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Thread: pH / EC / TDS / PPM

  1. #11

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    Wink What is a pH buffer?

    Chemical equilibrium
    To more deeply understand pH, we must first explore the concept of chemical equilibrium. The pH of pure water is considered neutral, because this is the point at which the autoprotolysis of water is just as favorable of a process as the reverse reaction. The chemical equation for the autoprotolysis of water is as follows:

    H2O <---> H+ + OH-

    The equilibrium constant for this reaction is called Kw, which is equal to the product of the concentrations of hydrogen ion and hydroxide ion in solution in moles per liter (See Appendix 1 below to calculate the #moles/L)

    The dissociation constant
    Kw = [H+][OH-]

    At this point, it is valuable to understand the p function itself. The p function of some value is equal to the negative logarithm of that value. So,

    pH = -log[H+]
    pKa = -log(Ka)

    The value for Kw is 0.00000000000001 at 24°C, which is easier to write as pKw = 14. So, for pure water, we know that all H+ and OH- came from water molecules, and thus they are equal in number throughout the solution. Since they have the same value, we can use the above equilibrium expression as follows:

    Kw = [H+][OH-] = [H+]2 = 10-14

    Thus, [H+] = 10-7, and pH = 7.

    Acids and Bases
    Any chemical that increases the concentration of hydrogen ion in solution, or lowers the pH is an acid. Likewise, any chemical that increases the hydroxide concentration in solution is a base.

    When an acid and its conjugate base are present in solution together, that solution is said to be a buffer, since it may react with acid or base without significant changes in pH. A hydroponic nutrient solution contains several conjugate acid-base pairs, since there are so many species present.

    For a solution containing an acid HA and its conjugate base A-, the following equilibrium exists:
    HA + H20 <---> H3O+ + A-

    For this protolysis equilibrium, the acid dissociation constant is given by:
    Ka = [H3O+]*[A-]/[HA]

    The pH is given by the Henderson-Hasselbalch equation:
    pH = pKa + log([A-]/[HA])

    The term pKa refers to the p-function of the dissociation constant for that acid in water, similar to pKw for water. Notice from the equation above that as long as the acid and conjugate base are within one order of magnitude in concentration, additions of acid or base will not greatly affect the pH.

    Buffering
    The buffering capacity, or ability to resist change in pH, is greatest within one pH unit of the pKa for the acid. A complex equilibrium exists between the concentrations of all of the species present in the nutrient solution and the concentration of available hydrogen ions, making the nutrient solution a buffer over a very large range. This is why adding acid to pure water decreases the pH much faster than adding acid to the mixed nutrient solution.

    Any species added to solution that can be either a proton donor (acid), or a proton acceptor (base), sets up a buffer.

    ex)
    You may have found that pure water you leave out in the air becomes slightly acidic over time. This is due to the absorption of CO2 from the atmosphere. The chemical process is as follows:

    CO2 + H2O ---> H2CO3 <---> H+ + HCO3-

    Carbon dioxide reacts with water to form carbonic acid, which dissociates in water to hydronium and bicarbonate anion. This increases the concentration of H+ in solution, reducing the pH. The pKa of carbonic acid is 6.4, which is about the pH of pure water that has been exposed to the air.

    ex) Potassium bicarbonate
    Potassium is K+, bicarb is HCO3-. usually with diprotic acids like carbonic and sulfuric, the first H comes off pretty easily but since the ion has a -2 charge it holds onto the second proton fairly strongly.

    Potassium bicarbonate is KHCO3, which dissociates to K+ and HCO3-. The bicarbonate anion can act as either an acid or a base. This makes it amphoteric.

    Chelation
    In chemical fertilization, EDTA salts are used as “chelators”. The purpose is to form a more stable species in solution by using bidentate bonds. This means that the metal ion (such as Mg2+) will have two bonds for each EDTA molecule attached. This entropy of formation is higher for the EDTA complex, preventing the metal ions that you want to stay in solution from reacting to form insoluble compounds. Chelation makes the nutrient species more soluble, and thus more readily available for uptake.

    What effect does pH have on elements in solution?
    The element of interest to the plant must be present in an ionic form that can be transported by the roots. Changes in pH mean changes in concentration of H+ and OH-, which drive changes in equilibrium between various salt forms. For example... if the pH is too high, any available OH- will react with manganese or magnesium, or any of the various components of the nutrient solution.

    Mg2+ + 2 OH- ---> Mg(OH)2

    Magnesium hydroxide is not available for passive transport into the root system, but Mg2+ is. On a similar note, contamination by chlorine is bad for your solution, because MgCl2 is insoluble as well, and has a high rate constant of formation.

    Appendix

    1. Calculating Molar concentration
    The molar concentration of a substance in solution is calculated by converting the mass of the substance into moles, and dividing that number by the liters of solution.
    To make the conversion, you add up the atomic masses (from the periodic table of elements) for each atom in a single molecule of that substance. This is the molar mass. Divide the mass of the substance added to solution by the molar mass. This result is the number of moles. Divide this by the volume to get the molar concentration.

    Let's do an example:
    We add 2.5g Epsom salts to 2 liters of water. The chemical formula for Epsom salt is MgSO4·7H2O.

    The atomic masses are as follows:
    Mg = 24.3 g/mol
    S = 32.1 g/mol
    O = 16.0
    H = 1.01

    Now remember to multiply each mass by the number of that species present in the molecule.
    Total mass = 24.3 + 32.1 + (11*16.0) + (14*1.01) = 246.5 g/mol.

    Now we convert grams to moles: 2.5g / 246.5 g/mol = 0.0101 mol.

    Since we used two liters, we divide number of moles by 2, and the result, [MgSO4·7H2O] = [Mg+] = [SO4-] = 0.00507 mol/L = 0.00507 M.

    Note: since Epsom salt is an ionic species, it dissociates in solution.

  2. #12

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    Default PPM-EC-CF Chart




  3. #13

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    Thumbs up How long does the seal last inside the probe?

    Something to note for all pH meters is the age of the meter; the gel seal inside the meter is only usually guaranteed to keep its seal for 2 years after being manufactured, although the gel seal may last much longer this.

    Digital meters such as the pHscan 1 have the month and year of manufacture on the inside of the battery lid. I suggest checking it when purchasing as some hydro stores may have slow stock turnover.

  4. #14

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    Wink What does pH mean?

    The degree of acidity/alkalinity of a solution is identified on the ph scale of 0 to 14, with a pH of 7 representing the neutral point. The pH scale is logarithmic, meaning small changes in pH represent large changes in the degree of acidity or alkalinity. For example, a solution with a pH of 5 is ten times as acidic as a solution with a pH of 6, but a solution with a pH of 5 is 100 times as acidic as a solution with a pH of 7. The pH of the nutrient solution is a major determinant of nutrient uptake by the plant.

  5. #15

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    Cool What is a TDS meter and what does it measure?

    Total Dissolved Salts meters are essentially little voltmeters that look at the voltage produced by a sensor, usually a couple of metal pins. The nute solution acts like a battery electrolyte and the pins function as do plates (electrodes) in a battery. The idea is that a nutrient solution is more electrically conductive when there are more nutrient salts in solution, so more salts means more voltage. A little math is done in the machine to convert the voltage to ppm (parts per million of dissolved solids).

    There is a calibration adjustment so this math can be touched up to compensate for various factors. You will need a test solution to verify your meter once a week. Usually you will find a single measurement at about 1500-1700ppm is enough to verify it's reading what it's supposed to.

    You need one that will read at least 0-2000ppm (or 0-1999ppm). You could use a 0-999ppm meter in a pinch if you added an equal volume of plain water to a sample from your tank-- you'd just double the meter reading.

    It's best to simply get the correct meter.

    There are other scales of measurement of nutrient concentration. In Europe, the "EC" (electrical conductivity) meters are preferred. They measure in units of millisiemens or mS instead of parts per million (ppm). The numbers are convertible one scale to the other, but most references and discussion here cite the ppm scale.

    Waterproof meters are both more expensive and worth it.

  6. #16

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    Arrow How do I figure out the ppm of my fertilizer mix?

    To figure out the ppm of your fertilizer (or fertilizer mix), you need to be able to measure grams and liters. Look at the 3 numbers on the side of a fert bag. These are the percent content of the nutrients. For every one gram of said fertilizer in one liter of water, it contributes 10 ppm of the given nutrient per percentage point. A 20-20-20 gives 200 ppm (10 ppm X 20) of each nutrient for each gram in a liter of water.

    The formula is this:
    grams of fert per liter = A/B
    A=your desired ppm
    B=10 ppm X the % of nutrient in mix
    or
    your ppm = C X B
    B=10 ppm X the % of nutrient in mix
    C= grams of fert per liter

    So to make a 200 ppm-100 ppm-200 ppm NPK mix using a 13-0-44 (potassium nitrate), a 12-62-0 (monoamonium phosphate), and a 33-0-0 (ammonium nitrate) you would work backwards from your sole P and K sources (it makes it easiest in this case), and make up the N at the end. I have rounded numbers to the nearest 0.1 g for the following. You would use 0.5 g of potassium nitrate (200 ppm/(10 ppm X 44 K)) and 0.2 g of monoammonium phosphate (100 ppm/(10 ppm X 62 P)) in one liter. This would give you 89 ppm N (10 ppm X 13 N X 0.5 g + 10 ppm X 12 N X 0.2 g), 124 ppm P (10 ppm X 62 P X 0.2), and 220 ppm K (10 ppm X 44 K X 0.5 g). 111 ppm are needed to raise the N to the 200 ppm level, so we can use 0.3 g of the ammonium nitrate (111 ppm/(10 ppm X 33 N)) to bring us up to finish.

    The actual mix would yield a 188 ppm N, 124 ppm P, 220 ppm K mixture in one liter of water. To get more precision, you need to mix larger batches or get a better scale (you would need to make a 10 liter batch of the above with a scale that is only accurate to the gram).

    If you mix your own fertilizer, you can adjust your N source to meet your pH needs, rather than being dependent on adding acid or base, which is nice.

    This works for formulating hydro mixes, as well as for us dirt farmers

  7. #17

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    thanks for all the info on meters and krap ive soil grown b4 but hydro is relitivly new to me
    and i was having trouble figuring out what meters to buy but this thread has help immensly this is a truly wonderfl site
    thanx again >chronic

  8. #18

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    Ebay have some good cheap TDS/PPM meters,i got mn for &#163;15 and its pretty acurate to.
    Plant a Seed
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    Free the Weed

    https://www.thctalk.com/gallery/showg.../500/ppuser/11

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  10. #19

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    Those 'HM Digital' brand ones that are often on there are good.. might be the ones you mean as they're usually &#163;15 ish. Had mine a couple of years and the accuracy still seems within a few ppm.

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  12. #20

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    very informative, cheers for taking the time and effort to do this IKDJ2003
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