ࡱ> KMJ "bjbjAA .:++rrrrr<$2j2222222$36:2r:2rrO2rr22P/|1Nu^02e202 0\37j370|137r|1:2:2S\237 8: 29. pH meter use and measurement of organic soil pH Preliminary step: check electrode Does the internal solution come to about 1 cm from top? If yes, go to Step A. If not, turn white collar (step 4 below) to open position (or simply just open the plug on the side of the probe - different probes have different opening mechanisms, e.g. a plug or a collar), add fresh filling solution (note: specific solution varies between probes) until about 1 cm from top of electrode. Turn collar to closed position (or close plug). Press down on top of probe so that internal solution is released at the bottom/bulb interphase, conditioning the probe (but do not release all of the internal solution). Step A: Two-point Calibration Press stdby button on pH meter. Press mode key until display indicates pH mode. Press the setup key twice and then press enter this clears the previous standardization. Turn white collar on electrode from its closed (all the way to the right) to its open (all the way to the left) position (or simply open plug, see text above). Remove electrode from storage solution; rinse tip by spraying water on lower 5 cm of electrode; gently blot (but do not dry) electrode tip. Put electrode tip into pH 7 buffer. When reading stabilizes (the word stable appears), and this will happen fairly fast. However, the reading may continue to change therefore, wait at least 30 seconds (preferably longer, e.g. 1-2 min) to make sure reading is genuinely stable. When reading is stable, press std button once; wait until the word stable appears again, then press std again. Rinse electrode tip; put it into either pH 4 or pH 10 buffer -- pH 4 if you expect the samples to be acidic; pH 10 if you expect the samples to be basic. Repeat step 6. Rinse electrode tip. The meter is now ready to read the pH of samples. Step B: Use No need to calibrate again. Simply rinse electrode between samples. If temporarily not in use, leave electrode tip in pH 7 buffer. When finished, press stdby on meter; turn white collar on electrode back to its closed position (or put plug back in); return electrode to storage solution1. See usage notes overleaf 1 storage solution: This depends on the actual probe, see probe manual for instructions) Measuring pH of organic soils (and mineral soils) Weigh out 6 g fw of soil and add 30 ml deionized water in a glass beaker (size: 50 ml) If you need a larger volume slurry you can use e.g. 10 g soil to 50 ml water - as long as you keep the soil:water ratio constant (I use a ratio of 1:5) If needed, e.g. when minimal soil material is available, its also possible to use other soil:water ratios up to 1:10 soil:water ratio. But always use the same ratio across samples! Gently mix by hand (using a glass rod) for 10-15 seconds, then put onto shaker table for 30 minutes. After shaking, let the slurries rest for another 30 minutes. Calibrate the pH probe (see overleaf). Rinse the probe sensor by spraying deionized water using a spray bottle. Carefully blot the probe sensor with Kimwipe tissue. Immerse calibrated probe into the solution so that the sensor section is covered by the supernatant, but not so deeply that it is touching the precipitated organic/mineral soil. The very first measurement with the probe after its calibration should be taken with caution, letting the pH probe sit for at least 6 minutes in the slurry to ensure the reading is sufficiently stable. Then, after five samples have been measured, re-measure the first sample slurry to ensure proper reading was achieved. All other samples should be measured after 3 minutes of immersion (as described below). For each sample, let the pH probe sit in the soil slurry for 3 minutes before reading the pH value. If the value is not stable after 3 minutes, wait until the pH meter reads stable, then record the value. Note the critically important cautionary note relating to first measurements below. Re-measure at least one sample slurry after every 10 samples measured (or even more frequently, e.g. after every 5 samples) to ensure that readings are reproducible. Pauls cautions and tips: First measurements: Conditioning of the pH electrode is a major issue in pH measurement of such samples. If the probe is immersed in deionised water for even 15 seconds, it will tend to return to neutral pH, and most importantly the sensor will start to recondition to the deionised water. Consequently, the pH value of the next sample that will probably not have stabilised within the 3 minute wait period, resulting in artificially high recorded pH values (e.g. +1.0 pH unit). Therefore, the very first measurement with the probe after its calibration should be taken with caution, waiting several minutes to ensure the reading is sufficiently stable. One approach would be to immerse the probe in the first sample, and leave it there for 10 minutes to condition it to the organic soil solution. Then remove it, rinse with deionised water by spraying the probe tip with deionised water using a spray bottle, and then to make a measurement of the same solution (recording the pH after 3 minutes). Accordingly, once you start making measurements on a set of samples, try to complete them in one run so the conditioning issue does not arise within the run. Be careful to randomly select the samples across treatments for pH measurement. Tests indicated that there is a gradual slow decline in pH values over time (after the initial 2-3 minute wait period), so that samples that are left standing for relatively long periods will tend to have slightly lower pH values (i.e. 0.1-0.3 pH units) than samples measured immediately after the 10 minute wait period. There may be a concern with plant and soil organic matter fragments actually touching the sensor and influencing the readings. This was not tested, but could be done by pouring off a subsample of the supernatant and measuring it separately. Most pH measurements described in Methods books use glass instruments. Although not tested rigorously, it did appear that there was an interaction between the probe in the state where it was conditioned to the organic soil and the plastic sample cup, resulting in higher pH values (0.4-0.6 units) for deionised water in the plastic sample cup than when the same probe when immersed in deionised water in glass beaker. However the latter was always measured first, and so the pattern may have been caused by progressing conditioning of the probe as it adjusts from organic soil solution to deionised water. 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