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Agarose gel vials are often used as calibration phantoms, but as a naturally occurring substance, agarose may exhibit unfavourable qualities relating to instabilities, inconsistencies and heterogeneity. To contribute towards standardisation and methods harmonisation of quantitative <jats:sup>23<\/jats:sup>Na MRI, the objective of this study was to develop and test a novel, standardisable synthetic polymer calibration phantom for in\u00a0vivo quantitative <jats:sup>23<\/jats:sup>Na MRI. Seven crosslinked polyacrylamide gel (PAG) samples were prepared, doped with sodium chloride (NaCl) at nominal concentrations of 10\u2013150\u2009mM. The sodium concentrations of all samples were estimated by volumetrics using high\u2010precision mass measurements. Relaxation time constants () of all samples were measured at 3\u2009T with a non\u2010localised pulse\u2010acquire sequence.  was measured longitudinally over 14\u2009months to assess stability. Finally, in\u00a0vivo TSC quantification with PAG phantoms was demonstrated in the human brain and calf muscle on different systems, with different imaging sequences. The measured sodium concentrations of phantoms were on average 5% lower than nominal ones, owing to the unknown volumetric contribution of the solid fraction. Hence, they were reported as apparent sodium concentrations, and the apparent TSC (aTSC) was quantified in\u00a0vivo. Mean relaxation time constants of <jats:sup>23<\/jats:sup>Na in PAG were in the following ranges: \u2009=\u200927\u201339\u2009ms, \u2009=\u20094.8\u20137.1\u2009ms, \u2009=\u200916.8\u201318.8\u2009ms, short fraction \u2009=\u20090.64\u20130.77. Over 14\u2009months, relaxation time constants were stable within 10% (above sodium concentrations of 25\u2009mM). In\u00a0vivo aTSC measurements were in the expected ranges. 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