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Total organic carbon (TOC) is an important indicator for measuring the total amount of organic matter in water. It reflects the water quality by measuring the carbon content in water. Currently, the methods for measuring total organic carbon in water mainly include low-temperature oxidation method, combustion method and conductivity method. Combustion oxidation-non-dispersive infrared absorption method (NDIR) is a commonly used TOC determination technology. It can completely oxidize the organic carbon in water and obtain accurate results through precise infrared absorption detection. This method not only has a short measurement time but also has high accuracy. Therefore, it is widely used in water treatment and water quality monitoring in many countries around the world.

This article will share how to use the WTOC200 Total Organic Carbon Analyzer to determine the total organic carbon content of water samples with different salinity. By using the subtraction method, six water samples were accurately tested to ensure the accuracy and reliability of the results.

Experimental part

Instrument and reagents

Instrument: WTOC2000 total organic carbon analyzer

Reagents:

Potassium hydrogen phthalate (reference reagent)

Sodium carbonate (reference reagent)

Sodium bicarbonate (superior purity)

Phosphoric acid (superior purity)

Distilled water without carbon dioxide

Experimental method

Preparation of standard solution [p(organic carbon, C) = 1000 mg/L]

Weigh 2.1254g potassium hydrogen phthalate, 4.4085g anhydrous sodium carbonate and 3.5000g sodium bicarbonate, place in a beaker, add pure water to dissolve, transfer to a 1000 mL volumetric flask, dilute to the scale with pure water and mix well to obtain a standard solution with a TOC value of 1000 mg/L.

Phosphoric acid solution (volume fraction 10%)

Measure 10 mL of high-grade pure H₃PO₄ and dilute to 100 mL with ultrapure water (valid for one month).

Experimental conditions

Experimental methods and experimental data

Drawing of standard curve

Based on the standard solution, a series of mixed standard solutions of different concentrations were prepared, with total carbon concentrations of 0.0, 4.0, 10.0, 20.0, and 40.0 mg/L, and inorganic carbon concentrations of 0.0, 2.0, 5.0, 10.0, and 20.0 mg/L. The same volume of solutions of different concentrations were used for injection to draw the standard curve. With the mass of carbon as the horizontal coordinate and the integrated area signal as the vertical coordinate, the following curve equation was obtained by quadratic fitting:

TC curve equation: Y = -5024648.3X² + 324998.2X + 1.4, correlation coefficient R = 1.0000

TIC curve equation: Y = -5995021.6X² + 334757.0X + 6.4, correlation coefficient R = 0.9998

Quality Control

A mixed standard solution with a total carbon concentration of 20.0 mg/L and an inorganic carbon concentration of 10.0 mg/L was prepared to reversely verify the standard curve. By calculating the reverse test results, the relative standard deviation (RSD) of TOC was 0.88% and the indication error was 3.00%, both of which met the standard requirements.

Sample pretreatment

According to the salt content information of the samples, the salt content of "Sample 1", "Sample 2", "Sample 3" and "Sample 4" is about 15%, and the salt content of "Sample 5" and "Sample 6" is about 26%. In order to ensure the accuracy of the test, reduce the interference of high-salt samples on the test results, and reduce the loss of instrument consumables, ultrapure water is used to dilute "Sample 1" to "Sample 4" 20 times before injection testing, and "Sample 5" and "Sample 6" are diluted 50 times before testing.

Sample test results

By analyzing the test results of six diluted samples, it was found that the inorganic carbon content in all samples was low and the proportion in the total carbon was also small. Based on this feature, the use of subtraction method for testing can effectively ensure the accuracy of the data and is suitable for the analysis of such samples.

The sample test results are shown below:

Note: The test results in the above table are all converted test data.

Analysis and Summary

In this experiment, the samples with high salt content were appropriately diluted to reduce the salt concentration before testing. The experimental results show that the WTOC2000 total organic carbon analyzer can effectively test water samples with high salt content, and the test results are good. The repeatability and indication error of the method meet the requirements of relevant standards, proving that the instrument is suitable for the analysis of high-salinity samples.