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I bought a 2 pH meters several months back from Amazon for roughly $10 a piece. I couldn't tell you how they work, but here's the link on Amazon: http://www.amazon.com/gp/product/B0087LOWIA/ref=wms_ohs_product?ie=UTF8&psc=1

I also got a neutral buffer solution for calibration. It worked fine initially but I'm thinking it's off. It's now being used for pH testing in the salt water of an floatation tank (147 gallons of water + 750 lbs pharmaceutical grade epsom salts and 50 pounds of dead sea salt). One meter shows a pH of 6 and another of the same kind at about 8.

My question is, does the salt effect the accuracy of the meter? Are the meters inherently bad, or do they need periodic recalibration? I didn't want to keep buying litmus paper.

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    $\begingroup$ You should recalibrate your pH meter often. Also clean it with [much]$_n$ distilled water in between uses. Check it against solutions you know the pH of. Do not measure directly in the tank (take a sample first). Always make sure, that your solution is stirred very well. Measure multiple times. $\endgroup$ – Martin - マーチン May 26 '14 at 2:53
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    $\begingroup$ For people who have not the slightest idea of these ridiculous units. (147 gallons of water $\approx 556.5~\mathrm{L}$ + 750 lbs pharmaceutical grade epsom salts $\approx 340.2~\mathrm{kg}$ and 50 pounds of dead sea salt $\approx 22.7~\mathrm{kg}$) $\endgroup$ – Martin - マーチン May 26 '14 at 2:56
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The answer is yes, ionic force does influence real pH signal. Because pH meter is sensitive to activities (instead of concentration of H+ only) of ions that modifying it junction potential you must take in account ionic force when it is strength enough to significantly impact activities. There is a lot of limitation with pH-meter: above pH>12 or bellow pH<2 they are not precise at all (see for acidic and basic errors). Or if they are used in different solvents than water. Organic chemists should tell you what pH could mean in hexane. Assessing pH is a complex task that must be addressed with the care it deserves.

Anyway, you should find the user-guide of your pH meter on internet. I am sure the manufacturer will provide limitation and use tips.

There is a detailed chapter on pHmetry in Skoog and West Analytical Chemistry. It may help you to deeper understand those phenomena.

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