|MadSci Network: Chemistry|
For the sake of this response let’s consider “heavy water” to be pure D2O. A strict interpretation of matters would tell us that its pH would be positive infinity or perhaps undefined since it contains no H from which to form hydrogen ions. If by the question “What would its pH be?” we really are asking for the negative Log of the Deuterium ion concentration then based on a pK for dissociation of 14.869 at 25C (CRC Handbook 1971 pg D-122 from NBS, now NIST, Tech Note 400) we would get a “pH”, (should we say pD?), of 7.435 . These numbers tell us that under standard conditions heavy water is less dissociated than ordinary water or, as it is sometimes stated, that heavy water is a weaker acid than ordinary water.
Many of the properties of heavy water differ from those of ordinary water. For processes not involving nuclear reactions these differences are due to the greater mass of D compared to H. A fairly well referenced comparative list of many of these properties can be found at http://www.lsbu.ac.uk/water/data.html .
The smaller dissociation constant, and thus pH, for heavy water is a reflection of the somewhat stronger bond between D and O. It may not be immediately obvious why simply increasing the mass of what would otherwise be just another H atom should make this bond a bit stronger. Since it is determined by the electric charges the shape of the potential energy well is not all that different for H-O or D-O but the reduced mass of the oscillator they form determines how deep in the well the zero point energy of the oscillator is located. The greater reduced mass of the D-O puts it deeper. This makes the D-O bond length shorter as the well narrows toward the bottom, all of which means it will take a greater amount of energy to separate D from O.
If by your question you meant “What pH do I measure if I insert a standard glass pH meter probe into heavy water?” other problems arise. I’ve never tried it but presumably you’d get some number greater than 7 that would change with time as the heavy water becomes contaminated by H ion exchange as well as the water within the glass electrode becoming contaminated with deuterium. Exactly where this diffusion limited process might lead is not obvious but it would not appear to be very useful as a way of finding the deuterium ion concentration.
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