What is the density of sodium trimethylsilylpropanesulfonate (DSS)?
The reported crystallographic density for sodium 3-(trimethylsilyl)-1-propane-sulfonic acid monohydrate is $\pu{1.32 g cm^-3}$ [1].

The title compound which is used as reference for some NMR spectra in $\ce{D2O},$ was obtained casualy during an attempt to crystallize a cephalosporin of a serie we are interesting for […].
The X-ray analysis was undertaken because the expected cephalosporin and the two independent molecules of the title compound which were in fact present in the unit cell have nearly identical molecular weights.
The compound crystallizes from water in the triclinic space group $P\bar{1}$ with unit cell dimensions $a = 5.914(3),$ $b = 10.816(7),$ $c = \pu{18.869(9) Å},$ $α = 97.96(5),$ $β = 95.46(4),$ $γ = 89.76(5)^\circ,$ $V = \pu{1190(1) Å^3}.$
The calculated density is $\pu{1.32 Mg m^-3}$ for $Z = 4.$


Figure 1. Unit cell of sodium trimethylsilylpropanesulfonate monohydrate (CSD: BEHNIL).
Reference

Tinant, B.; Coene, B.; Declercq, J. P.; Germain, G.; Meerssche, M. van. X-Ray Crystal Structure of 3-(Trimethylsilyl)-1-Propane Sulfonic Acid Sodium Salt Hydrate $\ce{C6H15O3SSi^-Na^+ · H2O}.$ Bulletin des Sociétés Chimiques Belges 1982, 91 (1), 95–96. DOI: 10.1002/bscb.19820910116.