Why do the preparation of the medium in original paper of Eagle needs to put the salts in two different bottles for storage? I guess it has something related to valence but I can't put it in a clear way.

The reason why the preparation of the medium in the original paper of Eagle requires the salts to be stored in two separate bottles is to ensure the stability of the medium.

The medium used in cell culture often contains a complex mixture of various salts, nutrients, and other components necessary for cell growth. These salts can interact with each other, leading to the formation of chemical compounds or changes in pH, which could alter the concentration and composition of the medium.

By storing the salts in separate bottles, the original paper of Eagle ensured that the individual components remained stable and would not react with each other during storage. When it was time to prepare the medium, the salts could then be properly mixed at the appropriate concentrations to achieve the desired composition.

Regarding the role of valence, it is possible that the salts used in the medium had different valence states. Valence refers to the combining capacity of an element in a compound, indicating the number of electrons that an atom can gain, lose, or share in order to form chemical bonds. Some ions, such as calcium (Ca2+), have a higher valence than others, like sodium (Na+), which may affect their stability or reactivity.

To gain further insights or confirm this hypothesis, it would be helpful to refer to the specific original paper by Eagle or related literature, as they may provide more detailed information on the rationale behind storing the salts separately.