# What the difference in data if calibrate pH-meter with 2 and 3 solutions? [closed]

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.

• 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? – Karl Jul 10 '20 at 12:08
• @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. – Buck Thorn Jul 10 '20 at 12:48
• 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. – Buck Thorn Jul 10 '20 at 12:57
• @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. – R S Jul 10 '20 at 13:19
• @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. ;-) – Karl Jul 10 '20 at 13:49

As EdV said, three points might be better for the $12 pH meter. • Better 3 even if I know that pH of solution is somewhere between pH of those 2 calibration powders, is that correct? – R S Jul 11 '20 at 14:19 • 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. – M. Farooq Jul 11 '20 at 15:52