柠檬酸钠又称枸橼钠、枸橼酸钠或柠檬酸三钠,分子式为Na₃C₆H₅O₇·2H₂O,分子量294.1。它是一种无色晶体或白色结晶性粉末,无臭、味咸凉,在150℃时失去结晶水,更高温度下则分解。在湿空气中微有潮解性,在热空气中有风化性,易溶于水及甘油,不溶于乙醇,难溶于其他有机溶剂。柠檬酸钠无毒性,具有pH调节性能和良好的稳定性,因此广泛应用于食品工业中。
优良性能柠檬酸钠是目前最重要的柠檬酸盐之一。其主要由淀粉类物质经发酵生成柠檬酸后与碱类物质中和而成,具有以下特点:
此外,柠檬酸钠还能提高乳化效果,并具有可生物降解性及分散能力和抗再沉积能力强的特点。因其环保特性,在日用化工洗涤剂中替代三聚磷酸钠生产无磷洗衣粉和液体洗涤剂时表现出色;在食品加工过程中用作调味、稳定剂。
食品添加剂最大允许使用量与残留标准柠檬酸钠作为食品添加剂,其最大允许使用量按生产需要适量使用,具体数值根据不同食品种类而定。我国规定可用于各类食品中,并按需添加。此外,在电镀工业中用作络合剂和缓冲剂;在医药工业中用于制造抗血凝药物、化痰药及利尿药等。
化学性质与生产方法柠檬酸钠的化学性质为无色结晶或白色结晶性粉末,无臭味咸凉感。没有熔点,相对密度1.857。常温下在空气中稳定,在加热至150℃时失去结晶水,继续加热则分解。不溶于乙醇,易溶于水,其5%的水溶液pH值为7.6~8.6。大白鼠腹腔注射LD₅₀为1549 mg/kg,ADI无需特殊规定。
柠檬酸钠的主要生产方法是将碳酸氢钠溶解在热水中,在85-90℃下加入柠檬酸并调整至pH 6.8后,加入活性炭脱色并趁热过滤。随后,进行减压浓缩、冷却结晶、离心分离、洗涤和干燥以获得产品。
化学反应方程式如下: C₆H₈O₇ + 3NaHCO₃ → C₆H₅Na₃O₇·2H₂O + 3CO₂↑ + H₂O
The [Zn3(CitH)2] (1) (CitH4= citric acid), was dispersed in sodium lauryl sulphate (SLS) to form the micelle of SLS@[Zn3(CitH)2] (2). This material 2 was incorporated in hydrogel made by hydroxyethyl-methacrylate (HEMA), an ingredient of contact lenses, toward the formation of pHEMA@(SLS@[Zn3(CitH)2]) (3). Samples of 1 and 2 were characterized by UV-Vis, 1H-NMR, FT-IR, FT-Raman, single crystal X-ray crystallography, X-ray fluorescence analysis, atomic absorption and TG/DTA/DSC. The antibacterial activity of 1–3 as well as of SLS against Gram-positive (Staphylococcus epidermidis (St. epidermidis) and Staphylococcus aureus (St. aureus)) and Gram-negative (Pseudomonas aeruginosa (PAO1), and Escherichia coli (E. coli)) bacteria was evaluated by the means of minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC) and inhibitory zone (IZ). 2 showed 10 to 20-fold higher activity than 1 against the bacteria tested. Moreover the 3 decreases the abundance of Gram-positive microbes up to 30% (St. aureus) and up to 20% (PAO1) the Gram-negative ones. The noteworthy antimicrobial activity of the obtained composite 3 suggests an effective antimicrobial additive for infection-free contact lenses.