The Concept of Osmolarity: Problems and Resolutions
Authors: Kuang Serena Y. , Walter Stefan , Yang Xiaoqi , Li Xiaonan 
Published: April 2020

IntroductionIn this presentation, we address and resolve the problems with the current definitions of osmolarity (conventional osmolarity) that result in great confusion about the definition of tonicity. These problems are partially rooted in issues described in our first abstract in the EB2020, “The Problems with the Concepts of Osmole and Osmotically Active Particles”.Methodlogical reasoning.Results1) CTSP (the molar concentration of the total solute particles (TSP) in a solution measured using mM,not mOsm/L) is defined as the first prerequisite to understanding what true osmolarity should be. CTSP is a missing concept in current teaching and research on solutions and osmosis. 2) A thorough differentiation of the vague term “solute” (Fig
1) is the second prerequisite. 3) Our amended definition that reflects true osmolarity (bolded osmolarity) is the impermeant fraction of CTSP (or the concentration of the impermeant SP) in the solution (S) compartment of a simple osmosis system (S‐m‐H2O consisting of a S compartment and a water compartment separated by a selectively permeable membrane (m) (irrespective whether the m is water‐permeable (only permeable to water) or SP‐permeable (permeable to water and some species of solute particles (SP)). The unit of osmolarity is mOsm/L. This single definition of osmolarity eliminates the use of two types of osmolarity (conventional and effective) that are problematic. 4) Osmolarity is a variable during osmosis. In keeping with the recommendation by the International Union of Pure and Applied Chemistry (IUPAC) Gold Book 1996 to use “osmotic concentration” to replace “osmolarity”, we promote the use of bolded osmotic concentration (OC) to present bolded osmolarity. The initial OC or OC0 is of practical use. 5) A solution has one CTSP but may interact with many different membranes and result in many values of OC0 (Fig
2). 6) A composite osmosis system (S1‐m‐S2) has two solution compartments separated by an m, which can be deconstructed into two simple osmosis systems that are mirrored: S1‐m‐H2O and H2O‐m‐S2. 7) The application of OC0 produces two by‐products: First, the multiple forms of van ‘t Hoff’s Law can be unified into one single general form because OC0 is not only membrane‐dependent, but also embraces multiple factors that influence its value and thus is of complexity; and second, by using OC0 and S1‐m‐S2 to illustrate tonicity, all confusion about tonicity is eliminated in our third abstract titled “The Problems with Tonicity and Resolutions” in EB2020.Conclusion1) OC0 is a membrane‐dependent system parameter affiliated with S‐m‐H2O (not S). 2) By replacing conventional osmolarity with CTSP, effective osmolarity with OC0, and osmotically active particles with the membrane‐dependent osmotically effective SP (irrespective whether the membrane is water‐permeable or SP‐permeable), all problems with conventional osmolarity are resolved.Solute differentiationFigure 1The one‐to‐many relationship between CTSP and OC0: facing different 3 membranes, 3 different values of OC0 emerge in the SA compartment of the 3 simple osmosis systems. σ: reflection coefficient.Figure 2