Magnesium Hydroxide – the Most Powerful and the Most Gentle

This is a common discussion I’ve had over the years with wastewater operations folks that struggle to understand this reality.  I’ve decided to put my text into a conversational form in order to reflect the standard way these discussions have gone over the years.  Enjoy!

The superintendent mentioned that when 50% NaOH (caustic soda) was dosed into their wastewater, the pH climbed to a higher level than when dosed with the same amount of 60% Mg(OH)₂ (ALKA-Mag⁺).  Therefore, he concluded that caustic soda is the more powerful additive, as it drove the pH up higher.  In addition, he saw the hazardous nature of caustic soda as further proof that it is much more powerful than “milk of magnesia”.

I agreed with him that while caustic is dramatically more reactive and hazardous, I could show him that magnesium hydroxide is much, much more powerful – and it’s not even close!

To demonstrate, I went to my favorite little benchtop test – the Vinegar Test.  We started with two beakers each holding 200 mL of grocery store vinegar at a pH of 2.3, and added 12 mL of each alkaline product into a beaker.

Once we added 50% NaOH into the 1^st beaker, the pH rapidly shot up to 12.2.  “Wow, that was really powerful!”, he exclaimed.  “No,” I corrected, “that was dramatically reactive, but not powerful.  There’s a difference.  Let’s go on.”

We then added ALKA-Mag⁺ into the 2^nd beaker, and stirred and waited, and stirred and waited.  After 5 minutes the pH had only climbed to 8.9.  He said, “See, your milk of magnesia barely took the pH above neutral!”  My response was, “Uh-huh, let’s keep going.”

Since in both cases the alkaline additive took the pH above neutral, we decided to dose more vinegar into each beaker and titrate until the pH dropped back down to 7.

We first dosed 40 mL of vinegar into the Caustic Soda beaker.  The pH rapidly dropped from 12.2 to 6.9 (below neutral).  Hmmm . . . it didn’t hold its pH very well.

We then dosed 40 mL of vinegar into the ALKA-Mag⁺ beaker.  The pH went immediately from 8.9 down to 5.5.  He gave me a smirk.  But with stirring for 3 minutes, the pH climbed back up to 8.7!  I returned his smirk.

We dosed another 40 mL of vinegar into the same beaker, and again the pH immediately dropped to 5.5, but then rose with stirring back to 8.5!

Again, we added 40 mL of vinegar into the ALKA-Mag⁺ beaker.  And again, the pH immediately dropped to 5.5, but then rose with stirring back to 8.2!

One more time we added 40 mL of vinegar into the ALKA-Mag⁺ beaker.  And again, the pH immediately dropped to 5.5, but then rose with stirring back to 7.6 – still above neutral!

In summary, two beakers were dosed with equal amounts of alkaline products.  We then back-titrated with 40 mL of vinegar into the Caustic Soda beaker and it dropped the pH from 12 all the way down to 7 (neutral).  Very little buffering strength – not very powerful.

Alternatively, it took an additional 160 mL of vinegar to bring the ALKA-Mag⁺ beaker down to neutral – and still it held a slightly alkaline pH!

He looked at me and agreed, “Okay, I get it – the most powerful and most gentle!”.