The water content of water-based paints is mostly determined by Karl Fischer volumetric method. This paper combines the practical work and discusses the influencing factors in the process of water determination in light of the particularity of water-based paints, and puts forward feasible suggestions.
There are many methods to determine the moisture content, such as gas chromatography, Karl Fischer method, extraction method, titration method, etc., and there are few reports about the use of Karl Fischer method to determine the moisture content of water-based paints. The Karl Fischer measurement method includes the volumetric method and the Coulomb method. Since the Coulomb method is only suitable for the determination of trace amounts of water, it is not suitable for water-based paints with a water content of about 40%. The application of the method in the determination of water content in water-based coatings was studied.
1 Experimental part
1. 1 Experimental principle of Karl Fischer volume method
Karl Fischer reagent is composed of iodine, sulfur dioxide, organic base and solvent, etc. The solvent is generally alcohol, such as methanol, ethylene glycol, 2-methoxyethanol or mixtures thereof. As long as there is free water in the titration cup of Karl Fischer titrator, the free iodine in Karl Fischer reagent will participate in the reaction, as shown in Equation 1.
H2O I2 SO2 R′OH 3RN → (RN H) SO4R ′ 2 (RN H) I (Formula 1) where: RN refers to organic bases such as pyridine, imidazole, etc .; R′OH refers to alcohols.
Once all the water has reacted with iodine, there will be free iodine in the mixed solution in the titration vessel. This free iodine will promote ion conduction and lower the voltage to stabilize the polarization current. At the set value, the titration is terminated.
1. 2 titration steps
Karl Fischer reagents are divided into single-component reagents and two-component reagents. Because single-component reagents are easy to handle, inexpensive, and fast to operate, they are easy to use when determining the water content of water-based coatings.
In this study, single-component reagents were mainly used. The basic steps of the card · Fischer volumetric titration are:
â‘ Add Karl Fischer reagent to the burette;
â‘¡Add suitable solvent to the titration cup;
â‘¢Pre-titration and drift value determination with Karl Fischer reagent;
â‘£ Calibration of Karl Fischer reagent;
⑤Add samples to the titration cup;
â‘¥ Titrate the water content with Karl Fischer reagent.
2 Results and discussion
It can be seen from the above titration principle and steps that when the Karl Fischer volumetric method is used to determine the moisture content, the influence of various factors needs to be considered.
2. 1 laboratory
When performing Karl Fischer titration, there is ubiquitous water, and the experimenter needs to have practical experience beyond the ordinary volumetric titration technique. For the determination of water content in water-based paints, ubiquitous water is bound to be a potential source of interference. Karl Fischer titration should be performed under "ordinary conditions", and the temperature in the laboratory should be avoided above normal temperature [13].
2. 1. 1 Ambient humidity
Ambient humidity is one of the most common factors causing errors in Karl Fischer titration. Moisture can enter the sample, titrant and Karl Fischer titrator, and the humidity can be reduced by using an air conditioner to lower the indoor temperature in the laboratory. However, it should be noted that the Karl Fischer titrator cannot be installed in front of the air conditioner [14]. In the case of high humidity, a dehumidifier should be installed in the laboratory. It is recommended not to place equipment such as water baths in the laboratory, because such equipment may increase the humidity of the indoor environment.
2. 1. 2 Temperature
Has a clear influence on the Karl Fischer titration rate. At low temperatures, it can reduce the reaction rate and reduce the occurrence of side reactions that affect the titration results; at high temperatures, it can accelerate the dissolution of the sample and the extraction of water, which shortens the titration time. Karl Fischer reagent contains more than 90% of methanol or ethanol, its volume increases due to temperature increase, and its concentration decreases accordingly. According to ISO760-1978, for every 1 ℃ increase in temperature, the concentration decreases by 011% [11] .
2. 2 Calibration of Karl Fischer reagent
Accurate calibration of Karl Fischer reagent is one of the factors to ensure accurate and reliable results of Karl Fischer titration. The stabilization procedure of Karl Fischer reagent depends on the chemical stability of Karl Fischer reagent, the sealing degree of Karl Fischer reagent container and the change of ambient temperature. How often to calibrate the concentration of Karl Fischer reagent depends on the selected Karl Fischer reagent, the sealing procedure of the titrator, and the degree of accuracy required. It is best to calibrate Karl Fischer reagent every day during the measurement [12]. Before each calibration, the Karl Fischer reagent in the plastic tube in the titration line should be discharged 2 to 3 mL, because the plastic tube cannot be strictly waterproof. After a period of time, the Karl Fischer reagent in the tube will absorb the environment The moisture in [14]. Karl Fischer reagent can be calibrated using sodium tartrate dihydrate, standard water or pure water, which is generally deionized water or water that meets the ISO3696 standard. Most laboratories use pure water to calibrate Karl Fischer reagents. Because the amount of pure water taken during calibration is very small (5 ~ 10μL), this requires skilled and careful operation to obtain correct results with good repeatability. When using a micro-syringe (10μL) to take water, please note: the syringe needle cannot be immersed in the solvent of the titration cup; after determining the volume of water, wipe off the water droplets on the needle with a paper towel; each injection of pure water should be reduced Method, measured with an electronic balance with a resolution of 011 mg, it is recommended not to directly read the volume of the syringe, otherwise the deviation will be large; always add pure water in the same way. The repeatability value of Karl Fischer reagent calibration should be controlled within 1%, which can further reduce the error caused by Karl Fischer reagent calibration for moisture content determination.
2. 3 Sample preparation
The following rules should be followed when sampling: the sample taken must be representative, and the sample to be tested should be fully stirred before sampling; the exchange of moisture between the sample and the surrounding environment should be prevented, and the sample should be weighed and processed quickly. The amount of sample taken by the titration has a lot to do with the accuracy of the final result. For titration, the water content of each sample is preferably consistent with the amount of water used during calibration. The optimal water content of each sample is 10 mg [13]. Increasing the sample volume will increase the accuracy, because the influence of the environmental moisture absorbed during sampling and sample addition on the measurement results is relatively weak, but the sample volume contains up to 100 mg of water, because too much sample volume will cause the sample and The blank solvent in the titration cup is not miscible or dispersed, which affects the accurate determination. The water content of water-based coatings is generally about 40%. When Karl Fischer reagent with a titer of 5 mg / mL is used, the amount of sample taken is about 011 to 014 g. As the titer of the reagent decreases, the sample volume can be increased appropriately. Since water-based coatings are generally in a viscous state, it is recommended to use the decrement method when adding samples to reduce errors. It is best to directly add the water-based paint sample to the titration cup for titration, but if the sample is not well miscible or dispersed in the solvent, so that accurate titration cannot be performed, external extraction method can be used, that is, the sample is dissolved or dispersed in a suitable In an anhydrous solvent, then take an aliquot of the solution and add it to the titration cup for titration. At this time, a blank titration of the same amount of the selected solvent should be done. For specific operations, see Appendix X1 [9] in ASTM D4017-02.
2. 4 Solvent
According to the experimental principle of Karl Fischer volume method, to determine the water content in water-based paint, the water-based paint must be dissolved or dispersed in a solvent, and its water must be in a free state, so that it can react with Karl Fischer reagent. Part of the solvent must be alcohol (preferably methanol) to ensure that the Karl Fischer reaction proceeds stoichiometrically. The solvent's ability to dissolve or extract is the most important source of error that affects Karl Fischer volumetric titration. Since there are many types of raw materials and additives used in water-based coatings, it will inevitably affect the solubility of water-based coatings, so the choice of solvent becomes the most important task. Due to the particularity of water-based coatings, when the solvent is only methanol, the dissolution effect is often not very good. Methanol and other solvents are often used together. After a large number of verification experiments, it is found that such as methanol, methanol-chloroform mixed solvent, methanol-pyridine mixture Solvents, methanol-formamide mixed solvents, methanol-2-chloroethanol mixed solvents, etc. are very effective for the determination of moisture in water-based paints. However, in any case, the content of methanol should be at least 25% [13], otherwise the titration reaction cannot be performed stoichiometrically; at the same time, it should be noted that if the amount of water to be extracted exceeds the water capacity of the solvent used, the extraction may not be complete. The general principle is: 1 mL of methanol can extract up to 1 mg of water. In actual work, a more suitable mixed solvent containing methanol should also be selected according to the particularity of the sample. At the same time, it should also be noted that the water content of the solvent used should be as small as possible, otherwise the titration duration is too long, and the Karl Fischer reagent is wasted.
2. 5 Titration control
At present, there are many manufacturers of Karl Fischer titrators. Better ones include Metrohm, Swiss METTLER, Zhejiang University, and Beijing Pioneer. However, no titrator can completely eliminate the surrounding water interference, which requires Take some effective measures to reduce the impact of surrounding water. If the titrator will not be used for a long time, the titration cup should be completely disassembled, the parts should be washed in methanol, and then dried, generally in the air to dry. During titration, close all openings of the titration cup. Use a desiccant (such as silica gel, molecular sieve) to protect the titration cup, Karl Fischer reagent, solvent and waste bottle to avoid the invasion of moisture. However, it should be noted that due to the limited life of the desiccant, it should be replaced in time.
2. 5. 1 Pre-titration and determination of drift value
After each addition of solvent, the titration cup should be shaken 3 to 4 times, and then pre-titrated with Karl Fischer reagent. Moisture remains in the empty space of the titration cup, and there is often a layer of solvent film on the inner surface of the titration cup. This solvent film absorbs moisture, which will cause slow titration. After the pre-titration, the drift value should also be measured, that is, the total amount of water that enters the titration cup in a period of time, in μg / min. The drift value of the titration should be as low and stable as possible to obtain accurate measurement results.
2. 5. 2 Stirring
Because the surface tension of the coating is greater than the surface tension of the solvent, the water-based coating samples tend to coalesce into agglomerates after being added to the titration cup, which often leads to titration drift. Therefore, extraction and stirring immediately after the sample is added are critical. The author has a small trick that can extract the water in the sample very well, that is, the initial stirring speed is adjusted to the fastest. After about 5 minutes, the stirring speed is adjusted to a small vortex in the titration solution without bubbles. , That is, the optimal stirring speed is reached, and the titration can be started after about 2 minutes; it can also be processed according to the method of Appendix X2 in ASTM D4017-02. In this method, the Karl Fischer titrator is equipped with a homogenizer, Disperse the insoluble sample before titration, extract the water in the sample into the anhydrous solvent within an appropriate time, and then titrate the extracted water [9]. Full stirring is an important condition for rapid and accurate titration. The solution must be thoroughly mixed during the titration to ensure that the ions conduct electricity constantly, rather than intermittently. Stirring is too slow, the reaction speed is slow and irregular, and it is easy to drip; too fast stirring, it is easy to produce bubbles, which affects the accuracy of the final measurement result.
2. 5. 3 End point control
The current Karl Fischer titrator generally adopts the principle of double voltammetry. When free iodine is detected in the mixed solution, Karl Fischer titration is terminated, that is, when the voltage at the polarization end of the double platinum electrode drops to a certain setting The titration end point is reached when the value is set. The value depends on the polarization current, the type of electrode, the selected solvent, etc. Each end point has a specific polarization current. Increasing the polarization current can shorten the titration time, but the polarization current is too high, which is easy to cause electrode contamination. The polarization current measured in the water content of the water-based paint is generally 20 to 30 μA, preferably 25 to 28 μA. At present, a special end point technology has been introduced, that is, when a specified voltage or current is reached and can be maintained for a long period of time (about 20 s), it is considered that the titration end point has been reached [13].
2. 6 Side effects
The Karl Fischer titration method has many substances that interfere with the determination of moisture. For a detailed description, see ASTM E203-01 [10]. These disturbances are generally accompanied by polycondensation reactions or redox reactions. Due to the many components in water-based coatings, there may be substances that interfere with the normal Karl Fischer reaction. The occurrence of these side reactions will affect the titration results. For these substances, independent analysis and appropriate calibration are required. Common side reactions [14] are: active carbonyl compounds and methanol can produce acetal or ketal and water. For these samples, special reagents for aldehydes and ketones can be used. It should be noted that at this time, the sample volume should be small and the titration is 2 ~ After 3 times, the solvent must be changed; the organic acid and methanol start the esterification reaction. At this time, a 1: 1 (volume ratio) methanol-pyridine mixed solvent can be used; react with water. In the presence of aldehyde and water, disulfite is added Water is consumed, in this case, it should be titrated immediately after adding the sample; react with iodine, iodine is easy to react with the oxidized substances, which will cause more iodine to be consumed, which can be controlled by changes in temperature, Such as the use of a drying furnace; the amount of methanol in the solvent is insufficient, pyridine sulfite will react with water, it is recommended to use 20 to 40 mL of methanol as the titration solvent. In short, special samples should be analyzed and processed to minimize the effects of side reactions.
3 Conclusion
In this paper, the application of Karl Fischer volumetric method in the determination of water content in water-based coatings is discussed in detail from the laboratory, Karl Fischer reagent calibration, sample preparation, solvents, titration process, side reactions, etc. Factors, and corresponding solutions are given.
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