What is pH and how do you Measure it?

The pH scale is a way to measure the Acid or Alkaline quantities in water. The official definition of pH is : a unit of measure which describes the degree of acidity or alkalinity of a liquid solution. It is measured on a scale of 0 –14. Acids are in a range from 0 –7, with the lower numbers being a stronger acid Alkaline is in the range from 7 to 14, with the higher numbers being a stronger Alkaline. The term pH is derived from ‘p’ the mathematical symbol of the negative logarithm, and ‘H’ the chemical symbol of Hydrogen. The technical definition of pH is the negative logarithm of the Hydrogen ion activity { pH = -log[H+] } As pH ions are added the pH gets more acidic, thus having more of a positive charge. Similarly when substances with more hydroxide ions are added the pH gets more alkaline, thus having more of a negative charge. These charges are present in the nutrients, the water, the trace elements, the growth mixture and the gases at the roots and all have different electrical charges. Theses then surround the roots and exchange positive and negative charges allowing for the absorption of nutrients into the roots. For this reason the pH must be carefully monitored during the growth cycle of the plants to maintain the maximum healthy uptake of nutrients. The pH of the nutrients will affect how well each element passes through the root wall and nourishes the plant.

A pH of 7.0 is considered to be neutral, and this is very safe to handle. Actually if the pH of a chemical is in the range of 3.0 to 10.0 these can be handled fairly safely, at least from the point of view that the acid or alkaline levels will not be harmful to exposed skin. It should be noted that any chemical with a pH of lower than 3.0 or higher than 10.0 should be handled with care because these pH ranges can be harmful. It is recommended that if you are dealing with chemicals in these ranges that you wear gloves and eye protection.

When growing soil less it is very important to control the pH of the water. The recommended pH range for plants is 5.0 to 7.0 with 6.0 to 6.5 being ideal for the vegetative stages. For flowering and fruiting stages a pH of 5.5 – 6.0 is ideal. The problem with giving a specific value to keep the pH at is that different plants need different pH. The pH of the water can be changed by adding up or down. These chemicals are usually called pH up and pH down with potassium silicate being the best used to increase the pH and phosphoric acid being the best to decrease the pH. Both of these products are plant additives.

The nutrient we use in soil less gardening can also affect the pH of the plant. When adjusting the pH in the water, it is recommended that you add the nutrient first then measure the PH to find out if the pH needs correction. It is recommended that during the growth cycle of the plants that the pH be checked regularly. The pH has to be monitored to see if its drifting out of the desired range, this can be caused by many factors. Some things that can change the pH are: when nutrient is replenished to the system, the type of growing medium used, the plants themselves affect the pH by consuming and depleting nutrient from the reservoir, as well as evaporation of water from the reservoir.

A liquid pH Test Kit

One of the least expensive ways to measure the pH of a sample of water is to use liquid pH kit. The pH range that can be measured with the liquid pH kit is usually around 5-8 with .2 increments on the scale. To test the pH you simply add two or three drops of the provided solution to about ½ of a vial of the water you wish to test and compare the color of the water to the color bar on the test kit.

Litmus Paper

Another very economical method for pH measurement is the litmus paper. The scale of pH that can be measured with litmus paper is usually around 5 –7. Litmus paper is a small piece of paper that has been chemically treated; when dipped into a water sample it will change color to indicate the pH level. Then you compare the color on the paper to the color on the chart provided.

A pH Metering Device

The most accurate pH measurements are obtained with a pH meter, It is much easier to get an accurate reading because the user is not required to do color comparisons. A pH measurement system consists of two parts: a PH measuring electrode and a high input impedance meter. These may be included in one device or the probe may be attached by a cable to the meter. The pH electrode can be thought of as a battery, with a voltage that varies with the pH of the measured solution. The pH measuring electrode is a hydrogen ion sensitive glass bulb, where the measured value varies with the changes in the relative hydrogen ion concentration inside and outside the bulb. PH probes accuracy are between (+ or – ) 1 to 2% depending on the value spent.

Notes when looking for a pH meter

The pH probe needs to be replaced over time. They are like a battery and will wear out especially when the probe is allowed to dry out or the solution in the probe has become contaminated. The pH probes usually have a glass bulb in the measuring end. The bulb is delicate and can be broken, making the pH probe unusable. Replace the pH probe if this happens. A pH meter may need to be recalibrated to keep it accurate. pH value will drift due to the pH probe aging or from battery power lost on pH pens. To get the most accurate readings it is recommended that the pH meter be AC powered since the power will remain constant and the meter will remain in calibration for a longer period. The size of the pH probe and amount of solution in the probe will make a difference on how long it will last. It is recommended to buy a pH meter which has a probe separate from the meter, this way when the probe wears out it can be replaced without having to replace the most expensive electronic parts of the meter.

Buffer Solutions and Calibration

Buffers are solutions that have constant pH values and the ability to resist changes in that pH level. They are used to calibrate the pH measurement system. There can be small differences between the output of one electrode and another, as well as changes in the output of electrodes over time. Therefore, the system must be periodically calibrated. Buffers are available with a wide range of pH values, and they come in both premixed liquid form or as a convenient dry powder capsule. Most pH meters require calibration at several specific pH values. One calibration is usually performed near the isopotential point (the signal produced by an electrode at pH 7 is 0 mV at 25 degrees Celsius. And a second is typically performed at either pH 4 or pH 10. It is best to select a buffer as close as possible to the actual pH value of the sample to be measured. A pH meters calibration should be checked every month to insure that the displayed value is the actual pH value of the solution. Most hydroponics stores that sell pH meters will sell the buffer solutions. The pH buffer solutions that are recommended for calibration of the pH meter are pH 4 and pH 7. These pH buffer solutions are very close to the recommended pH values for plants and will ensure that your meter continues to display an accurate value.

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