Uncertainty of determination of water content in w

Evaluation of uncertainty in the determination of water content in water-based coatings by gas chromatography

evaluation of uncertainty in the determination of water content in water-based coatings by gas chromatography

January 1, 2019

0 Introduction

with the promulgation and implementation of national energy conservation and emission reduction policies, water-based coatings have developed rapidly in recent years and have been widely used in the fields of architecture, interior decoration and other fields. Since 2001, China has issued more and more environmental protection standards for coatings [], strictly limiting the content of volatile organic compounds (VOC), triphenyl, glycol ether esters and other toxic and harmful substances. The accuracy and reliability of water content determination in waterborne coatings have a great impact on VOC content [3] Chinese coatings. Test environment and measuring party in February this year, the company's controlling shareholder group signed a letter of intent with Dongfang Hengtai and its relevant shareholders. Five sources of uncertainty, including the law, testers, testing standards, and testing instruments, will affect the measurement results, so it is necessary to evaluate the uncertainty of water content measurement. At present, the determination methods of water content mainly include gas chromatography, Karl Fischer (KF) method [4] and pneumatic method [5]. According to GB 18582-2008 Appendix B method [6], this paper systematically studies the various factors that affect the test results by using gas chromatography, and describes in detail the evaluation method of the uncertainty of the determination of water content in water-based coatings, so as to provide a reference for evaluating the impact of various uncertain sources on water content, so as to better evaluate the reliability of the determination data

1 determination principle and method

according to the method in Appendix B of GB 18582-2008 "limits of harmful substances in interior wall coatings of interior decoration materials", weigh 0.2 g of distilled water (M standard) and 0.2 g of isopropanol (M standard), accurate to 0.1 mg, put it into a glass bottle with a stopper, add 2 ml of dimethylformamide, seal it, shake it well, and use it for standby. Determine the sample under the same chromatographic conditions as the determination of the relative correction factor, record the chromatogram and chromatographic data of each component on the chromatographic column, and calculate the water content with the internal standard method

2 establishment of mathematical model

where:

r1 - the average value of the peak area ratio of water and internal standard in the standard solution

r2 - the average value of the peak area ratio of water and internal standard in the sample solution

m standard - mass of standard sample, G

m sample - mass of sample, G

p - mass fraction of water in the standard sample,%

a standard -- peak area of water in standard sample solution

a sample - peak area of water in sample solution

a internal standard - peak area of internal standard in standard sample solution

a - peak area of internal standard in sample solution

m in the sample - the mass of the internal standard in the sample solution

m internal standard - mass of internal standard in standard sample solution

3 determination and quantification of uncertainty source

according to the above mathematical model, the uncertainty source of water content in paint is mainly composed of the following parts: (1) quality m standard of standard sample; (2) Mass of sample m; (3) The mass of the internal standard in the standard sample solution is m within the standard; (4) The mass of the internal standard in the sample solution is m in the sample; (5) The ratio of the peak area of distilled water and internal standard in the standard solution, and the ratio of the peak area of water and internal standard in the sample solution; (6) The uncertainty introduced by the purity of the standard sample

4 evaluation of standard uncertainty

the hard copper strip of the standard sample and the sample are weighed by the same balance, so the sensitivity of the balance can be ignored. The uncertainty of the balance is limited to the linear uncertainty of the balance. The balance measurement certificate proves that the linearity is ± 0.0001 g, which is the maximum difference between the actual mass and the balance reading. The rectangular distribution is used to convert the linear component into the standard uncertainty, so the standard uncertainty of the linear component of the balance can be expressed as:

(4) the control accuracy of the test is not inferior to the uncertainty component introduced in the internal standard mass m sample in the color sample solution. The weighing of the internal standard in the sample solution is 0.3 g, and its relative standard uncertainty is:

Table 1 the determination results of water content in the sample

in order to calculate the synthetic standard uncertainty of the multiplication expression, each component of the relative standard uncertainty is substituted into the following formula:

5 evaluation of the synthetic uncertainty

the source of the uncertainty and the relative standard uncertainty are shown in Table 2

Table 2 list of uncertainties

6 evaluation of expanded uncertainty

expanded uncertainty u can be calculated by using the inclusion factor. According to experience, take the inclusion factor kp=2, then:

u=kp UC（ ω）= 0.34% × 2 = 0.7%

7 expanded inaccuracy gbt17394 ⑴ 998 metal Leeb hardness test method calibration report and expression

the mass fraction and determination result of water in the waterborne coating are expressed as: (35.118 ± 0.7)% kp=2

8 results and discussion

to sum up, different parameters and influence quantities have different contributions to the measurement uncertainty. This paper systematically analyzes the influence of the quality of the standard sample, the quality of the sample, the quality of the internal standard in the standard solution, the quality of the internal standard in the sample solution, the repeatability, the purity of the standard sample and other factors on the uncertainty. It can be seen from table 2 that the repeatability experiment is the most important component of all sources of uncertainty, so the quality of the repeatability experiment is the key to the correct application of gas chromatography to evaluate the uncertainty of water content in water-based coatings, which has practical guiding significance for future testing work