Resolutions to the Problems Caused by Introducing both Osmolarity and Effective Osmolarity
Authors: Kuang Serena Y. , Yang Xiaoqi , Li Xiaonan , Walter Stefan 
Published: May 2022

Introduction

In the literature, 2 types of osmolarity are introduced: osmolarity (OS, the concentration of the total solute particles (SP) of a solution (S)) and effective osmolarity (EOS, the concentration of the impermeant SP (imp‐SP) in S). Introducing 2 types of osmolarity causes 3 problems in dealing with osmosis. In this presentation, we address these problems and resolve them.

Method

Logical reasoning.

Results

1) Addressing the 3 problems

i. The fact that both OS and EOS are introduced in the literature is one of the four causes of inconsistency in the definitions of tonicity addressed in our first abstract1 to EB2022.

ii. The fact causes a great confusion between the meanings of the following terms: hyper‐osmotic or iso‐osmotic or hypo‐osmotic (referred to as x‐osmotic) vs. hypertonic or isotonic or hypotonic (x‐tonic). Some literature specifies that x‐osmotic is used to compare 2 OS and x‐tonic to compare 2 EOS. Consequently, x‐osmotic and x‐tonic compare two different properties (apples and oranges) and knowing one relationship does not mean we know the other, e.g., “iso‐osmotic is not always isotonic”2. Moreover, if S1 (with an OS of 600 mOsm/L) is hyper‐osmotic to S2 (its OS = EOS = 300 mOsm/L), S1 can be hypertonic or isotonic or hypotonic to S2 if it contains 200 mM permeant SP (p‐SP) + 400 mM impermeant SP (imp‐SP), or 300 mM p‐SP + 300 mM imp‐SP, or 400 mM p‐SP + 200 mM imp‐SP, respectively.

iii. It is unclear what real osmolarity is.

2) The resolutions

i. As illustrated previously3, real osmolarity is the osmotic concentration (OC, the concentration of the imp‐SP, a variable during osmosis in a simple osmosis system (S‐m‐H2O) where m refers to a semipermeable membrane separating S and H2O compartments); OC0is the initial OC before osmosis occurs (time = 0) and of practical use. OC0 is membrane (m)‐dependent: Facing different m, the resulting fraction of the imp‐SP in S‐m‐H2O is different. Hence, real osmolarity is neither OS nor EOS: OS contains permeant SP that do not exert an osmotic pressure; OS is a misconception of the term CTSP (the molar concentration of the total SP in S that is inherent to S and m‐independent)3. It is unclear whether EOS is a variable or a constant. OC0 is a system parameter owned by a simple osmosis system (S‐m‐H2O), whereas OS and EOS are owned by a solution.

ii. If OS is replaced by CTSP and EOS is replaced by OC0, then all problems addressed in this and our first abstracts to EB2022 are eliminated and x‐osmotic has the same meaning as x‐tonic, so that if S1 is x‐osmotic to S2, it is also x‐tonic to S2. See our third EB2022 abstract titled Origin of the Term “Isotonic”.

Conclusions

By applying CTSP to replace OS and OC0 to replace EOS, we have proved the effectiveness of CTSP and OC0 in eliminating the problems presented in this and our first abstracts1.

References

1. Kuang et al., “A Resolution for the Inconsistency in the Definitions of Tonicity” [Abstract], submitted to EB2022

2. Silverthorn, DU. Adv. Physiol Edu 40:499‐500, 2016

3. Kuang et al., The Concept of Osmolarity: Problems and Resolutions. The FASEB Journal, 2020, 34(S1)