在酸性试液中加入捕捉剂,以二甲酚橙(XO)为指示剂,经实验证明可用标准硝酸铋直接滴定试液中的PO_4~(3-)离子。滴定在较强的酸性溶液中进行,可消除能与PO_4~(-3)生成难溶盐的阳离子的干扰。XO在该滴定条件下不封闭Al~(3+)(200mgAl_2O_3)。Fe~(3+)(50mg)用抗坏血酸还原为Fe~(2+)可消除干扰。As~(3+)离子严重干扰测定,当用王水溶样,将As~(3+)氧化为AsO_4~(3-)的形式,则可使砷 Addition of trapping agent to acid test solution and xylenol orange (XO) as indicator indicate that PO_4 ~ (3-) ions can be directly titrated with standard bismuth nitrate solution. Titration is carried out in a strong acidic solution, which can eliminate the interference with PO_4 ~ (-3) to form cation of hardly soluble salt. XO does not block Al ~ (3 +) (200mgAl_2O_3) under this titration condition. Fe ~ (3 +) (50mg) with ascorbic acid reduced to Fe ~ (2+) to eliminate interference. As ~ (3+) ions seriously interfere with the determination. When As ~ (3+) is oxidized to the form of AsO 4 ~ (3-) with aqua regia,