Laboratory tank mix compatibility testing presents the challenge of predicting the ultimate physical fate of mixtures under a large variety of often poorly known field use conditions. Most procedures, as dictated by practical necessity, involve only a few test conditions, are easy to perform, and utilize common laboratory equipment. By reason of their simplicity, however, these tests may fail to adequately mimic, hold fixed, or consider many of the field variables which may influence compatibility.

Studies were conducted with two formulations of each of two pesticides to explore the effects of some of the variables generally known or perceived to affect formulation performance and physical compatibility. Fractional factorial experimental designs were conducted in stages to study the effects and interactions of water hardness, temperature, copesticide ratio, order of addition, degree of agitation, residence time of the mix in the tank, and dilution rate. For one of the formulation pairs multiple regression and analysis of variance were used to construct a model which was able to adequately predict the failure or success of experiments not yet performed. The results indicate that all these factors can affect tank mix compatibility. Complex interactions between the factors were observed. The effects and interactions of the factors appeared to vary both qualitatively and quantitatively from the other formulation pairs, indicating that the major effects and critical ranges of factors are different for different tank mixes. These results indicate the types of difficulties which can be expected to be encountered in devising simple standardized tests for tank mix compatibility.