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The glass sensor has became red inside like something melted. I stored it in a liquid with pH that mentioned in the manual. What is that and why could that happen, how often that happens with pH-meters?
(10 bucks pH-meter from China) enter image description here enter image description here enter image description here

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    $\begingroup$ Be more precise ! What is the meaning of "red like something melted" ? Which "liquid" did you use to store your electrode ? How long did you store it ? What is the "pH mentioned in the manual" ? What goes wrong, the pH-meter or the glas sensor ? $\endgroup$ – Maurice Aug 29 at 9:00
  • $\begingroup$ @Maurice some reddish substance appeared on the inner of the glass sensor (pretty sure it wasn't like that). I used liquid with pH=4, I stored it for about couple of weeks. The meter can't be calibrated anymore, always the same value. $\endgroup$ – R S Aug 29 at 9:54
  • $\begingroup$ @Buttonwood I uploaded images. $\endgroup$ – R S Aug 29 at 10:30
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    $\begingroup$ This is still not precise enough, what is the liquid you used? You should disassemble the electrode and clean it from the inside if possible. These glass electrodes sometimes have built-in salt bridges maybe it got contaminated there, but based on the picture the damage is more likely due to the long soaking. Probably something absorbed on its surface (glass membrane on the bottom), hindering its functionality. It is most likely you have to buy a new one, this one seems hopeless. You can try soaking it in dilute mineral acids. Also these liquids are for calibration not for storage. $\endgroup$ – Andrew Kovács Aug 29 at 11:58
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    $\begingroup$ I assume the pH is neutral. The concentration is 4 M KCl. That's always been what the manufacturer suggested. $\endgroup$ – user137 Sep 10 at 18:31

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