CN103439279B - Spectrophotometry quantitative analysis method of iodine-starch color-developing system - Google Patents

Abstract

The invention belongs to the technical field of chemical and spectral analysis testing, and specifically discloses a spectrophotometric quantitative analysis method of an iodine-starch color development system. The specific steps are: at room temperature, (1) take a certain amount of I ^- containing solution in the volumetric flask, and fully react with the excess _FeCl3 solution at room temperature; (2) add a certain amount of starch solution into the volumetric flask, shake (3) Immediately add distilled water to the volumetric flask to dilute until it is close to the neck of the volumetric flask, and shake well; (4) Immediately add an appropriate amount of H ₂ O ₂ , shake well, make up to volume with distilled water, and shake well; (5) Use FeCl ₃ as a blank control, and draw an absorption curve at 400-700nm; determine the measurement wavelength; (6) According to the method (1)-(4), use I ^- standard solution to prepare color development systems with different iodine concentrations, and use the measurement wavelength Determine the absorbance A, use the corresponding relationship between I ^- concentration c and A value ^, establish a standard curve, and obtain a linear regression equation and correlation coefficient; A of the chromogenic system, calculate the concentration of I ^- in the measured system according to the regression equation in (6). The chromogenic system of the method is stable and can be directly and indirectly used for iodine analysis. The method has the advantages of simplicity, accuracy, trace amount, quickness, low price and the like, and has wide applications.

Description

A Spectrophotometric Quantitative Analysis Method of Iodine-Starch Chromogenic System

technical field

The invention relates to the technical field of chemical and spectral analysis testing, in particular to a spectrophotometric quantitative analysis method of an iodine-starch color development system.

Background technique

The iodine-starch color reaction is a traditional and very classic chemical reaction. I ^- is oxidized to I ₂ by an oxidant in aqueous solution, and I ₂ molecules penetrate into the helical structure of starch to form I ₂ -starch inclusion complex, which absorbs blue Complementary color light, the solution is blue. The color reaction between I ₂ and starch is reversible (I ₂ +starch I ₂ -starch clathrate), reach equilibrium under certain conditions. If the reaction conditions (concentration, temperature, etc.) are changed, the balance will be broken, the concentration of the inclusion compound will change accordingly, and the absorbance will also change accordingly. The disproportionation of I ₂ is the fastest in an alkaline environment, followed by neutrality, and slower in acidity; even in acidic conditions, it is still disproportionated, and the concentration continues to decrease, making it difficult to maintain an equilibrium state; the iodine-starch color reaction is unstable, making it color The system is difficult to meet the spectrophotometric quantitative analysis. This color reaction is often used for qualitative identification.

At present, there are few reports on iodine-starch color reaction spectrophotometry in France and abroad, and there are many domestic reports, mainly including: using H ₂ O ₂ , Cr ₂ O ₇ ^2- , MnO ₄ ^- , Br ₂ , BrO ₃ ^- , IO ₃ ^- , Cu(II) as an oxidant, oxidize I ^- to I ₂ and then use spectrophotometric quantitative analysis for color development of starch, for example, use bromine water to oxidize I ^- to generate I ₂ and then use spectrophotometric quantitative analysis for color development, but Related reports have failed to pay attention to and solve the core problem of unstable iodine-starch color reaction. After bromine water oxidizes I ^- to generate _I2 and starch color development, the change in absorbance measured by spectrophotometry is shown in Figure 1. Many oxidants are used for The color development of iodine-starch has the same characteristics. Therefore, the reliability of the analysis results is low when the quantitative analysis is carried out by the above-mentioned method.

Contents of the invention

The purpose of the present invention is to provide a simple, accurate, trace, rapid and cheap iodine-starch color system spectrophotometric quantitative analysis method.

The present invention has taken following technical measures, and concrete steps are:

(1) In a volumetric flask, take a quantitative amount of I ^- containing solution and react fully with _excess FeCl solution at room temperature;

(2) Add a certain amount of starch solution into the volumetric flask and shake well;

(3) Immediately add distilled water to the volumetric flask to dilute until it is close to the neck of the volumetric flask, and shake well;

(4) Immediately add an appropriate amount of H ₂ O ₂ , shake well, make up to volume with distilled water, and shake well;

(5) Use FeCl ₃ as a blank control, and make an absorption curve at 400-700nm; determine the measurement wavelength;

(6) According to the method (1) to (4), use I ^- standard solution to prepare color development systems with different iodine concentrations, measure the absorbance A with the measurement wavelength, and use the corresponding relationship between I ^- concentration c and A value to establish the standard Curve to obtain the linear regression equation and correlation coefficient;

(7) According to the method (1)-(4) above, measure the A of the color system of an unknown I ^- containing solution, and calculate the concentration of ^I- in the measured system according to the regression equation in (6).

In order to adapt to the detection range and sensitivity of the spectrophotometer, the present invention regulates the concentration of ^I- in the measured system to be 0.02-0.10mmol/L.

The _FeCl3 solution used in the present invention is a high-concentration microvolume. For example, 50 μL of a 1.5 mol/L _FeCl3 solution is used in the specific embodiment. The purpose is to make the reaction of Fe3 ⁺ and ^I- reach equilibrium in a short time, and the room temperature is 18°C About 30 minutes, the room temperature is higher than 18 ℃ can shorten the time of this step; the reaction time of Fe ³⁺ and I ^- is not enough, which will prolong the time for the color development system to reach equilibrium (Figure 2).

The starch used in the present invention is ordinary analytical pure starch, using a 25mL volumetric flask, take more than 2.00mL of 1% starch solution, and the amount of starch increased on this basis has no effect on the absorbance of the color development system.

The present invention uses an appropriate amount of H ₂ O ₂ as an important substance to stabilize the color development system. Using a 25mL volumetric flask, taking 50 μL of 1.5% H ₂ O ₂ can stabilize the color development system within 240 minutes (Figure 3); When the amount of H ₂ O ₂ is reduced, the stable time of the color development system is shortened. If the amount of H ₂ O ₂ exceeds 2 times, the color development system collapses immediately (Figure 4).

Using the absorption curve of the chromogenic system of the present invention, the measurement wavelength is determined to be 600 nm (Figure 5).

Containing I ^- test solution and I ^- standard solution to develop color together and measure together, the precision and accuracy are the best.

The spectrophotometric quantitative analysis method of the iodine-starch chromogenic system of the present invention has a chromogenic system that is stable within 240 minutes, ensuring maximum reliability of the measurement results.

The spectrophotometric quantitative analysis method of the iodine-starch chromogenic system of the present invention does not use expensive instruments, uses less amount of various reagents, and has low analysis cost and low price; it can be completed quickly by ordinary analysts in ordinary laboratories, and the operation is simple and convenient The chromogenic system is stable within 240min, the reproducibility is good, the standard curve is linear, and the accuracy of the analysis results is high; the present invention has the advantages of simplicity, accuracy, trace, rapidity and low price, and can be directly and indirectly used for iodine analysis , has a wide range of applications.

Description of drawings

Figure 1 is a schematic diagram of the unstable absorbance of the bromine water oxidation iodide ion and starch color development system.

Figure 2 is a schematic diagram of the longer time for the co-construction system of color development and H ₂ O ₂ to reach equilibrium when the reaction between Fe ³⁺ and I ^- is not sufficient at 20°C.

Figure 3 is a schematic diagram of the stability at room temperature of the equilibrium system jointly established by Fe ³⁺ and H ₂ O ₂ and iodine-starch.

Fig. 4 is a schematic diagram of excessive H ₂ O ₂ destroying Fe ³⁺ and H ₂ O ₂ and iodine-starch to build an equilibrium system.

Fig. 5 is a schematic diagram of the absorption curve of the equilibrium system jointly established by Fe ³⁺ and H ₂ O ₂ and iodine-starch.

Detailed ways

The following embodiment describes the spectrophotometric quantitative analysis method for the equilibrium system jointly established by Fe ³⁺ and H ₂ O ₂ and iodine starch established by the present invention. The important chemical reaction formula of the color development system of the present invention is as follows:

response inhibition The reaction decomposes, reaching inhibition reaction disproportionation, maintenance The co-construction equilibrium system is stable for a long time, and a small amount of H ₂ O ₂ can methylate a small amount of I ₂ -starch inclusion complex, which increases the hydrophilicity of the I ₂ -starch inclusion complex, and does not precipitate for a long time, which is better Satisfy the absorbance photometry conditions; use H ₂ O ₂ excess reaction according to The I ₂ -starch clathrate disintegrates, and the equilibrium system jointly established by Fe ³⁺ and H ₂ O ₂ and iodine starch collapses immediately (Figure 4); a small amount of H ⁺ also maintains the stability of the system, and the system requires weak acid conditions. The stable mechanism of the color development system of the present invention for a period of time is very complicated. Because there are Fe ²⁺ and H ₂ O ₂ in the system, under acidic conditions, the free radical reaction is as follows:

Fe ²⁺ + H ₂ O ₂ =Fe ³⁺ +OH ^- + OH

Fe ³⁺ + H ₂ O ₂ =Fe ²⁺ +HO ₂ +H ⁺

The oxidation ability of these free radicals is very strong, and it can non-selectively oxidize organic and inorganic substances, which can also explain the reason why the color system collapses immediately when the amount of H ₂ O ₂ in the system is slightly larger. The above is only a general reasoning, the color system of the present invention can be stable for a period of time, and its mechanism needs to be further explored.

Embodiment 1: Fe ³⁺ and H ₂ O ₂ and iodine-starch co-build the stability test of the equilibrium system and the establishment of the standard curve, its operation steps:

(1) Pipette 0.00, 0.20, 0.40, 0.60, 0.80, 1.00 mL I ^- standard solution (2.50 mmol/mL) into 25 mL volumetric flasks (number 0~5), add 1.5 mol/L FeCl ₃ solution 50 μL, shake well, let stand for 30 min, add 3.00 mL of 1% starch solution, shake well, add distilled water near the neck of the volumetric flask, shake well, add 1.5% H ₂ O ₂ 50 μL, shake well, distilled water to volume, shake well ;

(2) At room temperature, use No. 0 bottle as a blank, use No. 4 bottle as a wavelength scan at 400-700nm, draw an absorption curve (Figure 5), and obtain a maximum absorption wavelength of 600nm;

(3) At room temperature, with 600nm as the measurement wavelength, measure the absorbance of bottles No. 1 to No. 5; measure once every 30 minutes, draw a standard curve with the iodide ion concentration as the abscissa, and the absorbance of the solution as the ordinate to obtain the corresponding linearity The regression equation and correlation coefficient, measurement data and calculation results are shown in Table 1; the results show that the equilibrium system jointly established by Fe ³⁺ and H ₂ O ₂ and iodine-starch is stable within 240 minutes (Figure 3), and the linear coefficient of the standard curve is r ﹥ 0.999, The linear range is 0.0200-0.1000mmol/L, following Lambert-Beer's law, the color system is measured several times within 120min, the RSD is between 0.6-1.5%, the method has good reproducibility and high accuracy;

Table 1 (measured at 18°C)

(4) Put part of the color development system of bottle No. 5 into an ice-water bath to cool down and then heat up naturally (room temperature 20°C). The results of absorbance changes during the measurement process are shown in Table 2. ), the results of absorbance changes during the measurement process are shown in Table 3, the results show that temperature changes have a significant impact on absorbance; when the temperature fluctuates in the range of ±0.1 °C, the absorbance has no obvious change; will absorb the heat released by the instrument, therefore, rapid determination can overcome this unfavorable factor during measurement

Table 2

table 3

Embodiment 2: use Fe ³⁺ and H ₂ O ₂ and iodine-starch co-build equilibrium system spectrophotometry to measure the iodine content of a certain commercially available potassium iodide, its operating steps:

(1) Accurately weigh 0.1g (accurate to 0.0001g) of commercially available KI (Chengdu Kelong Reagent Factory), dissolve with appropriate amount of distilled water, transfer to a 100mL volumetric flask to constant volume, pipette 5.00mL of the solution into a 50mL volumetric flask, Constant volume with distilled water to obtain the test solution;

(2) Pipette 0.00, 0.20, 0.40, 0.60, 0.80, 1.00 mL of I ^- standard solution (2.50 mmol/mL) into 25 mL volumetric flasks (number 0~5), respectively pipette 0.60 mL of test solution into Into a 25mL volumetric flask ( n = 6); respectively add 50 μL of 1.5mol/L FeCl ₃ solution, shake well, let stand for 30 min to fully react, add 3.00 mL of 1% starch solution, shake well, add distilled water near the neck of the volumetric flask, shake Mix well, add 1.5% H ₂ O ₂ 50 μL, shake well, distilled water to volume, shake well;

(3) At room temperature, with 600nm as the measurement wavelength and No. 0 bottle as the reference solution, measure the absorbance of the standard solution and the test solution; take the iodide ion concentration as the abscissa, and the absorbance of the solution as the ordinate to establish a standard curve to obtain Linear regression equation A = 9.445 c - 0.101 , r = 0.999, linear range 0.020 ~ 0.10m mol/mL, the absorbance of the test solution is substituted into the linear regression equation for calculation and related measurement conversion, the iodine content of commercially available potassium iodide is 76.4% ( n =6), RSD <1%.

Claims (3)

1. an iodine-starch color development system spectrophotometric quantitative analysis method, its color development system is by FeCl ₃solution, KI solution, starch solution and H ₂o ₂solution composition, containing following step:

(1) 2.50 m mol/mL I of certain volume are pipetted ^-standard solution, in 25mL volumetric flask, pipettes I ^-the volume range of standard solution is at 0.20 ~ 1.00mL;

(2) in volumetric flask, 1.5mol/L FeCl is added ₃solution 50 μ L, shakes up, places 30 min;

(3) in volumetric flask, add 1% starch solution 3.00 mL, shake up;

(4) in volumetric flask, at once add distilled water to close to neck, shake up;

(5) 1.5% H is at once added ₂o ₂50 μ L, distilled water constant volume, shakes up;

(6) FeCl is used ₃make blank, make absorption curve at 400 ~ 700nm; Determine to measure wavelength;

(7) according to the method for (1) ~ (5), I is used ^-standard solution prepares I ^-concentration, in a series of color development system having concentration gradient of 0.020 ~ 0.100 m mol/L, measures absorbance with measurement wavelength a, use I ^-concentration cwith athe corresponding relation of value, sets up calibration curve;

(8) by the method for above-mentioned (1) ~ (5), certain the unknown is measured containing I ^-the color development system of solution a, calculate I in tested systems according to calibration curve ^-concentration.

2., based on the method for claim 1, it is characterized in that I in tested systems ^-concentration be 0.020 ~ 0.100 m mol/L.

3. based on the method for claim 1 or 2, it is characterized in that at room temperature, when temperature fluctuates within the scope of ± 0.1 DEG C, color development system can be stablized in 240min, and absorbance is stablized, calibration curve related coefficient r﹥ 0.999.