pH meter

[Xylene]

I located the suggested pH meter, the Milwaukee pH600 model in a big Gardening Supply for $35.
Also a 500 ml. bottle of pH 7 calibration solution for $5 (distributed by Rambridge Wholesale Supply in Calgary Alberta Canada).

Back home again I found my three tiny 25 ml beakers fit the pH probe nicely, when filled to 10 ml mark on the beaker... this submerges the probe about halfway between the suggested minimum to maximum level marks. Thus allowing calibration with minimal waste of the Standard solution. Calibration adjustment is with the tiny supplied screwdriver through a little hole in back of yellow case, to right of the pocket clip. (You are turning a trimming potentiometer on the internal circuit board) CW is increase reading, CCW is decrease reading; only a small fraction of a turn was necessary.

Carefully prying the black top off the yellow plastic body with a small blunt table knife, I exposed the battery compartment and found three #357A "button cells". A phone-call to London Drugs confirmed they stock this type. So then, no problem when the cells wear down after 700 hours of use. I scratched a + mark at the top side of the battery holder as a future reminder of polarity. (The outer circumference of a button cell is + positive ... the inner radial contact is - negative) In this pH meter the three cells stack in "series circuit" arrangement: all + facing up towards the on/off switch, and all - facing down towards the interior of the instrument.

To check the instrument for repeatable readings; I washed my tiny 25 ml beakers with a splash of acetone, then hung each in turn upside down from a ring-stand clamp to air dry. I shook the pH Standard solution then filled each beaker up to 10 ml mark (as stated above). For each of 8 tests, I used fresh solution (NEVER return used pH standard solution to bottle) and got these readings:
7.0 / 7.0 / 7.1 / 7.0 / 7.0 / 7.1 fluctuating to 7.2 / 7.0 / 7.1
So yes, it seems test readings are reasonably within the advertised 1/10th pH accuracy.

[Xylene]

I've found that pH meter SO much better for doing acid/base titrations ! I can now see the meter pH approaching the sharply defined neutral "end point" and slow down my drip rate to a crawl. This is critical because near end point, single tiny drops begin to make a profound difference due to the non linear pH scale. It is therefore VERY easy to overshoot the end point and spoil your experiment. (Of course you could consider this first failed attempt a "rough titration" and now know where on the burette scale the end point approaches...and slow down the drops.)

Now if you go to Wikipedia and search "pH Indicator" you will see an interesting Table (middle of page opening) that compares the color transition of most indicator solutions. I was startled to see that the most common titration indicator Phenolphthalein changes color at:
clear 8.3 pH-------------------fuchsia 10.0 pH
And not closer to 7.0 neutral as I had supposed.

http://en.wikipedia.org/wiki/PH_indicators

[GoatRider]

I bought the Checker®1 pH meter. It has 2 calibration screws, and needs to be calibrated in both pH 7.01 and either 4.01 or 10.01 buffer solutions. Not sure why the .01, why not .00? Not sure what's in the cleaning solution and storage solutions, but I bought those too. The probe unplugs from the unit, and a minced garlic jar is just the right size to put it in with the storage solution.

It seems quite accurate- for example in lab 10.1, the .01M HCl solution should be pH 2- I measured 1.98.

[pgarvan]

I'm betting that the requirement for pH's of 4.01, 7.01, and 10.01 is just to emphasize that the known pH of each calibration solution needs to be accurate to within 0.01 pH. Although, I guess it would have been simpler for them to just say that.