Use of ammonium or potassium dihydrogen phosphate to protect pesticides in spray mixtures prepared with alkaline waters

Abstract

A practical procedure was developed to reduce the possibility of alkaline hydrolysis of pesticides in spray tanks when naturally alkaline waters are used. Natural alkaline water samples collected from the Holland Marsh area and Niagara Region in Ontario had pH values in the range of 7.60-9.68, and their buffer capacity ranged from 0.21 X 10'3 M to 2.00 X 3 M. Adding 0.5 g of NH4H2P04 or KH2P04 per liter of water lowered the pH of all samples tested to 6.7 ± 0.3. The majority of pesticides are most stable at this pH and the probability of alkaline hydrolysis occurring can be reduced by this simple procedure.Water plays a major role in controlling the rate of decomposition of pesticides and on their environmental impact because most pesticides are applied as emulsions or suspensions in water and some of the applied pesticides eventually end up in streams, rivers, and lakes. Past studies revealed that many organophosphorus pesticides (Eto, 1974), particularly malathion (Wolfe et al., 1975) and carbamate pesticides (Kuhr and Borough, 1976) are susceptible to hydrolysis under alkaline conditions.The rate of hydrolysis depends on the concentration and chemical structure of the pesticide and on factors such as solvent used, pH, and temperature. In aqueous solutions, rate of hydrolysis of organophosphorus pesticides increases sharply at pH levels higher than 7 (Eto, 1974). Because hydrolysis is primarily catalyzed by the hydroxide ion under alkaline conditions, the rate increases approximately tenfold with each additional pH unit (O'Brien, 1967).It is appropriate, therefore, to recommend that pesticide mixtures prepared in alkaline water should be used immediately. Any delay in the use of spray mixtures after preparation-whether because of sickness, equipment breakdown, or unexpected weather conditions-could result in poorer pest control than expected. Consequently, extra pesticide applications are needed, resulting in increased crop production costs (Valk, 1975) and perhaps further environmental pollution.To prevent pesticide decomposition in alkaline water and to maintain biological activity, Johansen and Eves (1972) attempted to stabilize these chemicals with acids and nutrient spray acidifiers. They were, however, unable to maintain a constant pH and found that some pesticides were hazardous to honey bees if the pH spray mixtures was too low. Some pesticides are also susceptible to acid hydrolysis (Eto, 1974) and highly acidic water may damage plant tissues (Kemp, 1975). It seems reasonable, therefore, to conclude that the water most suitable for preparing pesticide mixtures before application will be neutral or slightly acidic. This paper reports the values of pH and buffer capacity of certain natural waters in southern Ontario. The effect of NH4H2P04 or KH2P04 in neutralizing naturally alkaline waters and in buffering them at pH 6.5-6.9 is demonstrated. The significance of using these chemicals in a routine procedure for preparing pesticide spray mixtures is discussed.