固定pH滴定法测定混合食用色素

SIMULTANEOUS DETERMINATION OF FOOD PIGMENTS IN MIXTURES BY pH FIXED TITRATION

传统的滴定分析方法以当量定律为依据，必须确定化学计量点．对于离解常数小于１０ － ７ 的极弱酸碱，由于化学计量点ｐＨ 没有突变，传统的确定化学计量点的方法失效，无法在水溶液中直接滴定．本文提出的固定ｐＨ 滴定法是以水溶液中弱酸（ 碱） 各种存在形式的分布系数（ 数值上等于中和百分数） 在ｐＨ 值固定时是常数为依据的新型滴定分析方法，不以当量定律为依据，不需要确定化学计量点，不要求滴定反应定量完成，也不需要已知滴定剂浓度．只要温度、离子强度及滴定剂浓度不变，滴定到达某一固定ｐＨ 值时，滴定剂消耗体积与弱酸（ 碱） 物质的量呈正比．当溶液中存在多种弱酸（ 碱） 时，滴定到达某一ｐＨ 值，滴定剂消耗体积与各弱酸（ 碱） 物质的量之总和呈正比．因此，只需测定滴定至多个ｐＨ 值时所消耗的滴定剂体积，就可求出各组分含量．将本法应用于弱酸性合成食用色素胭脂红和柠檬黄混合物的同时测定，结果满意，回收率９９２ ％ ～１０２５ ％ ．还测定了的酸离解常数，其ｐ Ｋａ 分别为１１１９ 和９４３ ．

The traditional titration methods are based on the law of equivalent proportions.When applying to a weak acid or base whose dissociation constant is less than 10 -7 ,there is no perceptible break of pH at the stoichiometric point,so the end point of the titration cannot be ascertained.The traditional titration methods,therefore,are frustrated.A novel titrimetry to very weak acids and bases,pH fixed titration,is proposed in this paper.It is not based on the law of equivalent proportions but on the fact that the distribution coefficients of various species of a weak acid or base in aqueous solution are constant at a fixed pH value.It is not necessary to ascertain the stoichiometric point of the titration and to know the titrant concentration.Provided the temperature and ionic strength of the solution,and the titrant concentration are not varied,the volume of titrant consumed to reach a certain pH value will be proportional to the moles of the weak acid or base being titrated.If the solution is a mixture of weak acids or bases,the volume of titrant consumed to reach a certain pH value will be proportional to the sum of the moles of the components.To determine every component in the solution,it is only needed to measure the titration volumes at several different fixed pH values.Two weakly acidic synthetic food pigments permitted in many countries,Ponceau 4R and Tartrazine,in their mixtures have been selected for determination by the present method simultaneously.The results are satisfactory and the recoveries obtained are 99.2%～102.5%.The pK a values of Ponceau 4R and Tartrazine measured in this paper are 11.29 and 9.43 respectively.