## 1 Introduction  
The formation of free radicals in living organisms is natural and most organisms are able to neutralize these reactive oxygen species themselves [1, 2]. Nevertheless, in addition to their own neutralization system, they also use the antioxidant properties of certain nutrients for this purpose. Hence, the role of antioxidants in nutrition is an area of growing interest [3]. Supplementing the body with food containing substances with antioxidant properties is the basic way to protect it against the harmful effects of reactive oxygen species, including free radicals, which are the activators of many dangerous diseases [4, 5]. Scientific research carried out around the world proves that reducing the number of free radicals with the help of a proper diet increases the body's immunity, stops the development of some cancers and, most importantly, prolongs life [6].  
Most of the food which is consumed by humans occurs in the form of complex mixtures containing at least several antioxidants, each of which may have a different antioxidant capacity. It is known that in the case of mixtures of antioxidants, their antioxidant potential is not always an additive value consisting of the antioxidant properties of individual components [7; 8; 9]. The results of performed experiments often indicate an antagonistic or synergistic interaction of antioxidant compounds [10; 11; 12]. Moreover, other components of mixtures which occur in nature, not only the antioxidants present in them, may affect the overall antioxidant properties of the entire mixture. The experiments carried out in simple measuring systems containing only single antioxidants have shown that factors such as metal ions, water or reaction solvent, as well as pH, influence the estimation of antioxidant activity of compounds in spectrophotometric methods (for example in DPPH, ABTS and \(\beta\)-carotene bleaching assays) [13; 14; 15]. The presence of the above-mentioned factors differentiating the measurement systems may also affect the resultant antioxidant effect of the mixture. There is no information in the literature about research performed in this regard. Therefore, the aim of this paper is to check how the selected factors affect the resultant antioxidant effect of a simple mixture containing only two antioxidants. Due to the fact that fruits and vegetables, which are components of the human diet, are a rich source of polyphenic compounds (exogenous antioxidants), two popular compounds from this group were used in the research, i.e., kaempfer and chlorogenic acid. The choice of these antioxidants was dictated by the fact that they are present in commonly consumed plant products. For example, kaempfer is present in large amounts in onions, broccoli, chives, tea, grapes, tomatoes or strawberries. In turn, chlorogenic acid is a bioactive phenolic compound found in potatoes, eggplants, apples, plums or coffee beans [16; 17]. Both of these antioxidants occur side by side in cherry and elderberry fruits [18; 19; 20]. The study of the influence of the presence of hydrogen ions, water, and metal ions on the properties of these compounds and their binary mixtures, as well as the temperature and type of solvent on these properties, was carried out using the ABTS method. The experiments used the concentrations of antioxidants in which these compounds typically occur in plants. In order to assess the impact of changing the concentration of each of them on the resultant antioxidant effect of the mixture, the tests were carried out at different volume ratios of the components of the measurement system.