Method experiment and error analysis of TOC in water by TOC folding meter

author: Otis

2023-01-05

Method experiment and error analysis of TOC in water by TOC folding meter

Abstract: TOC, namely total organic carbon, is a comprehensive index of the total amount of organic matter in water body by the content of carbon. Because the determination of TOC uses combustion method or wet oxidation method, which can oxidize all organic matter, it can directly represent the total amount of organic matter more than BOD 5 or COD, so it is often used to evaluate the pollution degree of organic matter in water.

1. Preface
TOC is the total organic carbon, which is a comprehensive index of the total amount of organic matter in the water body represented by the content of carbon. Because the determination of TOC uses combustion method or wet oxidation method, which can oxidize all organic matter, it can directly represent the total amount of organic matter more than BOD 5 or COD, so it is often used to evaluate the pollution degree of organic matter in water.
The instrument used herein is TOC-3000 Total Organic Carbon Analyzer of Optosky, determined by wet oxidation-nondispersed infrared absorption. Using sodium persulfate as an oxidant to oxidize all kinds of organic matter in the monitored water in the solution, so that the organic matter is completely oxidized, and then the total amount of carbon dioxide is measured by infrared absorption, so as to indirectly determine the content of total organic matter in the water.

2. Experiments
2.1 Instruments and reagents
TOC-3000 Total Organic Carbon Analyzer sodium persulfate (100-200 g / L), phosphoric acid (5%), potassium hydrogen phthalate (KHP) stock (1000 ppmC), sodium carbonate stock (1000 ppmC). Reagent water: containing TOC less than 200 ppbC distilled or pized water.
2.2 Analysis method
A continuous set of numbering can be set up for the sample in advance. According to the compiled procedures, the instrument can repeatedly test the blanks, samples and standards. The analysis order of blank, sample and standard can be set, and the sample analysis and standard curve verification can be conducted automatically according to the compiled program. The reaction time and detection time of TIC, TOC and TC can be extended for different samples.

3. Results and discussion
3.1 Analysis results
(1) Detection limit: According to the calculation of 10 batches of blank test, the test of this method is obtained
The exit limit was 0.10 mg/L.(Subject to 5ml conventional injection tube)
(2) Precision: measurement 2.0 mg/L standard 10 times, calculated RSD of 0.032. The stability of the instrument is good.

(3)Spiking recovery: 8.2 mg/L sample in water sample. The spike recovery rate of water samples is 91.2% -106.8%. Ten batches of spiked water samples were determined in parallel, and the accuracy control map is drawn as follows:

3 .2 Error analysis
(1) The wet oxidation-non-dispersed infrared absorption method compared with the combustion method, the wet oxidation method does not interfere with the detection of soluble substances in water, while the latter is greatly disturbed by the coexisting particles in the water, which requires pretreatment. And the range of interference ions is very wide, all ions with high content in natural water bodies, such as sulfate, chloride ion, nitrate, phosphate, sulfur ion, etc. (take the test results of the Yellow River source water on April 2,2003, sulfate 68 mg/L; chloride ion 40 mg/L; nitrate (in N) 1.45 mg/L; total phosphorus (by P) <0.02 mg/l) and the more system error. Second wet oxidation method can determine TOC, TIC (total inorganic carbon), TC (total carbon, the sum of TOC and TIC), does not interfere with each other, unlike the TIC before removing solution, but this process will cause the loss of volatile organic compounds in water and measurement error, so the determination result is not blow the organic carbon (NPOC), rather than TOC, resulting in negative error.

(2) When the tested water samples are collected in natural water bodies, because the water samples have bacterial activity, the phosphoric acid pH value of 2 should be added, and the refrigerator should be stored at 4℃. However, if there are some or some certain kinds of acid-resistant bacteria in the water body, the acidification can not well inhibit the bacterial activity, resulting in the continuous reproduction and respiration of the bacteria, resulting in a large negative error of this water sample preservation method. In view of this, the water samples should be tested as soon as possible. If not timely detection, phosphoric acid sufficient to inhibit bacterial activity should be added and cryopreserved and tested as soon as possible.

(3) Method interference
Carbon is found in nature, so reagents, water, and glass devices cannot completely exclude carbon. Method interference (positive deviation) can be caused by gas, dilute water, reagents, glass devices, or sample processing hardware (e. g. homogenizer). These materials must be demonstrated by analytical reagent blanks and can minimize interference using high purity or purified reagents and gas.

(4) Non-CO2 gas interference: the infrared detector is sensitive to CO2 and can completely eliminate the interference of other gases.

(5) Interference of the sampling process: to obtain the most accurate analysis results. The sampling vessel must contain no machine carbon. Plastic containers will permeate the organic carbon into human water samples. Especially when they are new, or analyze low concentration samples (less than 200 ppbC). All new bottles are be placed with clean water for a few days before use. Or boil it in water for a few hours. Before the first use of the refractory glass bottle, it is best to wash and dry at 400℃. The TIC and POC of the samples will be affected by the atmosphere, and the TOC below 50 ppbC will also be affected. Therefore, so in this case, the sampling bottle should be kept as closed as possible, and the automatic injection bottle can be equipped with puncture diaphragm.

(6) The desiccant in the drying pipe must be replaced in time. Otherwise, the blank will drift, produce large measurement error. The flow rate must be re-measured after each change of desiccant. The filter layer on both sides of the drying pipe must be replaced regularly, otherwise the gas path will be blocked, which will seriously affect the detection.

(7) The cleaning water should use the deionized water prepared on the same day, otherwise the blank is increased and unstable.

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