Measuring pH of bark samples

1:2 Dilution Method of Extraction

1. If necessary, air-dry the sample at room temperature, or below 80. Unless the sample is very wet, it should be dry enough in 24 hours to test.

2. Combine one volume of air-dried bark with two volumes of sterile distilled (de-ionized) water. This means if you fill one 3 oz. cup with bark, then you need to fill two 3 oz. cups with distilled water and mix them together in a larger sterile beaker.

3. Mix the sample and distilled water thoroughly by swirling the jar and then allow it to stand for 30 minutes.

4. Properly calibrate the pH meter. Follow the instructions with your meter. Don't forget to rinse off the buffer.

Calibration of Pens

All of the commonly available pens and more expensive types of pH meters work on the same basic principles and if they are calibrated properly should give the same readings.

Calibration procedures differ somewhat depending on the brand of pen. On some you push a button, on others you turn a small screw to set display at the proper value. For pH pens, calibration is done with the pH 7 buffer solution or by a "two point"calibration using the pH 4 and pH 7 buffers. Calibration involves placing the pH pen in a buffer solution, allowing a stable reading to develop, and then, if necessary, adjusting the displayed value to the pH of the buffer. After calibration the used solutions should be discarded.

Calibration is a must to get any useful information from pens and meters! How often it needs to be done depends on the type of meter and frequency of use. The readings of even the most expensive laboratory meters tend to "drift" over time and must be brought back to the proper reading fairly often. When you first start using the pens, plan on calibrating at the beginning of every testing session until you find out how much the readings drift between sessions.

5. Remove the bark from the container with sterile forceps. DO NOT use your fingers!

6. Place the pH meter into the sample so that the sensor of the pen will be completely immersed in the extract. The pen should be swirled in the extract to dislodge any air bubbles and then leave the pen still until a stable (unchanging) reading appears. The stable pH reading is your result and should be recorded.

This procedure has been modified by Trish Smith for secondary students to measure the pH of bark from the following document:

FACT SHEETS > GREENHOUSE MANAGEMENT > HOW TO USE pH AND EC PENS

How to Use pH and EC "Pens" to Monitor Greenhouse Crop Nutrition

http://www.umass.edu/umext/floriculture/fact_sheets/greenhouse_management/phecpens.html

Prepared by: Dr. Douglas Cox, Plant and Soil Sciences, University of Massachusetts, Amherst