Titration is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of a known reactant. Because volume measurements play a key role in titration, it is also known as volumetric analysis. A reagent, called the titrant or titrator[1], of known concentration (a standard solution) and volume is used to react with a solution of the analyte or titrand[2], whose concentration is not known. Using a calibrated burette to add the titrant, it is possible to determine the exact amount that has been consumed when the endpoint is reached. The endpoint is the point at which the titration is complete, as determined by an indicator (see below). This is ideally the same volume as the equivalence point - the volume of added titrant at which the number of moles of titrant is equal to the number of moles of analyte, or some multiple thereof (as in polyprotic acids). In the classic strong acid-strong base titration, the endpoint of a titration is the point at which the pH of the reactant is just about equal to 7, and often when the solution permanently changes color due to an indicator. There are however many different types of titrations (see below).

    Many methods can be used to indicate the endpoint of a reaction; titrations often use visual indicators (the reactant mixture changes colour). In simple acid-base titrations a pH indicator may be used, such as phenolphthalein, which becomes pink when a certain pH (about 8.2) is reached or exceeded. Another example is methyl orange, which is red in acids and yellow in alkali solutions.

    Not every titration requires an indicator. In some cases, either the reactants or the products are strongly coloured and can serve as the "indicator". For example, an oxidation-reduction titration using potassium permanganate (pink/purple) as the titrant does not require an indicator. When the titrant is reduced, it turns colourless. After the equivalence point, there is excess titrant present. The equivalence point is identified from the first faint pink color that persists in the solution being titrated.

    Due to the logarithmic nature of the pH curve, the transitions are, in general, extremely sharp; and, thus, a single drop of titrant just before the endpoint can change the pH significantly — leading to an immediate colour change in the indicator. There is a slight difference between the change in indicator color and the actual equivalence point of the titration. This error is referred to as an indicator error, and it is indeterminate.

    滴定是定量化學分析中常見的實驗室方法，用于測定已知反應(yīng)物的未知濃度。因為體積計量在滴定中發(fā)揮關(guān)鍵作用，所以又稱為容量分析。一種叫作滴定液的濃度和體積已知的試劑（標準溶液）與未知濃度的被測物或被滴定物反應(yīng)。用經(jīng)過校正的滴定管加入滴定液，就有可能測定到達終點時滴定液的確切消耗量。終點是滴定完成時所處的點，由指示劑（見頁底）來測定。理想中，這與等當點的體積相同。等當點是滴定液的摩爾數(shù)等于或幾倍于（比如，多元酸）被測物摩爾數(shù)時所加入的滴定液的體積。在古典強酸－強堿滴定中，滴定終點為反應(yīng)物PH值約等于7時的點，也就是由于指示劑作用，溶液顏色變化穩(wěn)定之時的點。然而，滴定有多種不同型式。

    多種方法都能用于指示反應(yīng)終點：滴定常常使用視覺指示劑（反應(yīng)混合物顏色發(fā)生變化）。在簡單酸－堿滴定中，可使用PH指示劑，如酚酞。當達到或超過某一PH（約為8.2）時，它變成紅色。另一個例子是甲基橙，它在酸性溶液中為紅色而在堿性溶液中為黃色。

    并非每一種滴定都需要指示劑。有些情況下，反應(yīng)物或生成物都有很強烈的顏色，可用作“指示劑”。例如，一種以高錳酸鉀（粉紅或紫色）為滴定液的氧化－還原滴定就不需要指示劑。當?shù)味ㄒ罕贿�原時，它就變成無色。等當點之后，有多余的滴定液出現(xiàn)。可從第一次出現(xiàn)在正在被滴定的溶液中的穩(wěn)定的微紅色來鑒別等當點。

    由于PH曲線的對數(shù)性質(zhì)，一般而言轉(zhuǎn)變是非常明顯的，因此，等當點之前的一滴滴定液能使PH發(fā)生顯著變化，引進指示劑顏色突變。指示劑顏色的改變與真實等當點之間存在著微小差異。這個誤差被稱為指示劑誤差，而這個誤差是不確定的。