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pH-meter (12 bucks one) can be calibrated with 2 different solutions (let's say 4.0 and 6.86), but as an option 3 can be used (4.0, 6.86 and 9.18). If I use 2 instead of 3, what error I'm going to get. For instance I need to determine if liquid higher or lower than 5.5.

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    $\begingroup$ You are the only one who can answer this question, because you have the instrument in your hands and can compare. Of course a third calibration point outsinde the range you are interested in should be quite useless, don´t your think? $\endgroup$ – Karl Jul 10 at 12:08
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    $\begingroup$ @Karl I agree with the gist of your comment, but the data at pH 5.5 is intermediate to the two extreme calibration points in the 3-point calibration. $\endgroup$ – Buck Thorn Jul 10 at 12:48
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    $\begingroup$ The instrument might have internal electronic settings that are adjusted when it encounters voltage input in different ranges. This is a question of electronics as much as of math and chemistry. It would be interesting to have a link to the product in question. $\endgroup$ – Buck Thorn Jul 10 at 12:57
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    $\begingroup$ @Buck Thorn you must be right, 9.18 is in the kit for calibration, I think they know what they doing. A typical yellow pH-meter that you can find on Aliexpress or Amazon. $\endgroup$ – R S Jul 10 at 13:19
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    $\begingroup$ @BuckThorn 5.5 is very much in the middle of 4.0 and 6.9. A third point well in the alcalic range cannot improve accuracy here. Of course it shouldn´t hurt also, but I have not seen the firmware source nor any calibration curve of that instrument. ;-) $\endgroup$ – Karl Jul 10 at 13:49
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You cannot "predict" any error by thinking that if we calibrate with two buffers the error will be x, and if we calibrate with three buffers the error will be y.

Ideally, in each case, the pH of the sample should be identical. Good quality pH meters tell you how close they are to the theoretical slope in terms of percentage. It should be close to 100%.

For example, at 25°C the theoretical slope is ~ -59 mV/pH. If your calibrate with two buffers and is close to -58 mV/pH, the slope percentage is 98.3%. It is good to go. As electrode age, the percentages goes down.

As EdV said, three points might be better for the $12 pH meter.

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  • $\begingroup$ Better 3 even if I know that pH of solution is somewhere between pH of those 2 calibration powders, is that correct? $\endgroup$ – R S Jul 11 at 14:19
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    $\begingroup$ It all depends on the slope. If the slope is close to the theoretical slope, 2 buffers are fine. A $12 pH meter will not show the slopes. $\endgroup$ – M. Farooq Jul 11 at 15:52

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