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Although I think a pH meter is pretty much essential for a reef tank, its not much use unless the meter is correctly calibrated. General Comments Never wipe the probe tip, to remove water droplets, blot the tip with kitchen paper or shake the liquid off. A new pH probe Before using a pH probe for the first time, check that the sleeve that the probe tip came in is full of liquid. If it is dried out then return it to your supplier. To condition the probe before use, soak the probe in either pH 4 or pH 7 buffer for 20 minutes, then rinse with RO or DI water. Then you are ready to calibrate. pH probe cleaning The safest way is to soak the probe in white vinegar for an hour or more. The easiest way is to soak the probe in 0.1M HCl for 20 minutes. Every so often it is worth removing the bacterial film build up. To do this, mix 1 part household bleach with 10 parts water and soak for 10 minutes. Rinse with RO or DI water and you are ready to calibrate. pH probe calibration Having cleaned your pH probe, you need a good thermometer, fresh calibration fluids and a small jug of RO or DI water. I much prefer pH sachets to bottles as they are always fresh. As soon as you open a bottle of pH calibration buffer, it starts to age. Stick with sachets. I use those from Hanna as they have both a use by date and a chart that shows pH vs. Temperature. Next, throw the unopened pH calibration sachets in the sump to come up to tank temperature, this only takes a few minutes. At the same time get a mug and fill it full of tank water. After five minutes empty the mug and refill with more tank water, the mug of water helps minimise temperature change. Carefully open the pH sachets and place them in the mug of tank water. Put your accurate thermometer in the water in the mug. Put the probe into the pH 7 solution, stir gently for a few seconds and then go make a cup of coffee or something that keeps you occupied for 10 minutes. Come back and adjust the meter to the correct pH. Rinse the probe in RO or DI water, blot or shake dry, then place the probe in the pH 4 or pH 10 solution, again stir gently for a moment and have another coffee. After 10 minutes adjust the pH meter (check your thermometer in case of temperature changes!). Rinse the probe, dry and stick it back in the pH 7 solution, having stirred and left for 2 or 3 minutes it should read correctly within 0.01 or so. If so then rinse and put in service, otherwise repeat the calibration.
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