Photometric titration device by spectrophotometer. The titration cell with micro mixer will put light absorbing pool in the dark room in the photometer, micro burette is placed above the titration cell. Titration process, variation of the solution absorbance A Lambert - Bill's law. Titration, titration agent each added, record the absorbance at certain wavelength, in more than the equivalent point, also need to add 6 ~ 8 times the titrant, and record the absorbance. Then the absorbance of A as the ordinate, the volume of V standard solution as abscissa, draw the photometric titration curve, from the point of intersection of two lines can be derived by titration.
Acid-base titration, redox titration and titration are using photometric titration. On the reduction reaction of oxidation, photometric titration is mainly used to dilute solution colored, such as Potassium Permanganate standard solution titration can be ferrous solution containing nickel sulfate. By complexometric titration by spectrophotometric titration method, can obtain more accurate results.
Photometric titration should be performed in the spectrophotometer chamber. Because the non monochromatic light will make the solution absorbance from Lambert Bill law; darkroom light is not good, the stray light interference on absorbance will. Added titrant will produce dilution error, however, such as the titrant addition of not more than 1- of the total volume, you can ignore.
Absorbance titration processAChanges follow Lambert Bill law:
Type inBAs the absorbing layer thickness;CThe concentration of absorbing light material; E as a constant of proportionality, it with a light absorbing material, the wavelength of incident light and temperature.
In the process of titration, titration agent each add a certain amount of absorbance, records, until the absorbance jump. Then the titration of 6 ~ 8 times, and record the absorbance.
The absorbance as ordinate, the volume of standard solutionVAs the abscissa, draw the photometric titration curve, from the point of intersection of two lines can be derived by the end point of titration, 1, 2, 3 are the titration curve spectrophotometric titration.
The available photometric device the simplest gauge or modified by spectrophotometric and filter design, in the light absorbing parts placed titration pool, into the micro mixer (or electromagnetic stirrer), burette titration pool on the top. Semi automatic photometric titration meter is the incident light through the solution after irradiation in light current battery, to reach the end point absorbance due to jump, so the titration suddenly stop through the relay action, the results calculated by the burette readings. Automatic titration device is used in electronic circuit assembly of automatic addition of titrant and record type record instrument plotting the titration curve.
In the titration of very dilute acid solution or alkali solution or weak acid and weak base solution is by spectrophotometric titration. For example, the carbon dioxide measurement is 10 mu with sodium hydroxide solution after absorption, using phenolphthalein as Indicator , was photometric titration for the rest of the sodium hydroxide with 0.002 M hydrochloric acid. Also, with bromothymol blue as indicator, titration with sodium hydroxide solution of benzoic acid solution at a wavelength of 615 nm, can accurately detect the content.
Photometric titration is mainly used in dilute solution, the colored for example, ferrous solution containing nickel sulfate standard solution for titration Potassium Permanganate. In sulfuric acid solution of arsenic (III) can be at the wavelength of 320 nm with 10 ~ 10 mu cerium sulfate by spectrophotometric titration. Cerium (IV) on the wavelength of light absorption, before arsenic has not been completely oxidized absorbance unchanged, the absorbance increased gradually after the end, from the titration curves can be found at the end of the titration of arsenic. Determination of 50 micrograms to an average error of 35 mg of arsenic is less than 0.2-.
By spectrophotometric titration can get more accurate results. For example, with Eriochrome Black T as indicator, in pH9.7 and 620 nm wavelength with EDTA titration of magnesium, high accuracy than visual method (Figure 2). The Cu-EDTA complex at the wavelength of 745 nm light absorption is very strong, and Fe (III) -EDTA absorption is very weak. The copper and iron solution to pH 2, use EDTA titration of iron, the absorbance unchanged, to iron after the reaction is completed with Cu-EDTA generation, the absorbance increases gradually, after the reaction to the copper absorbance titration curve tends to a constant, as shown in Figure 3, from the intersection of the two tangent lines can be calculated the amount of iron and copper. In the presence of large amounts of magnesium, visual titration of calcium is not possible. Such as the EGTA (ethylene glycol ether amine two two four acetic acid) as the titrant, murexide indicator, in pH10 and 505 nm wavelength by spectrophotometric titration of calcium, 100 times in the presence of magnesium can be sudden abrupt curve.
The non monochromatic light does not follow the Lambert Bill law, the use of spectrophotometer small error than using filter photometer. The added titrant will produce dilution error must be corrected. However, if the titrant addition of not more than 1- of the total volume, you can ignore. The material of the reaction and mixing will generate heat, the solution temperature, affect the absorbance. However, titration mostly in very dilute solutions, this effect is small, can be neglected. The titration cell part as far as possible in the darkroom, influence from stray light.