Supplementary Information S1 (pdf 140K)

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1

Supplementary Information

2

S1. Development and validation of the method detecting pK

a

3

Selection of the detection wavelength

4

Solutions (5×10

-5

mol/L) of

p

-nitrophenol were prepared in 0.01 mol/L NaOH,

5 0.01 mol/L HCl and 0.01 mol/L NaHCO

3

, and a 0.1-mol/L imperialine-ethanol

6 solution was also prepared. The resulting samples were scanned using an

7 ultraviolet-visible spectrophotometer (Cary 100 Bio UV-VIS, Varian Medical Systems

8 Inc., USA) to select the optimal wavelength for analyte absorbance detection.

9 The UV-VIS spectra of

p

-nitrophenol and imperialine were merged, as shown in

10 Figure S1

, illustrating that at a detection wavelength (λ) of 401 nm, the

p

-nitrophenol

11 absorptive difference between the ionized and non-ionized forms was the most

12 significant, while imperialine did not exhibit any absorbance.

13

14

Figure S1.

The UV-VIS spectrum of o-nitrophenol and imperialine. (A) UV-VIS spectrum of

15 o-nitrophenol in different media: 1. 0.01 mol/L NaOH; 2. 0.01 mol/L NaHCO

3

; 3. 0.01 mol/L HCl.

16

(B) UV-VIS spectrum of imperialine.

17

18

Measurement of the p-nitrophenol pK a

19

Solutions (5×10

-5

mol/L) of

p

-nitrophenol were prepared in solutions of 0.01

20 mol/L NaOH, 0.01 mol/L HCl and PBS (with pH values of 7.0, 7.4, 7.8 and 8.5).

21 Their absorbance values were then obtained using a UV-VIS spectrophotometer at the

22 selected wavelength; meanwhile, the pH of the PBS solution was precisely

23 determined. The p

K a

of

p

-nitrophenol was calculated using Eq. (S1) ,

24

(S1)

25 where

A

is the absorbance of the solution containing a certain total concentration of

26 the acid-base mixture;

A

1

is the absorbance of the acid form (100 % non-ionized type);

27

A

2

is the absorbance of the base form (100 % ionized type) at the same concentration;

28 and

γ

is the activity coefficient.

29 The

γ

was calculated according to the Debye-Huckel Formula, Eq. ( S2 ),

30 (S2)

31 where

I

represents the ionic strength of the solution, and

Z

represents the charge

32 number of the ion.

33

I

was calculated using Eq. (S3) ,

34

(S3)

35 where

m

B

is the molality of each ion in the buffer solution, and

Z

B

is the charge

36 number of that ion.

37 The absorbance (

A

) of

p

-nitrophenol at different pH values is shown in Table S1 ,

38 as measured by the spectrophotometric indicator method with a detection wavelength

39 of 401 nm. The calculation was carried out using Eq. (S1) ; the p

K a

of the chosen

40 indicator,

p

-nitrophenol, was calculated as 7.16, which corresponds well with the

41 reported value of 7.17

[S1] . Therefore, our proposed method was accurate and

42 reliable.

43

Table S1.

The pK a

of

p

-nitrophenol as determined by the spectrophotometric indicator method a

pH A lg γ pK

a

T

Int

7.00

7.39

7.80

8.50

0.5550 ± 0.0001

0.7262 ± 0.0001

0.8339 ± 0.0001

0.8987 ± 0.0002

-0.3354

-0.3361

-0.3367

-0.3371

44

45

All results are the mean of measurements from three replicates. a

A

1

= 0.0282

A

2

= 0.9170

λ

= 401 nm

7.17

7.16

7.15

7.16

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