D-(−)-Tartaric acid, CAS 147-71-7, molecular formula C₄H₆O₆, molecular weight 150.09 g·mol⁻¹, is supplied as article code DTA-PG/100 for pharmaceutical applications requiring the (−)-enantiomer. The material is a white to almost white crystalline powder manufactured under ICH Q7 conditions and released against pharmacopoeial specifications aligned with Ph. Eur. 0476, USP–NF Tartaric Acid, and JP 17. The substance behaves as a diprotic acid with pK_a1 2.98 and pK_a2 4.34 at 25 °C. Aqueous solubility exceeds 1000 g·L⁻¹ at 25 °C, and specific optical rotation is controlled at −12.0° to −12.5° (c = 20, H₂O). For tablet, capsule, and granule operations, the acid functions as an acidifying agent, effervescent acid source, pH modifier, or chiral counterion; for injectable manufacture, it is used as an acidic buffering agent when terminal sterilization and particulate control are defined.

Pharmacopoeial monographs for tartaric acid are often enantiomer-non-specific; DTA-PG/100 therefore uses Ph. Eur. 0476 and USP–NF Tartaric Acid as the baseline, with additional polarimetric and chiral chromatographic controls to establish the (−)-enantiomer. Material labeled simply as “tartaric acid” without rotation may be racemic or partially resolved; it is not interchangeable with D-(−)-tartaric acid in chiral salt resolution. Three particle-size grades are available for direct compression, wet/dry granulation, and powder filling: D50 75–150 µm, D50 150–250 µm, and D50 250–425 µm. Residual moisture is controlled to ≤0.2% by Karl Fischer titration because free moisture above 0.3% causes punch filming on rotary tablet presses and capsule shell embrittlement. Sulfated ash is limited to ≤0.1%, chloride to ≤0.01%, and sulfate to ≤0.015%. When injectable grade is specified, bacterial endotoxins are controlled to <0.5 EU/mg by Ph. Eur. 2.6.14, and sub-visible particulate matter complies with USP <788> after 0.22 µm filtration.

Specification and Release Parameters

Compacting and Granulating with the (−)-Enantiomer

In effervescent granule manufacture, the stoichiometric ratio is 1 mol D-(−)-tartaric acid to 2 mol sodium bicarbonate, equivalent to approximately 1.00 g acid per 1.12 g sodium bicarbonate. Carbon dioxide release at STP is approximately 0.30 L·g⁻¹ acid. Fluid-bed granulation is conducted with inlet air at 45–55 °C and spray rate 10–20 g/min; granule loss on drying must remain below 0.2% before lubrication to prevent premature CO₂ evolution and sticking. Direct compression of acid-containing blends requires brittle excipients such as anhydrous dibasic calcium phosphate or microcrystalline cellulose. At acid loads above 25 wt%, tablet tensile strength decreases, and pre-compression force 3–5 kN and main compression 12–22 kN on a 10-station rotary press produce hardness values of 60–90 N. Friability is monitored per USP <1216>, with an acceptance limit of ≤0.8%. Production-scale compression of high-dose effervescent granules has shown that residual moisture above 0.3% produces punch filming on rotary presses and increased ejection force; tablet thickness and hardness show batch-to-batch variance when pre-compression force is not maintained within 3–5 kN. Punches require intermittent cleaning after 20–30 min if magnesium stearate level is above 1.0% because hydrophobic lubricant segregation occurs.

When Is the (−)-Enantiomer Preferred Over L-(+)-Tartaric Acid in Chiral Resolution?

Selection of D-(−)-tartaric acid over L-(+)-tartaric acid is dictated by the target amine enantiomer. In diastereomeric salt resolution of basic active pharmaceutical ingredients, D-(−)-tartaric acid is added at 1.0–1.1 molar equivalents to the racemic base in an ethanol/water 95:5 v/v solvent system at 40–50 °C. The less soluble diastereomeric salt crystallizes on cooling; after two recrystallizations, the enantiomeric excess of the liberated base can exceed 99.0%. The opposite enantiomer remains in the mother liquor and is recovered by pH adjustment. Because D-(−)-tartaric acid is the synthetic (−)-enantiomer, it precipitates the mirror-image salt from an (S)-configured base; the L-(+)-acid would precipitate the opposite diastereomeric salt. Chiral purity of the acid itself is therefore critical, since 1% of the opposite enantiomer reduces resolving efficiency and can produce mixed crystals with lower diastereomeric excess. The (−)-enantiomer is not obtained from grape fermentation; it is typically produced through resolution of synthetic racemic tartaric acid or through stereospecific synthesis. This origin affects residual chiral impurities, commercial availability, and the ICH Q3C residual solvent profile.

If Aqueous Granulation Is Required for High-Dose Tartaric Acid Tablets

The solubility of D-(−)-tartaric acid in water creates a narrow processing window when aqueous wet granulation is used. In a high-shear mixer with bowl capacity 10–25 L, impeller speed 150–250 rpm, and chopper 2000–3000 rpm, water addition above 10–15% w/w dissolves a portion of the acid. On drying, solute migration deposits a crystalline crust on granule surfaces. The resulting granule size distribution becomes bimodal, and dry granule hardness can vary by 30–50% across the bowl. Tablets compressed from such granules exhibit hardness variability of ±15 N and friability up to 1.2% under USP <1216>. Non-aqueous granulation with ethanol 96% or isopropanol at 20–40% w/w reduces acid dissolution but requires explosion-proof equipment and ICH Q3C residual solvent control. Residual ethanol or isopropanol is controlled to ≤0.5% total Class 3 solvents. When dry granulation is selected, slugging or roller compaction is performed at roll pressure 4–6 MPa and granule size 0.8–1.2 mm; this avoids the water-solubility conflict and preserves enantiomeric purity.

In parenteral manufacturing, DTA-PG/100 is dissolved in Water for Injection at concentrations of 5–50 mg/mL; the resulting pH is approximately 2.0–2.2. Terminal sterilization at 121 °C for 15 min may be acceptable at pH ≤3.0, but process validation must confirm no change in enantiomeric excess; published stability data for this specific concentration range is limited. Processing above pH 6.0 is avoided because neutralization increases the ionized fraction and reduces pH control. Solutions are filtered through 0.22 µm PVDF membranes and held in borosilicate glass or polyethylene-lined tanks; stainless steel contact is acceptable only for short durations at pH 2 because prolonged exposure can produce trace iron and chromium residues. Calcium-containing diluents must be avoided: calcium tartrate precipitates at neutral pH and increases sub-visible particle counts measured by USP <788>. The osmolality contribution is formulation-dependent and must be calculated from the acid's partial dissociation; published data for high-concentration single-component parenteral formulations is limited.

Elemental impurities are assessed per ICH Q3D; release data control lead ≤2 ppm, cadmium ≤1 ppm, arsenic ≤2 ppm, and mercury ≤1 ppm in oral and injectable grades. Residual solvents are controlled per ICH Q3C with ethanol and isopropanol as Class 3 solvents. For injectable use, batch records include bacterial endotoxin release per Ph. Eur. 2.6.14 and bioburden prior to sterilising filtration ≤10 CFU/100 mL. D-(−)-tartaric acid is incompatible with strong oxidizing agents, concentrated bases, and carbonates. In dry blending or granulation, contact with reactive metals and free water should be avoided to prevent premature effervescence and package bulging.

Compared with L-(+)-tartaric acid, D-(−)-tartaric acid differs in optical rotation and stereochemical configuration; its melting point is 166–170 °C versus 168–172 °C for the (+)-enantiomer. The racemic mixture melts at 206 °C and has lower aqueous solubility, approximately 206 g·L⁻¹ at 20 °C. Citric acid, by contrast, is achiral, triprotic, and has pK_a1 3.13, pK_a2 4.76, pK_a3 6.40. Citric acid generates approximately 0.35 L·g⁻¹ CO₂ when fully reacted with sodium bicarbonate, while D-(−)-tartaric acid generates 0.30 L·g⁻¹. The difference in proton count and pK_a profile means citric acid provides wider buffer capacity in solution but cannot provide chiral resolution. Fumaric acid is dicarboxylic but non-hydroxylated, has pK_a1 3.03, pK_a2 4.44, and is far less water-soluble; it is not interchangeable with D-(−)-tartaric acid in injectable formulations where high aqueous solubility is required.

In effervescent tablet and granule reformulation from citric acid to D-(−)-tartaric acid, the acid-to-bicarbonate ratio is adjusted because tartaric acid has one fewer reversible proton per molecule and lower CO₂ yield per gram. The lower pK_a1 also produces a faster initial pH drop in solution, which can affect taste-masking systems and enteric coating stability. For opaque capsule products, the acid is dry-blended with sodium bicarbonate and direct filled at RH ≤40% to prevent reaction; moisture sorption is monitored using ICH Q1A stability protocols. For parenteral formulations requiring a chiral acid counterion, DTA-PG/100 is specified over racemic tartaric acid because the absence of the opposite enantiomer avoids variable diastereomeric salt formation and simplifies X-ray diffraction and chiral purity analysis of the finished injection.