2 AQUEOUS SOLUTIONS OF LACTIC ACID

      Carl T. Lira and Lars Peereboom

2.1 INTRODUCTION

      Lactic acid is a three‐carbon alpha hydroxy carboxylic acid. This chapter summarized some key properties of pure lactic acid and lactide, but primarily focuses on aqueous lactic acid solutions, that are of importance in purification. Lactic acid is typically marketed as aqueous solutions.

2.2 STRUCTURE OF LACTIC ACID

Lactic acid is a chiral molecule with two enantiomers, L‐(+)‐lactic acid (also known as S‐lactic acid) and D‐(−)‐lactic acid (also known as (R)‐lactic acid). L‐lactic acid is dextrorotatory and D‐lactic is levorotatory. The racemic mixture is termed rac‐lactic acid. The L‐ and D‐forms of lactic acid have melting points near 53°C, while rac‐lactic acid has a melting point of 16.8°C (Table 2.1). The optically active forms will racemize slowly to rac‐lactic acid when held at 200°C for 500 h [1].

Lactide is the dilactone of lactic acid. Having two stereocenters, lactide has three unique configurations: SS (L‐lactide), RR (D‐lactide), and RS (meso‐lactide). Note that a blend of L‐lactide and D‐lactide is called rac‐ or DL‐lactide and is not the same as meso‐lactide. The lactides rotate polarized light in the opposite direction of the constituent lactic acids; SS (L‐lactide) is levorotatory (−) and RR (D‐lactide) is dextrorotatory. Meso‐lactide is optically inactive. Melting points are indicated in Table 2.2.

      The optical rotation for D‐ and L‐forms of lactic acid in water is complicated by oligomer equilibration. Literature values for the optical rotation of L‐lactic acid in water range from −13° to 3.9°. Bancroft and Davis [2] show that specific rotation ([α]_D) for L‐lactic acid as a function of apparent concentration changed linearly from 5.26° at 76 g/100 mL to 0.85° at 5 g/100 mL. Sodium lactate, on the other hand, exhibits nonlinear optical rotation with concentration and changes from −7.8° at 47 g/100 mL to −12.2° at 0.7 g/100 mL. They also showed that specific rotation of a freshly prepared lactic acid (76 g/100 mL) decreased from 5.2° to 1.37° over 17 days. The lactic acid was prepared by acidifying zinc lactate, filtering of zinc sulfate and removing the water under vacuum, keeping the temperature < 40°C. It should be noted that their structural interpretations were wrong. We recommend not to use optical rotation as a measure of the enantiomeric purity of lactic acid but instead rely on HPLC [3], GC [4], or NMR [5] methods.

2.3 VAPOR PRESSURE OF ANHYDROUS LACTIC ACID AND LACTIDE

Vapor pressure of anhydrous lactic acid has been measured by relatively few authors. Due to the challenges in purifying lactic acid, data are somewhat scattered. Table 2.3 summarizes data and the accepted data are plotted in Figure 2.1. Data are accepted based on consistency between values from multiple researchers by plotting with the expected approximate linear behavior on the coordinates of Figure 2.1. Many of the data are measurements provided by researchers using lactic acid for chemical synthesis. The accepted data are well represented by Equation 2.1 for vapor pressure P ^sat (in units of Pa) that is extrapolated in the figure for comparison with lactide vapor pressure data at higher temperatures. Care should be taken in using the extrapolation above 405 K because the vapor pressure may have a curvature on the log P ^sat vs. 1/T plot that is not captured by the extrapolation. Oligomerization and subsequent water evaporation are likely to occur during vapor pressure measurements, complicating the interpretation of the results.

TABLE 2.1 Physical Properties of Lactic Acid