A Resolution for the Inconsistency in the Definitions of Tonicity
Authors: Kuang Serena Y. , Li Xiaonan , Yang Xiaoqi , Walter Stefan 
Published: May 2022

IntroductionThere is great inconsistency in the current definitions of tonicity, which hinders research, teaching, and learning about osmosis across the cell membrane. In this abstract, we 1) show the inconsistency in the literature; 2) reveal what caused it and resolve it.MethodLogical reasoning.Results1) The inconsistencyTable 1 shows a collection of 14 descriptions/definitions of tonicity classified into 5 categories (Cat.), which indicates that the level of the inconsistency is high and the truth of tonicity is elusive.2) Causes and resolution of the inconsistencyAs we illustrated previously1, tonicity should be defined in a composite osmosis system, which can be deconstructed into 2 simple osmosis systems that are mirrored: S1‐m‐S2 = S1‐m‐H2O + H2O‐m‐S2, where S1 and S2 are two solutions separated by a membrane (m) that is only permeable to H2O in this abstract. Lack of this understanding is the first cause.Second, the owner of tonicity is wrongly reflected in the definitions of Cat. 1, 2, and 3 in the Table, phrased as “a solution’s tonicity” or “the tonicity of a solution”. Neither S1 or S2 in S1‐m‐S2owns tonicity because it reflects a comparison of the 2 initial concentrations of the impermeant solute particles in S1‐m‐S2(i.e., OC0(S1) and OC0(S2), where OC0 refers to osmotic concentration before osmosis occurs, i.e., time = 01, 2. As in a game that has two players, the result of the game (a qualitative statement of “player 1 won the game” or “player 2 lost the game” or a quantitative score of 12 to 9) reflects a comparison of the points (strengths) of the 2 players, so the game system owns the result. Osmosis in S1‐m‐S2 is like a water‐competing game between OC0(S1) and OC0(S2). Tonicity expressed qualitatively as “S1 is hypertonic to S2” or quantitatively as “300 : 200” if OC0(S1) = 300 mOsm/L and OC0(S2) = 200 mOsm/L, reflects a comparison of the osmotic strengths of OC0(S1) and OC0(S2), thus predicts the result of osmosis, i.e., a change in cell volume1. Hence, it is the composite S1‐m‐S2 system that owns tonicity (the comparison or prediction or the result of osmosis). “A solution’s tonicity” makes it impossible to define tonicity. Knowing the truth of tonicity, the definitions in Cat. 3 fail to define what tonicity is and reflect the result of osmosis inappropriately. The definitions in Cat. 5 obviously make no sense.The 3rd and 4th causes are addressed and resolved in our 2nd and 3rd abstracts titled Resolutions to the Problems of Introducing both Osmolarity and Effective Osmolarity and Origin of the Term “Isotonic”, respectively.ConclusionsTonicity can be expressed in multiple ways. The definitions in Cat. 4 express tonicity appropriately as something relational between S1 and S2 if OC0is the focus, not osmolarity or effective osmolarity. The last definition in Cat. 4 is accurate among all 14 definitions, with which, the result of osmosis (i.e., the osmotic pressure gradient after osmosis)3 is used to express tonicity.References1. The concept of tonicity: Problems and Resolutions. The FASEB Journal, 2020, 34(S1).2. The concept of osmolarity: Problems and Resolutions. The FASEB Journal, 2020, 34(S1).3. The concept of osmotic pressure: Two common misunderstandings and resolutions, The FASEB Journal, 2021, 35(S1).