HCl + Mg = decrease in pH? Why?

[shredder666]

I did an experiment today, I put some magnesium in HCl solution (pH of around 1 or so) , but for some reason the pH decreased (turned more acidic), and my teacher said that's expected but didn't tell me why it happens.

Enlighten me, please.

 

[symbolipoint]

This may come through as strange, but the Mg⁺² may behave like H⁺¹ to the glass electrode and by so doing, show a lowered pH reading.

Also, somehow, putting Magnesium (what form do you really mean?) into either water or anqueous solution seems like a bad idea. Am I missing something here?

 

[shredder666]

well, I'm supposed to find effect of surface area on the rate of reaction

used strip Mg powder Mg and ball Mg in HCl solutions, I really expected the pH to increase cus of the H2 gas escaping but that was not the case... btw Mg is like metal Mg (pure)

 

[Borek]

This may come through as strange, but the Mg⁺² may behave like H⁺¹ to the glass electrode and by so doing, show a lowered pH reading.

This is so called alkaline error or sodium error, compare

http://www.ph-meter.info/pH-electrode-selectivity

But I doubt that's what is happening here, these are mainly single charged cations that interfere.

Not sure what is going on.

Any changes to the temperature of solution?

--

 

[shredder666]

well i tried to keep the temperature controlled, but the overall reaction is exothermic, but then i used like 0.05grams of magnesium so the temperature change should be really small. BTW I looked up in some textbooks, and apparently due to the small ionic radius and the 2+ charge, the Mg ion acts as a electrophile and pulls the electron density in water in, and makes it give H+ to water thus making it into H3O+

I have another problem tho, the powdered magnesium just floats on the acid and doesn't look like its reacting

That sounds REALLY STRANGE to me, but then it happened in 5/5 trials I did (and yes, I made sure that the solution is HCl)

 

[Borek]

I have forwarded the question to one of the mailing lists I am on, that's what I got:

At low end of the pH the glass electrode doesn't read the pH very well. When the concentration of HCl decreased, probably the glass eletrode (if that was used) started to read the correct pH.

This was the only answer so far, and if things are obvious they are usually ripped to shreds in no time.

--
ChemBuddy chemical calculators - ,
www.ph-meter.info - , ph electrode

 

[shredder666]

glass electrode... hmm... I used Vernier Lab Quest Apparatuses and it had a glass bulb with a filament? inside on the end of the stick , is that what a glass electrode is?

 

[Borek]

http://www.ph-meter.info/pH-electrode-construction

 

[shredder666]

wow that's a real eye opener, thnx y'all

 

[shredder666]

got another problem...powdered magnesium doesn't react with HCl...

Not sure what could've happened, could contamination lead to zero reactivity?
or maybe there was a reaction, its just not obvious (no fizzing or anything, the powder just floats