BaadPuTthyKat

OK peeps.. I came across this & thought it would give a depth of understanding more than is really necessary for growing MJ.. so it might cover a lot of questions some might have... I know it did for me.. I tacked some hydro basics on the tail.

Introduction to pH

pH is a unit of measure which describes the degree of acidity or alkalinity of a solution. It is measured on a scale of 0 to 14. The term pH is derived from "p", the mathematical symbol of the negative logarithm, and "H", the chemical symbol of Hydrogen. The formal definition of pH is the negative logarithm of the Hydrogen ion activity.

pH = -log[H+]
pH provides the needed quantitative information by expressing the degree of the activity of an acid or base in terms of hydrogen ion activity.
The pH value of a substance is directly related to the ratio of the hydrogen ion [H+] and the hydroxyl ion [OH-] concentrations. If the H+ concentration is greater than OH-, the material is acidic; i.e., the pH value is less than 7. If the OH- concentration is greater than H+, the material is basic, with a pH value greater than 7. If equal amounts of H+ and OH- ions are present, the material is neutral, with a pH of 7. Acids and bases have free hydrogen and hydroxyl ions, respectively. Since the relationship between hydrogen ions and hydroxyl ions in a given solution is constant for a given set of conditions, either one can be determined by knowing the other. Thus, pH is a measurement of both acidity and alkalinity, even though by definition it is a selective measurement of hydrogen ion activity. Since pH is a logarithmic function, a change of one pH unit represents a ten-fold change in hydrogen ion concentration. Table 1 shows the concentration of both the hydrogen ion and the hydroxyl ion at different pH values.

The Molar Concept

A mole of a compound is defined as Avogadro's number of molecules (6.02 x 1023 molecules), which has a mass approximately equal to the molecular weight, expressed in grams. For example, sodium hydroxide, NaOH, which has a molecular weight of 23+16+1=40, would have 40 grams in a mole. Since the atomic weight of the hydrogen ion (H+) is one (1), there is one gram of hydrogen ions in a mole of hydrogen. A solution with a pH of 10 has 1 x 10-10 moles of hydrogen ions, or 10-10 grams in a one liter solution.

Ionization An ion is a charged particle, created by an atom or molecule which has either gained or lost electron(s). The presence of ions in solution allows electrical energy to be passed through the solution as a conductor. Different compounds form ions in solution in different amounts, depending on the ability of the atoms to gain or lose electrons. They will dissociate (or ionize) in solution to form hydrogen (H+) or hydroxyl (OH-) ions in the solution.

Molecules that dissociate easily will form strong acids or bases when in aqueous solution (water solvent). Examples of these are hydrochloric acid (HCI) or sodium hydroxide (NaOH): HCI + H2O ' H3O+ + Cl- NaOH ' Na+ + OH- In an aqueous solution, hydrogen ions normally combine with the water solvent to form the hydronium ion (H3O+). pH measurements of these solutions are therefore measurements of the hydronium ion concentration. Normally, the terms "hydronium ion" and "hydrogen ion" are used interchangeably in pH measurement applications.

Some compounds form weak acids or bases; only a very small percentage of the compounds dissociates into its constituent ions, so very few hydrogen or hydroxyl ions are formed. An example of this is acetic acid, which forms less than one hydrogen ion for every one hundred molecules:

H2O + CH3COOH ' H3O+ + CH3COO-

Pure water also dissociates weakly, with 10-7 hydrogen and 10-7 hydroxyl ions formed for every water molecule at 25°C: 2H2O ' H3O+ + OH- The addition of acid to water increases the concentration of hydrogen ions and reduces the concentration of hydroxyl ions. A base added to water has the opposite effect, increasing the concentration of hydroxyl ions and reducing the concentration of hydrogen ions:

H2O + HCl >H3O+ +Cl-
H2O + NaOH > Na+ + H2O + OH-

There is a wide variety of applications for pH measurement. For example, pH measurement and control is the key to the successful purification of drinking water, the manufacture of sugar, sewage treatment, food processing, electroplating, and the effectiveness and safety of medicines, cosmetics, etc. Plants require the soil to be within a certain pH range in order to grow properly,** and animals can sicken or die if their blood pH level is not within the correct limits. **Hydroponics of course, is where pH is totally central to the method's function.

pH Measurement
A rough indication of pH can be obtained using pH papers or indicators, which change color as the pH level varies. These indicators have limitations on their accuracy, and can be difficult to interpret correctly in colored or murky samples.

More accurate pH measurements are obtained with a pH meter. A pH measurement system consists of three parts: a pH measuring electrode, a reference electrode, and a high input impedance 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, with a millivolt output that varies with the changes in the relative hydrogen ion concentration inside and outside of the bulb. The reference electrode output does not vary with the activity of the hydrogen ion. The pH electrode has very high internal resistance, making the voltage change with pH difficult to measure. The input impedance of the pH meter and leakage resistances are therefore important factors. The pH meter is basically a high impedance amplifier that accurately measures the minute electrode voltages and displays the results directly in pH units on either an analog or digital display. In some cases, voltages can also be read for special applications or for use with ion-selective or Oxidation-Reduction Potential (ORP) electrodes.

Temperature Compensation
Temperature compensation is contained within the instrument, because pH electrodes and measurements are temperature sensitive. The temperature compensation may be either manual or automatic. With manual compensation, a separate temperature measurement is required, and the pH meter manual compensation control can be set with the approximate temperature value. With automatic temperature compensation (ATC), the signal from a separate temperature probe is fed into the pH meter, so that it can accurately determine pH value of the sample at that temperature.

Buffer Solutions
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 (electrode and meter). 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 convenient dry powder capsules. 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°C), 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.

Temperature Effects
As previously stated, the pH electrode is temperature dependent, and may be compensated for in the pH meter circuitry. The circuitry of the pH meter utilizes the Nernst equation, which is a general mathematical description of electrode behavior.

E=Ex + 2.3RTK log (ai)
nF
where:
Ex = constant depending upon reference electrode
R= constant
TK = absolute temperature (Kelvin)
n = charge of the ion (including sign)
F = constant
ai = activity of the ion
For pH measurement, we are interested in the hydrogen ion for
H+: 2.3RTk
______ = 59.16 mV
nF
where: n = 1 and T = 25°C. This term is commonly known as the Nernst coefficient. Since pH is defined as the negative logarithm of the hydrogen ion activity, the general equation at any temperature can be expressed as:

E = Ex- 1.98 Tk pH

Changes in temperature of a solution will vary the millivolt output of the glass pH electrode in accordance with the Nernst equation. Its variation in the electrode sensitivity versus temperature is a linear function, and most pH meters have circuitry designed to compensate for this effect (refer to Temperature Compensation). Figure 2 shows the effect on the glass pH electrode signal at various temperatures. In figure 2, all three slopes intersect at the point of 0 mV and pH 7.0; this implies no millivolt change with temperature at this, the isopotential point. Also, it can be seen that when working near 7.0 pH, temperature compensation is not a significant factor. However, when working at pH levels of 3.0 or 11.0, a temperature change of 15°C can result in an error of 0.2pH. Since the temperature effect on the electrode has been shown to be linear, the temperature dependence of pH can then be expressed as: 0.03 pH error/pH unit/10°C

The actual pH of the sample can change with temperature due to a change in the hydrogen ion activity in the solution, because ionization of compounds and hydrogen ion activity in the solution may be temperature dependent. Temperature compensation does not correct for this, and is not desirable, because an accurate pH measurement is desired at that particular temperature. Temperature compensation only corrects for the change in the output of the electrode, not for the change in the actual solution pH. Temperature will also affect the glass membrane's impedance. For each 8° below 25°C, the specified impedance approximately doubles. Depending on the original impedance of the glass membrane, the meter will have to handle a higher impedance at a lower temperature.

pH in Hydroponics.
In hydroponics, substrates need pH controlled within the max-min of around pH 6.3 ~ 5.3. The temperature range preferred by MJ roots is high enough that our pH shouldn't be affected much, but for reference 60F-75F is a range where MJ roots in hydro work best. ROckwool is commonly used, and needs soaking for 24hrs in pH 5.5 water or nutrient solution, dependant on whether it's for seed or cutting respectively. Different substrates will affect the pH buffering, and choice of nutrients.

Ideal pH changes with plant stage, but variation is beneficial, as it will allow the various minerals' own optimum pH for uptake. I find that vegging around 5.8 and flowering around pH 6.0 works best with my current nutrient, and most hydroponic nutrients will be similar. Anyone want to add a cherry, with StoNey's nutrient chart?

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grass_hopper

I'm having flashbacks to high school chemistry. Avogadro's number?? OMG I haven't heard those words for almost 25 years!

Too bad there's no smilie for "convulsing on the floor"

I hated chemistry, but I'm glad you posted this link

__________________
"Do not walk behind me, for I may not lead. Do not walk in front of me, for I may not follow. Do not walk beside me either. Just pretty much leave me the hell alone."


Ancient History:
Blueberry/Mazar (started in hydro, moved outdoors)...Skunk#1/Blueberry in Soil
Botanicare 42 Site Cloner - a Review...Grass_Hopper's Butane Honey Oil (BHO) Thread

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Xgone.

Hey GH, I just read through Kat's post, and without reading your response, I repped Kat saying "takes me back a few years to my chem classes". Well, I guess it does huh....?


Anyways, thanks for posting this Kat, I needed a little refresher course.

grass_hopper

I'm a mechanical kinda guy - I had to study my arse off in chem Give me a beam to size, on the other hand, and I'm all over it like white on snow

__________________
"Do not walk behind me, for I may not lead. Do not walk in front of me, for I may not follow. Do not walk beside me either. Just pretty much leave me the hell alone."


Ancient History:
Blueberry/Mazar (started in hydro, moved outdoors)...Skunk#1/Blueberry in Soil
Botanicare 42 Site Cloner - a Review...Grass_Hopper's Butane Honey Oil (BHO) Thread

Website goodies:
Grow Guide...Cecil_B's CO2 Enrichment Guide...Grower's Dictionary...Getting Females From Standard Seeds

BaadPuTthyKat

OK, I'll tidy up the subject with StOneY's chart.. Its just more math GH, & I bet the equations are easier than a site engineer's too. I now think the acceptable pH range is a little lower than the bracketed frame though....

Attached Thumbnails

 

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DickCheese

I agree... I often run my PH lower then the charts say is best,,, I like to start with my soil and Hydro that way and often run my hydro PH at 5.2-5.3

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Beener

Nice post Kat...very useful to a lot of people, I'm sure. Not nearly enough attention is paid to pH, and it's a huge part of the equation. pH effects more than meets the eye.

Interesting that a few use a lower range than mentioned. I tend to be on the high end in hydro 6.0/6.1.

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Beener

Lollie

Thank you for the information. I'll have to go over it a couple more times to make sense of it though.

I have a question for you, if you don't mind. I've read where you can grow with just a pH meter. I don't really understand that. I've always used my ppm meter and don't know what I'd do without out. But, I have tried to figure out how to grow with just the one meter and haven't succeeded yet. Maybe you have some thoughts on how to use the pH to help better tune up my garden???

All the best,
Lollie

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Conversions: 70 ppm to 50 ppm = 1 CF = 0.1 EC and 5 ml = 5 cc = 1 teaspoon

grnknight

Excellent information. For the truly mental masochists, I'd round out the discussion with an overview of how buffering agents affect ph. Note that most of this information is presented in the context of planted aquascapes, but the chemistry doesn't change. While most aquascaperes are adding CO2, we're adding oxygen, so we're tilting the other side of the equations.

http://www.thekrib.com/Plants/CO2/ha...arryfrank.html

http://fins.actwin.com/aquatic-plant.../msg00121.html

Beener

Hi Lollie, I grew for years in soil using only a pH meter. I learned to read the plants, and how to adjust my nutrient strengths by practice.

Now I also grow with hydro, so I bought a nice multi meter that reads nute strengths in EC or ppm. Since I've had it, I've also started using it for my soil grows. I wish I would have bought the meter years back when I was in soil alone. I get a better feel for my nute levels when I can see exactly what I'm feeding them, rather than using percentage variations of the label instructions.

All in all...I think learning without the meter made me a better fundimental grower. Using one now that I know what I know, has rounded out that experience by putting actual number values on those early observations.

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Beener