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Thread: Low pH after shocking

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    Low pH after shocking

    I've noticed that when I shock a pool, the pH drops way down. Is this a real drop in pH, or am I seeing a bleaching effect? I am testing with test strips. pH is typically 7.5 or so, and drops down to 6.6. It seems the higher the chlorine level, the lower the pH, but if it is bleaching out color, I would expect to see the same.
    TreeFiter

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    Saugerties, NY

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    Re: Low pH after shocking

    What type of chlorine are you using?
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    Re: Low pH after shocking

    The shock we use is Trichlor powder.
    TreeFiter

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    Saugerties, NY

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    Re: Low pH after shocking

    Quote Originally Posted by TreeFiter
    The shock we use is Trichlor powder.
    That will lower pH. How much depends on the pool volume and how much trichlor you added.

    Check out Effects of Adding Chemicals in http://www.poolcalculator.com/

    Every pound of trichlor you add per 10,000 gallons will lower pH by .59.
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    Re: Low pH after shocking

    So if I have a stubborn pool that needs a real kick in the ***, I should plan to kick up the pH too. I've seen it on a few pools where we were adding 8lbs to a 20x40 and the next day FC was zero again with the SWG at 50%. So I ended up having to hit it with 12lbs and bumping up the SWG.
    TreeFiter

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    Re: Low pH after shocking

    I assume that the pools you are shocking have a low CYA level since for every 10 ppm Free Chlorine (FC) you are adding with Trichlor, you are also increasing the Cyanuric Acid (CYA) level by 6 ppm. This is why one would normally use chlorinating liquid or bleach for shocking or perhaps Cal-Hypo if the Calcium Hardness (CH) is low.

    If there were no carbon dioxide outgassing, then every pound of Trichlor would need 10.8 ounces weight of caustic soda / lye / sodium hydroxide to compensate for the pH and TA (including the effects when the chlorine is used/consumed), but that is generally not available for pool use so the next best thing is 20 Mule Team Borax for which 52.5 ounces weight (about 50.3 ounces volume) would be needed. You could use 26.5 ounces weight (about 23.1 ounces volume) of Soda Ash / Washing Soda / Sodium Carbonate as found in most pH Up products, but that would increase the TA.

    At 80-100 ppm TA, adding 10 ppm FC would lower the pH from 7.5 to 7.1 after the chlorine got used up (it initially would drop to 7.3). Adding 20 ppm FC would lower the pH from 7.5 to 6.9 (it initially would drop to 7.2). The Pool Calculator is off with regard to the pH drop -- it's only an approximation and the actual drop from one pound of Trichlor in 10,000 gallons with the TA in the 80-100 ppm range (and no borates) is more like 0.40).
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    Re: Low pH after shocking

    Does the pH climb back up as the chlorine is consumed? In other words if I shock a pool and come back in a few days when FC levels have returned to normal levels, will the pH be back to where it was, or at least close to it?

    Also, if the pH drops, but the TA remains in range, won't the pH try to rebound?
    TreeFiter

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    Re: Low pH after shocking

    No, it's the other way around. Chlorine usage/consumption is a net acidic process. So the initial addition of Trichlor lowers the pH and the usage/consumption of chlorine lowers it even more.

    What you describe where the pH changes upon addition of chlorine and returns to normal after the FC has dropped is what happens when you use a hypochlorite source of chlorine such as chlorinating liquid, bleach, Cal-Hypo or lithium hypochlorite. The pH rises upon addition (i.e. when the FC rises) and drops back down upon chlorine usage/consumption (i.e. when the FC drops).

    If the TA is on the higher side, then it is true that the pH will try to rise and as the pH drops more carbon dioxide outgassing will occur. So the overall pH drop you see may not be as high as I described. In effect, it is like the TA lowering procedure since adding Trichlor is like adding a combination of chlorine and acid so if the pH does eventually come back up to where it started (without you adding additional pH adjusting chemicals), you would find that the TA was lowered. For every 10 ppm FC added by Trichlor, it lowers the TA by 7 ppm, again this is after the chlorine you added has been used/consumed since the lowering of TA actually occurs during this chlorine usage/consumption. Perhaps the following chart may be helpful to you in understanding what goes on:

    .......................... pH . TA . CYA . CH
    Trichlor added .... - .... 0 .... + .... 0
    Chlorine used ..... - .... -- ... 0 .... 0
    ------------------------------
    Net effect .......... -- ... -- ... + .... 0

    .......................... pH . TA . CYA . CH
    Dichlor added ..... 0 ... + ... ++ ... 0
    Chlorine used ..... - ... -- .... 0 .... 0
    ------------------------------
    Net effect ............ - .... - ... ++ ... 0

    .......................... pH . TA . CYA . CH
    Cal-Hypo added . + .. ++ ... 0 .... +
    Chlorine used ..... - ... -- .... 0 .... 0
    ------------------------------
    Net effect ........... 0 .... 0 ..... 0 .... +

    .......................... pH . TA . CYA . CH
    Bleach added ...... + .. ++ ... 0 .... 0
    Chlorine used ..... - ... -- .... 0 .... 0
    ------------------------------
    Net effect ........... 0 .... 0 ..... 0 .... 0

    In the above I am ignoring the small amount of excess lye in the hypochlorite sources of chlorine. Chlorinating liquid and lithium hypochlorite would be the same as bleach above.

    Now if you keep the TA higher so that there is more carbon dioxide outgassing, say because you are using Trichlor pucks/tabs on a regular basis, then you have the following where I assume the TA is high enough that the outgassing is enough to keep the pH constant (not usually the case, but useful to show):


    .......................... pH . TA . CYA . CH
    Trichlor added .... - .... 0 .... + .... 0
    Chlorine used ..... - .... -- ... 0 .... 0
    CO2 outgassing . ++ .. 0 .... 0 .... 0
    ------------------------------
    Net effect ........... 0 .... -- ... + .... 0

    In this extreme case, the pH is stable, but as you can see the TA is dropping so baking soda would need to be added. In practice, one usually has both the pH and TA drop and there is some carbon dioxide outgassing so pH Up is added instead:

    .......................... pH . TA . CYA . CH
    Trichlor added .... - .... 0 .... + .... 0
    Chlorine used ..... - .... -- ... 0 .... 0
    CO2 outgassing . + .... 0 .... 0 .... 0
    ------------------------------
    Net effect ........... - .... -- ... + .... 0

    With just the precise right amount of outgassing, one can balance everything using pH Up. Usually the TA level where this happens is higher than the 120 ppm normally recommended for Trichlor.
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