CO₂. At this point, pH and alkalinity are linked again; given a target pH value (such as 9.0), it is necessary to calculate the corresponding ammonia concentration that needs to be maintained, with conductivity (specific conductance) being used to indirectly determine this measure of weak alkalinity. In short, it involves using precisely measured amounts of mild alkalinity to accurately control the pH under extremely low buffering conditions.