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Effect of pH

EFFECT OF PH ON PESTICIDE STABILITY AND EFFICACY

Winand K. Hock
Extension Pesticides Specialist
Penn State University

Has a grower ever come up to you and complained that the insecticide you sold him or that you custom applied for him didn't do a good job of controlling his insect problem? You probably attributed the reduction or lack of control to either a bad batch of chemical, or poor application, or pest resistance, or, maybe the farmer just didn't know what he was talking about. But, how many of you ever bothered to check the pH of the water prior to mixing the chemical?

If you look closely at the pesticide label, chances are you will find a statement cautioning you against mixing the pesticide with alkaline materials such as lime or lime sulfur. The reason for this is that many pesticides, particularly the organophosphate insecticides, undergo a chemical reaction in the presence of alkaline materials which destroys their effectiveness. This reaction is called alkaline hydrolysis and occurs when the pesticide is mixed with alkaline water; water with a pH greater than 7. The more alkaline the water, the more rapid the breakdown of the pesticides.

Lime and lime sulfur are often mentioned on pesticide labels because they are sometimes added to spray tanks. However, they are not the only materials that provide sufficient alkalinity for this reaction to occur. Caustic soda, caustic potash, soda ash, magnesia or dolomitic lime, liquid ammonia--all of these provide alkaline conditions in which susceptible pesticides can readily be hydrolyzed to inactive organic compounds.

It has been shown recently that in many areas of the U.S., water supplies have sufficient natural alkalinity to cause hydrolysis of certain pesticides. This means that a pesticide may begin to break down as soon as it is added to the tank. In practical terms, this means that the degree of pest control may be somewhat less than desirable, or even nonexistent, because a certain amount of the active ingredient will be decomposed to an inactive form before it ever reaches the plant and the pest. And if a spray rig is allowd to stand several hours or overnight before spraying out the contents of the tank, as much as 50% or more of the active ingredient may be decomposed.

Chemistry of Alkaline Hydrolysis

To better understand the phenomenon of alkaline hydrolysis, let's take a brief look at the chemistry using one of the organophosphate insecticides as an example.

Trichlorfon (Dylox, Proxol):

The phosphorous atom sort of divides the compound into two parts. Organophosphate insecticides are effective when the two parts of the chemical are together. When the parts are separated the OP pesticides are generally ineffective.

As you already know, water is made up of H and O . . . 2 parts H, one part 0 = H20. You also find charged particles or ions in water; both H+ and OH, and depending on where the water comes from, there may be an abundance of either H+ in the water, or an abundance of OH ions. The more H+ in the water, the greater the acidity; the greater the OH, the more alkaline the water.

This may seem rather elementary to all of you, but I feel it is necessary to understand the chemistry of water in order to understand alkaline hydrolysis.

The OH ion reacts readily with the OP molecule and breaks the molecule into two parts. The more alkaline the water (more OH), the more rapid the breakdown. This is what happens to most of the OP and carbamate pesticides in the presence of alkaline water; the rate of breakdown varies according to the alkalinity and the temperature of the water, and the length of time the spray mix sits in the tank.

[ Diagram Of Dylox Molecule In Two Parts Shown Here (couldn't find original illustration) ]

pH of Natural Water Sources

If the pH of your spray water is higher than 7.5, it is alkaline enough to affect some pesticides. The next few tables show the pH ranges reported for natural water sources in different areas of the U.S. A pH of 7.5-8.5 is common in many areas of the U.S. and in many surface and ground water sources in Pennsylvania. There have been reports that 5% of the natural water supplies in the U.S. have a pH higher than 9.0.


                         pH  - Rivers in the U.S.
      Potomac (MD, PA, WV)        7.8-8.4        Ohio (OH, IN, KY)   7.0-9.0
      Delaware (PA, NJ)           7.4-7.6        Colorado (CA, AZ)   7.7-8.5
      Hillsborough (FL)           7.1-8.2        Snake (ID)          7.6-8.4
      Little (MA)                 6.2-6.5        Rio Grande (CO, NM,
      Arkansas (AR, OK, KS, CO)   7.4-8.6         (TX)               7.3-9.0
      Missouri (NE, KS, MO)       7.8-8.5        Brazos, Trinity,
      Mississippi (MN, WI,                        Colorado, Guadalupe
        IL, MO)                   7.6-8.9         (TX)               7.2-8.5
                                  pH - Great Lakes
                      Lake Michigan (MI, IN
                         IL, WI)                    7.5-8.5
                      Lake Ontario (NY)             7.9-8.3


    

Which Pesticides Are Affected by Alkaline Water?

Although there is a great deal of variability, in general we find that insecticides are affected more severely by alkaline water than fungicides and herbicides. And, we find among the insecticides that the OP and carbamates are decomposed much more rapidly than the chlorinated hydrocarbons.

Many manufacturers provide information on the rate at which their products hydrolyze. This rate is usually expressed as 'half-life' or the 'time it takes for 50% hydrolysis or breakdown to occur'. With trichlorfon or DYLOX, for example, the time for 50% hydrolysis at pH 8.0 is but 63 minutes; at pH 7.0 50 % breakdown occurs in 386 minutes. and at pH 6, 80 hours.

This means that if the pH of your spray water is 8 and one hour elapses between the time you add the insecticide to your spray tank and the spray dries on the foliage, 50% of the active ingredient has already decomposed. But if your water has a pH of 6, it is not likely that you will lose any significant activity during the process of application.

Let's take a look at a few more examples:


Carbaryl (Sevin)                           Imidan
pH        Half-life                 pH         Half-life (20 degrees C)
6          100-150 days             4.0             15 days
7            24-30 days             7.0              1 day
8              2-3 days             8.3              4 hours
9                1 day             10.0              1 min.


    

Lower the pH in Your Spray Tank

If your water supply is alkaline, especially if the pH is 8 or greater, and you are using a pesticide that is sensitive to hydrolysis, you should lower the pH of the water in the spray tank. A pH in the range 4-6 is recommended for most pesticide sprays. You can adjust your spray solutions to the 4-6 pH range by the use of adjuvants that are marketed as buffering agents. Examples are:

  • Buffer-X (Kalo Lab.)
  • Nutrient Buffer Spray
    • 0-8-0 Zn Fe
    • 0-16-9 Zn
    • 10-12-0 Zn
    • 8-8-2 Zn Mn
  • Spray-Aide (Miller)
  • Sorba-Spray(s) (Leffingwell) -- 6 different products
  • Unite (Hopkins)

A question that is sometimes asked is whether acidification increases the residual time of the pesticide on the plant, thus affecting such factors as re-entry time and pre-harvest intervals. Residue tests on foliage sprayed with acidified and unacidified parathion sprays have failed to show any differences in the rate of degradation of the parathion. This would be expected since the pH of the foliage runs around 7.

There are a few pesticide materials which should not be acidified under any circumstances. Sprays containing fixed copper fungicides (including Bordeaux mixture, copper oxide, basic copper sulfate, copper hydroxide, etc.) and lime or lime sulfur should not be acidified. But, if the product label tells you to avoid alkaline materials, chances are good that the spray mixture will benefit by adjusting the pH to 6 or slightly lower.

The major benefit from acidification is obtained during the time the pesticide is in the spray tank; that is, from the time the pesticide is added to the water in the tank to the time the spray has dried on the foliage. If your water source is alkaline, addition of a buffering agent to the spray preparation is an easy and economical way to guarantee maximum results from your pesticide applications.


                                             pH  HYDROLYSIS RATE 50%
      TRADE NAME      COMMON NAME                  HYDROLYZED IN
      Dylox           trichlorfon           8.0  6.3 minutes
                                            7.0  6.4 hours
                                            6.0  3.7 days
      Guthion         azinphos-methyl       9.0  12 hours
                                            7.0  20 days
                                            5.0  17.3 days
      Carzol          formetanate           9.0  3 hours
                                            7.0  14 hours
                                            5.0  17.3 days
      Imidan              --                8.3  Less than 4 hours
                                            7.0  Less than 12 hours
                                            4.5  13 days
      Dimecron        phosphamidon         10.0  30 hours
                                            7.0  13.5 days
                                            4.0  74 days
      Sevin           carbaryl              9.0  24 hours
                                            8.0  2-3 days
                                            7.0  24-30 days
                                            6.0  100-150 days
      Gardona         tetrachlorvinphos    10.5  80 hours
                                            7.0  44 days
                                            3.0  54 days
      Phosdrin        mevinphos            11.0  1.4 hours
                                            7.0  35 days
      DiSyston        disulfoton            9.0  7.2 hours
                                            5.0  60 hours
         --           EPN                  10.0  8.2 hours
                                            6.0  More than 1 year
         --           Parathion (Note:     11.0  170 minutes
                        Methyl Parathion   10.0  29 hours
                        hydrolyzes          7.0  120 days
                        several times       5.0  690 days
                        faster than
                        Parathion.)
        --            TEPP                 10.0  21 minutes
                                            9.0  3.5 hours
                                            6.0  6.8 hours
      Lannate         methomyl              At a pH of 9.1, loses 5.0% of its
                                            effectiveness in 6 hrs. at a
                                            rate of 8 oz. per 100 gal.
                                            water.  Stable in slightly acid
                                            solutions.
        --            malathion             Hydrolyzes rapidly at a pH above
                                            7.0 and below pH 3.0.
      Dibrom          naled                 Hydrolyzes 90-100% in 48 hours in
                                            alkaline conditions.
      DeFend, Cygon   dimethoate            Unstable in alkaline media.
                                            Stability is at a maximum at pH
                                            values between 4 and 7.
      Benlate         benomyl               Less soluble in alkaline
                                            solutions.