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.