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2025
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Determination of trace chloride ion content by fully automated potentiometric titration
1. Introduction
Trace chloride ions have always been a mandatory test item for many companies' products. Based on recent sample handling experience, the required trace chloride ion content is mostly between 1 ppm and 50 ppm. This paper uses sodium chloride aqueous solution as the sample to eliminate interference from other components in the sample on the electrode signal, and to determine its detection limit and quantitation limit. Experimental results show that the detection limit of the Shanghai Jiahang JH-T6 fully automatic potentiometric titrator is 2 ppm, and the quantitation limit is 5 ppm.
2. Instruments and Equipment
2.1 Instruments
JH-T6 fully automatic potentiometric titrator, 10m burette, Hamilton silver composite electrode,
2.2 Reagents
Silver nitrate solution (0.05 mol/L), sodium chloride solution (0.01, 0.0005, and 0.00005 mol/L)
3. Experimental Methods
3.1 Parameter Settings
3.2 Experimental Procedure
(1) Prepare three sodium chloride solutions (0.01, 0.0005, and 0.00005 mol/L), and then dilute them to 17, 10, 5, and 2 ppm solutions respectively.
(2) Titrate the corresponding concentrations of sodium chloride solutions with silver nitrate solution. If the electrode response signal is weak, try adding nitric acid or anhydrous ethanol.
4 Results and Discussion
4.1 Results
(1) Accurately pipette 5 mL of sodium chloride solution (0.01 mol/L), add 95 mL of water, and prepare a solution containing approximately 17.73 ppm chloride ions. Titrate with silver nitrate solution (0.0104 mol/L) to the endpoint. The results are shown in Table 1.
Table 1
Note: *The unit of content should be mg/kg. 95mL of water weighs 94.7g, the default is 95g. 5mL of sodium chloride solution is assumed to be 5g, the same applies below.
From the results of the three repeated experiments in Table 1, it can be seen that JH-T6 has good parallelism when detecting 17.7ppm chloride ions, so subsequent experiments need to further reduce the chloride ion concentration.
(2) Accurately pipette 2mL of sodium chloride solution (0.01mol/L), add 68mL of water, and prepare a solution containing approximately 10.13ppm chloride ions. Titrate with silver nitrate solution (0.0104mol/L) to the endpoint. The results are shown in Table 2.
Table 2
As shown in Table 2, the JH-T6 showed good parallelism in detecting 10.1 ppm chloride ions, so subsequent experiments need to further reduce the chloride ion concentration.
(3) Accurately pipette 1 mL of sodium chloride solution (0.01 mol/L), add 69 mL of water, and prepare a solution containing approximately 5.06 ppm chloride ions. Titrate with silver nitrate solution (0.0104 mol/L) to the endpoint. The results are shown in Table 3.
Table 3
As shown in Table 3, the JH-T6 detector exhibits good parallelism when detecting 5.0 ppm chloride ions. Therefore, subsequent experiments require further reduction of the chloride ion concentration.
(4) Accurately pipette 0.5 mL of sodium chloride solution (0.01 mol/L), add 80 mL of water, and prepare a solution containing approximately 2.2 ppm chloride ions. Titrate with silver nitrate solution (0.0104 mol/L) to the endpoint. The results are shown in Table 4.
Table 4
As shown in Table 4, the results of the two repeated experiments indicate that the JH-T6 detector deviated from the theoretical value when detecting chloride ions at approximately 2.2 ppm, and the peak shape of the real-time spectrum was not obvious near the endpoint. This may be due to the weak electrical signal caused by the low chloride ion concentration.
(7) Accurately pipette 0.5 mL of sodium chloride solution (0.01 mol/L), add 80 mL of water, and then add 10 mL of nitric acid solution* (1 + 10) to prepare a solution containing approximately 1.96 ppm chloride ions. Titrate with silver nitrate solution (0.0104 mol/L) to the endpoint. The results are shown in Table 5.
Table 5
Note: *Nitric acid solution provides an acidic environment.
From the results of three repeated experiments in Table 5, it can be seen that the measured results after adding nitric acid deviated from the theoretical values, and the peak shape was not obvious. Furthermore, if an autosampler is used, the endpoint can be determined prematurely.
(8) Accurately pipette 0.5 mL of sodium chloride solution (0.01 mol/L), add 60 mL of water, 10 mL of nitric acid solution* (1 + 10), and 20 mL of anhydrous ethanol to prepare a solution containing approximately 2.05 ppm chloride ions. Titrate with silver nitrate solution (0.0104 mol/L) to the endpoint. The results are shown in Table 6.
Table 6
Note: *Some literature states that nitric acid solution provides an acidic environment, and anhydrous ethanol can enhance the electrical signal. The density of anhydrous ethanol is 0.8 g/mL.
From the results of the two repeated experiments in Table 6, it can be seen that the measured results after adding nitric acid and anhydrous ethanol also deviated from the theoretical values, and the peak shape was not obvious. This indicates that nitric acid and anhydrous ethanol cannot enhance the electrical signal in the determination of trace chloride ions.
(9) Accurately pipette 5.0 mL of sodium chloride solution (0.0005 mol/L), add 45 mL of water, and prepare a solution containing approximately 1.77 ppm chloride ions. Titrate with silver nitrate solution (0.00104 mol/L) to the endpoint. The results are shown in Table 7.
Table 7
This experiment used lower concentrations of sodium chloride and silver nitrate solutions to explore the peak shape changes when the theoretical titration volume was greater than 1 mL (approximately 2.5 mL). After two repeated experiments, a white turbid precipitate was found in the beaker, but the instrument showed no obvious peak shape, making it impossible to determine the result.
(10) Accurately pipette 0.5 mL of sodium chloride solution (0.01 mol/L), add 80 mL of water, and prepare a solution containing approximately 2.2 ppm chloride ions. Titrate with silver nitrate solution (0.00104 mol/L) to the endpoint. The results are shown in Table 8.
Table 8
This experiment used a lower concentration of silver nitrate solution. After three repeated experiments, a white turbid precipitate was found in the beaker, but no obvious peak shape was observed at the theoretical titration volume of 5 mL, making it impossible for the instrument to determine the result.
(11) Accurately pipette 0.5 mL of sodium chloride solution (0.01 mol/L), add 60 mL of water, 5 mL of concentrated nitric acid, and 20 mL of acetone* to prepare a solution containing approximately 2.2 ppm chloride ions. Titrate to the endpoint with silver nitrate solution (0.0104 and 0.00104 mol/L). The results are shown in Table 9.
Table 9
Note: *Some literature states that nitric acid provides an acidic environment, and acetone can enhance the electrical signal.
In both experiments, a white suspended precipitate was observed in the beaker, but no peak shape was observed at the theoretical values of 0.5 mL and 5 mL, proving that the addition of nitric acid and acetone did not enhance the electrical signal. Combining the results in Tables 7, 8, and 9, it can be concluded that the T960 potentiometric titrator with a silver composite electrode cannot directly detect chloride ions at approximately 2 ppm. It also indicates that the instrument has a poor response to silver nitrate solutions with a concentration of approximately 0.0010 mol/L. It is recommended that this concentration of silver nitrate solution not be used in future chloride ion experiments.
(12) Accurately pipette 1.0 mL of sodium chloride solution (0.01 mol/L), add 69 mL of water (containing approximately 5.06 ppm chloride ions), and then accurately pipette 100 mL of sodium chloride solution (0.00005 mol/L), containing approximately 1.77 ppm chloride ions. Combine both solutions in a beaker, calculating a total chloride ion concentration of approximately 3.1 ppm. Titrate with silver nitrate solution (0.0104 mol/L) to the endpoint; theoretically, 1.5 mL should be consumed. Use this method to test whether the instrument can measure 2 ppm chloride ions. The results are shown in Table 10.
Table 10
Two repeated experiments showed that the instrument could directly detect chloride ions at a concentration of approximately 3 ppm, therefore the quantitation limit of the instrument with the silver composite electrode was determined to be approximately 3 ppm. Furthermore, by using a 5 ppm sodium chloride solution as an internal standard and subtracting the corresponding volume from Table 3, a rough estimate of 1.7 ppm could be obtained. Therefore, the instrument's detection limit was estimated to be approximately 2 ppm.
Calculation Formula
Where:
X -- chloride ion content, ppm;
V -- volume of silver nitrate solution consumed during chloride titration, mL;
m -- total mass of all solutions in the titration vessel, mL;
c -- concentration of silver nitrate solution, mol/L;
35.45 -- molar mass of chloride ions, g/mol.
4.2 Spectrum
（1） 17.7ppm
（ 2 ）10ppm
（ 3 ）5pp
（ 5 ）3ppm
4.3 Conclusion
This paper uses a single sodium chloride aqueous solution to explore the quantitation limit and detection limit of the instrument for chloride ions. Experimental results and spectra show that the quantitation limit is approximately 3 ppm and the detection limit is approximately 2 ppm. The results show good parallelism and the instrument can be used with an autosampler for large-scale detection. This paper only used one electrode in the experiment; therefore, other electrodes may be added in subsequent experiments for the determination of trace chloride ions.
Precautions
If the dropper and electrode are too close, the solution potential in the sample cup will be unstable, affecting the titration results. The stirring speed can be adjusted by observing the vortex generated when stirring the water sample.
Keyword:
automated potentiometric titration
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