*2.3. Analysis*

The concentration of sodium ions was measured based on microwave induced plasma with optical emission spectrometry (MIP-OES) using a PLASMAQUANT 100 (Carl Zeiss, Jena, Germany), which included a microwave energy generator and resonator delivered by Plazmatronika, Wroclaw. The resonant cavity was combined with a microwave energy generator operating with frequency equal

to 2.45 GHz. The generator has two cooling systems: aqueous (magnetron cooling) and pneumatic (external cavity wall cooling). More details regarding the MIP-OES used in that analysis was published elsewhere [52]. The analysis was performed using solutions with a concentration equal to 0.5000 g/<sup>L</sup> NaOH (microgranules, analytically pure, POCH, Gliwice, Poland), 0.500 g/<sup>L</sup> 4MgCO3 × Mg(OH)2 × 5H2O, 3.6, and 36.0 g/<sup>L</sup> C4H6O4 (purum p.a., >99.0%, Fluka Analytical, Sigma-Aldrich), which were appropriately diluted in order to prepare the analytical curve in the range of 0.5–30.0 mg/L. Standard solutions and samples were introduced to the excitation source (argon-helium plasma) with a rate of 1.5 × 10−<sup>3</sup> L/min delivered by peristaltic pump (SPETEC, Erding, Germany) and concentric atomizer (Meinhard Glass Products, Golden, CO, USA) equipped with cyclone cloud chamber (EPOND, Vevey, Switzerland).

The contents of succinic acid and succinates in feed solutions, retentate and permeate obtained during the NF processes were determined by high-performance liquid chromatography method using HP Agilent 1100 Series system (Germany). The apparatus was equipped with an autosampler, interface (HP 35900), RI Detector (HP 1047A), pump (HP 1050), and column Rezex ROA-OrganicAcid H+ (8%), Phenomenex ®. The carrier phase was a 2.5 mM H2SO4 solution with the flow of 0.5 × 10−<sup>3</sup> L/min. The column temperature at the input to the detector was equal to 50 ◦C and pressure of 5.6 MPa. Before measurement, all samples were acidified to pH ≤ 2 by addition of 25% H2SO4 in an amount of 100 × 10−<sup>6</sup> L to 1000 × 10−<sup>6</sup> L of investigated solution. The pH of solutions was measured by laboratory pH-meter CP-505 (Elmetron, Poland) equipped with EPS-1 electrode.

The obtained concentrations of components were used to calculate retentions ( *R*) according to Equation (1):

$$R = \left(1 - \frac{C\_{p,i}}{C\_{f,i}}\right) \cdot 100\% \tag{1}$$

All rheological studies were conducted using a rotational stress rheometer Physica MCR 501 (Anton Paar) with a double gap concentric cylinder measuring geometry (DG26.7, cup diameter 27.59/23.83 mm, bob diameter 26.66/24.66 mm). The experiments were conducted in the scanning shear rate measurements with increasing shear rates in order to obtain flow curves of succinic acid solutions in the range from 10 to 500 s<sup>−</sup>1.
