分光光度计法测定甲基红离解常数 实验数据处理

Part 1：Measure the maximum absorption

Table 1: Absorption of solution A Wavelength/nm absorption 420 0.075 440 0.172 460 0.41 480 0.762 500 1.14 510 1.286 520 1.353 530 1.332 0 1.263 560 0.905 580 0.295 600 0.046

1.41.21.0 absorptionabsorption0.80.60.40.20.04004204404604805005200560580600620wavelength (nm) Graph 1: Absorption of solution A

Conclusion：from the Graph 1，we can get the wavelength for the maximum absorption of solution A is 520 nm.

Table 2: Absorption of solution B Wavelength/nm absorption 410 0.52 420 0.3 430 0.552 440 0.552 450 0. 460 0.498 480 0.3 500 0.178 520 0.074 0 0.037

0.6 absorption0.50.4absorption0.30.20.10.04004204404604805005200560wavelength (nm)Graph 2: Absorption of solution B

Conclusion：from the Graph 2，we can get the wavelength for the maximum absorption of solution A is 430-440 nm.

Part 2：Test the Beer’s law

Table 3: Absorption of HMR in different concentrations No.(concentration) Absorption (520nm ) Absorption (430nm ) 0#-c(A) 1.353 0.147 1#-0.75c(A) 1.0 0.106 2#-0.5c(A) 0.68 0.0 3#-0.2c(A) 0.373 0.055 Note：because it is always acidic through the process, we can ignore the absorotion of MR.

Absorption (520nm ) Linear Fit of Absorption (520nm )1.4-1.2Absorption (520nm )1.00.8EquationWeighty = a + b*xNo Weightin0.003250.9912ValueAbsorption (5InterceptAbsorption (5Slope0.10121.2510Standard Erro0.046250.067960.6Residual Sum of SquaresAdj. R-Squar0.40.20.20.40.60.81.0concentration (c（A）)

Graph 3：Absorption of HMR in different concentrations（520nm )

0.16 Absorption（430nm） Linear Fit of Absorption（430nm）0.14Absorption（430nm）0.120.10Equationy = a + b*x0.08No WeightinWeightResidual 1.66667E-7Sum of Squares0.9999Adj. R-SquarValueStandard Err8.97527E-40.00115Absorption（InterceptAbsorption（Slope-0.01880.1660.060.50.60.70.80.91.0concentration (c（A）)

(the forth data is ignored due to its large derivation)

Graph 4：Absorption of HMR in different concentrations（430nm )

Conclusion：(C0=1.851*10-5) from the Graph 3，the slope is 1.2510, so §HNM520=1.2510/(C0=1.851*10-5)=6.79*104，from the Graph 4，the slope is 0.166, so §HNM430=0.166/(C0=1.851*10-5)=8.97*103 . Table 4: Absorption of MR- of different concentrations No.(concentration) Absorption (520nm ) Absorption (430nm ) 0#-c(B) 0.074 0.552 1#-0.75c(B) 0.0455 0.288 2#-0.5c(B) 0.0479 0.134 3#-0.2c(B) 0.0322 0.0 Note：because it is alkaline through the process, we can ignore the absorotion of HMR.

Absorption（520nm） Linear Fit of Absorption（520nm）0.080.07Absorption（520nm）0.060.05EquationWeighty = a + b*xNo Weightin0.04Residual Sum 4.08163E-10of Squares1Adj. R-SquareValueStandard Err2.31792E-53.5348E-5Absorption（5InterceptAbsorption（5Slope0.02170.05220.030.20.40.60.81.0concentration（c（A））

(the second data is ignored due to its large derivation)

Graph 5：Absorption of MR- in different concentrations（520nm )

Absorption（430nm） Linear Fit of Absorption（430nm）0.60.5Absorption（430nm）0.4Equationy = a + b*x0.30.2No WeightinWeightResidual 0.00202Sum of Squares0.987Adj. R-SquarValueAbsorption（InterceptAbsorption（Slope-0.30230.836Standard Err0.098730.127020.10.50.60.70.80.91.0concentration（c（A））

(the forth data is ignored due to its large derivation)

Graph 6：Absorption of MR- in different concentrations（430nm )

Conclusion：(C0=1.851*10-5) from the Graph 5，the slope is 0.052, so §NM-1520=0.052/(C0=1.851*10-5)=2.81*103，from the Graph 6，the slope is 0.836, so §NM-1430=0.836/ (C0=1.851*10-5)=4.52*104. Part 3：the ratio VS the pH：to get the k

Table 5: Absorption for the ratio VS the pH No.（pH is Absorption Absorption ratio(MR-/HMR) lg(MR-/HMR) different） (520nm ) (430nm ) 7# 0.024 0.056 4.099887863 0.612771978 8# 0.041 0.068 2.7778106 0.443716312 9# 0.055 0.05 1.447452569 0.160604341 10# 0.08 0.047 0.916005775 -0.038101788

Note：the ratio(MR-/HMR) is calculated from

pH 4.68 4.36 3.97 3.69 pK 4.067228 3.9162837 3.8093957 3.7281018

pK= pH - lg(MR-/HMR)

Conclusion：so the average number of pK is 3.8803 then we get the k=1.317*10-4

VI、Question and Discussion

 Q1：Would the measurement results be influenced by the temperature？how

to avoid or minimize the influences？ Answer：Yes, for the reason that the k value of the methyl red will be changed with the temperature, which will also result in the change of the ratio of (MR-/HMR) and the measurement of the absorption. So we should measure all the data at the same temperature to minimize the influences.

 Q2：Can we still use the similar method to measure the equilibrium

coefficient if the sample solution is colorless ？ Answer：Yes, this method is not limited in the visible light area, it can also be

used in the area of UV and IR areas for the reason that it is determined by the inter structure and interaction of substances. It also presents absorption in the area of UV and IR.

 Q3：Would it affect the measurement results if the solution volumes are not

accurately pipetted when you are preparing solutions 7#-10#？

Answer : There is no influences on the final results. According to the

equation pK= pH - lg(MR-/HMR), the value MR-/HMR corresponds to a Ph value, so the pK won’t be changed if the solution volumes are not accurately pipetted.

 D1: According to literature, the value pk of methyl red at the room

temperature is about 5.05, however, we get 3.8803. Here are some reasons which may contribute to the deviation：

 The temperature，which has been talked about in Q1.

 The preparation of solution in A and B may also result the error, we

should let one student to do the all pipettation and each time we should wish the pipette and any glass container with the solution.

 The errors comes from equipment, eg. We cannot get the total

monochromatic light.

 Actually, the Beer’s law just applies to the dilute solutions, it may also

cause some errors.

 We ignore the volume change when we prepare solutions of different

concentrations. It will also cause some error because the volume cannot be just added without the consideration of volume change.

 The fresh prepared solution should be placed for a period to let the

solution turn uniform.

 We also ignore the ignore the absorotion of HMR and MR-1 in the acidic

and alkaline solution respectively.

 D2: We use 520nm and 430nm to test the Beer’s law for the reason that the

maximum absorption led to high sensitivity. Actually, the maximum absorption of solution B is about 435nm in our experiment.