- 期刊
食鹽中氯化鈉之極譜定量法 第二報－氧化銀法
Polarographic Determination of Sodium Chloride in Common Salt: II. Silver Oxide Method
摘要
硝酸銀溶液(約0.2F)與氫氧化碱(約1F)反應而生氧化銀之褐色沉澱，溶液中尚懸浮有少量氧化銀-氫氧化銀之黃褐色膠體。用傾瀉法除去膠體，即可得氧化銀。將此濕狀氧化銀加入於氯化鈉、氧化鉀、氯化鈣及氯化鎂等各種氯化物混合液時，則生成定量之氯化銀及氫氧化物，2Cl^-+Ag_2O+H_2O□2AgCl+2OH^-。食鹽中影響氯化鈉極譜之鎂鹽，成為不溶性氫氧化鎂，可與過剩之氧化銀一同除去。由氫氧化鈉之極譜測定，即可求得食鹽中氯化鈉之含量。氯化鈉溶液內加入氧化銀，除去氯化銀及過剩氧化銀沉澱後，則氫氧化鈉留存於溶液中。雖氯化鈉及氫氧化鈉有不間之半波電勢，惟都含有同量之Na^+，故由氫氧化鈉之極譜可定量氯化鈉。在同一情形下，在氯化鈉-氯化鎂系或食鹽中加入氧化銀，則氧化銀、氫氧化鎂及過剩之氧化銀沉澱而氫氧化鈉及少量之氫氧化鉀殘留液中。經充分洗滌可減低氫氧化鈉被膠體之吸收至約1.0%。如氯化鎂含量超過0.7%，加1.0 mm於波高即可校正。如氯化鎂含量低於0.7%，利用波高──濃度曲線，亦可校正。本法能消除氫氧化鈉法之Cl^-及OH^-共存之不同影響，並能減低Na^+被膠體吸收至一極小限。校正值比較準確。食鹽中之鉀，在本法中雖未能分別定量，惟因含量僅約0.5%，將其看作鈉計算，並無大差。臺灣製鹽廠三種食鹽在25°C恒溫下供作實驗。所得值平均高0.7%於氫氧化鈉法，低1.0%於化學差減法。
關鍵字
無資料並列摘要
Silver oxide is made by the reaction of 0.2F silver nitrate with excess 1F alkaline hydroxide. Brown Ag_2O precipitated with a few yellowish brown Ag_2O-AgOH colloidal particles. Separate the Ag_2O from the colloid by decantation and use it in the wet state. When Ag_2O is added to the NaCI solution, AgCI and the excess Ag_2O precipitate and NaOH are left in the solution. In spite of NaCl and NaOH give different half-wave potentials, they have the same amount of Na+. From the polarogram of NaOH, therefore, can determine quantitatively the NaCl. In case of NaCI-MgCI_2 or common salt, when Ag_2O is added, AgCI, Mg(OH)_2 and excess Ag_2O precipitate and NaOH with a little amount of KOH are left in the solution. About 1.0% of NaOH is absorbed by the Mg(OH)_2 colloidal precipitate after washing throughly. Therefore, 1.0mm correction is added to the wave-height of polarogram in cases that MgCl_2 is more than 0.7% by weight in NaCI-MgCl_2 or common salt. When MgChl_3 content is less than 0.7% wave-height vs. MgCl_2 percent diagram is available for correction The Ag_2O method eliminated the different effects of CI^- and OH^- in NaCI and NaOH respectively and reduced the absorption of NaOH by Mg(OH)_2 colloidal precipitate to a limited value by washing throughly. Therefore, Ag_2O method after correction would give more exact value than chemical analysis and NaOH methods. Three kinds of common salt were used for experiments at constant temperature of 25°C. I got a value about 0.7% higher than that of NaOH method given in the first paper, and about 1.0% lower than that of chemical analysis substraction method now used here. Potassium is not eliminated by this method. As its content is usually 0.5%, it seems to be no trouble to consider potassium as sodium in common salt.