Titanium and titanium alloys are important metal materials widely used in aerospace, medical, chemical, metallurgical, marine engineering and other fields. China's titanium and titanium alloy production capacity ranks among the top in the world, but due to the overall low grade and unstable quality of titanium materials, high-end titanium and titanium alloys still need to be imported. The reason for poor quality of titanium materials is not only due to technical and technological factors, but also due to important factors such as testing methods, among which sample pretreatment is the most critical step. Developing a sample pretreatment method that can simultaneously determine multiple elements in titanium and titanium alloys has an important promoting effect on improving detection efficiency.
Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) can simultaneously determine multiple elements and is widely used. At present, the standard methods for determining the above elements in titanium and titanium alloys include GB/T 4698 series, SN/T 3910-2014 "Determination of Manganese, Chromium, Nickel, Aluminum, Molybdenum, Tin, Vanadium, and Copper in Sponge Titanium, Titanium, and Titanium Alloys by Inductively Coupled Plasma Emission Spectroscopy", YS/T 1262-2018 "Chemical Analysis Methods for Sponge Titanium, Titanium, and Titanium Alloys Determination of Multi element Content by Inductively Coupled Plasma Atomic Emission Spectroscopy", etc. These standard methods all use hydrochloric acid hydrofluoric acid nitric acid or sulfuric acid hydrofluoric acid nitric acid systems to dissolve the sample in a beaker, and require that the dissolution temperature should not exceed 70 ℃, otherwise it will cause the loss of volatile elements such as Si and Cr, resulting in low measurement results. Researchers used a mixed solution of hydrochloric acid hydrofluoric acid nitric acid perchloric acid as the digestion acid system, microwave digestion of titanium and titanium alloy samples, and simultaneous determination of 19 elements in titanium and titanium alloys by ICP-AES. The method not only reduces acid consumption, but also minimizes the loss of the tested elements, improves the accuracy of the measurement results, and is conducive to energy conservation and emission reduction.
1. Experimental method
Sample pre-treatment
Weigh the sample into a microwave digestion tank, add 33% (volume fraction, the same below) hydrochloric acid solution, 25% hydrofluoric acid solution, 50% nitric acid solution, and perchloric acid, place the microwave digestion tank in a microwave digestion instrument, digest according to the microwave digestion program, cool to room temperature, transfer the digestion solution to a volumetric flask, dilute with water, and prepare sample solution 1 for the determination of elements with a mass fraction not exceeding 1.0%. Take 5.0 mL of sample solution 1 and transfer it to a 50 mL volumetric flask. Dilute with water to obtain sample solution 2, which is used to determine elements with a mass fraction greater than 1.0%. Prepare a blank solution by conducting a blank test with high-purity titanium.
Preparation of standard solution series
Preparation of Multi Element Mixed Standard Solution Series I for Matrix Matching: Weigh an appropriate amount of 6 parts of high-purity titanium, pre treat them according to the above method, and transfer them to 6 50mL volumetric flasks. Add an appropriate amount of 1000mg · L − 1 Fe, Si, Mn, Mo, B, Al, Sn, Cr, V, Zr, Mg, Nb, Pd, Ni, Ta, W, Nd, Ru, Cu single element standard reserve solutions, and dilute them with 5% nitric acid solution to prepare 19 matrix matching multi element mixed standard solution series I with mass concentrations of 0, 0.2, 0.5, 2.0, 8.0, 20.0mg · L − 1 for the determination of elements with mass fractions less than 1.0%. The sentence is:.
Preparation of Multi Element Mixed Standard Solution Series II for Matrix Matching: Weigh an appropriate amount of high-purity titanium, transfer it to a 100mL volumetric flask after pretreatment, and make up to volume with 5% nitric acid solution. Take 5.0mL of Fe, Mn, Mo, Al, Sn, Cr, V, Zr, Nb, Cu single element standard reserve solution at an appropriate amount of 1000mg · L − 1 into 6 50mL volumetric flasks. Dilute with 5% nitric acid solution to prepare Fe, Mn, Cu mass concentrations of 0, 1.0, 2.0, 4.0, 6.0, 10.0mg · L − 1, Mo, V mass concentrations of 0, 1.0, 5.0, 20.0, 40.0, 80.0mg · L − 1, Al, Sn, Cr, Zr mass concentrations of 0, 1.0, 5.0, 10.0, 20.0, 40.0mg · L − 1, Nb mass concentrations of 0, 2.0, 10.0. Series II of matrix matched multi-element mixed standard solutions with concentrations of 30.0, 60.0, and 10.0mg · L − 1 are used to determine elements with mass fractions greater than 1.0%.
2. Results and Discussion
2.1 Selection of digestion methods
At present, when dissolving samples in a beaker, the digestion time is short and easy to observe, but the temperature is not easy to control, and volatile elements and trace elements are prone to loss. The experiment used 30mL of 50% hydrochloric acid solution+2mL of hydrofluoric acid+2mL of nitric acid as the digestion acid system in YS/T 1262-2018 to compare the determination values and relative standard deviations (RSD) of volatile elements and trace elements (Si, B, Ni, Cu) after digestion of the same sample using two digestion methods (open beaker digestion and microwave digestion). The results are shown in Table 1.
Table 1: The influence of digestion methods on the determination results of some elements (n=10)

The results showed that when microwave digestion was used, the RSD of the elemental determination values were all less than 4.0%, indicating good repeatability. Therefore, microwave digestion method was chosen as the sample digestion method for the experiment.
2.2 Comparison of Dissolving Acid Systems
At present, the standard methods for sample digestion of titanium and titanium alloys using ICP-AES are listed in Table 2.
Table 2 Comparison of different digestion methods

　　According to Table 2, the commonly used digestion acid systems in the standard method mainly include mixed solutions of hydrochloric acid hydrofluoric acid nitric acid, sulfuric acid nitric acid, and sulfuric acid hydrofluoric acid nitric acid. The use of hydrochloric acid hydrofluoric acid nitric acid mixed solution as a digestion acid system is suitable for sample pretreatment for the determination of Mn, Mo, B, Al, Cr, V, Zr, Nb, Pd, Ni and other elements. However, if the sample contains rare earth elements such as Nd, F - and Nd3+form neodymium fluoride that is difficult to dissolve in hydrochloric acid and nitric acid. Therefore, this digestion acid system is not suitable for sample pretreatment for the determination of rare earth elements. Adding sulfuric acid to the digestion acid system can be used for sample pretreatment in the determination of rare earth elements, but the reaction between sulfuric acid and titanium causes the solution to turn purple black, which is not easy to observe whether the sample is completely dissolved. In addition, the viscosity of sulfuric acid is high, which affects the atomization efficiency, so it is not suitable for use. Based on the characteristic that nitric acid can oxidize some low-priced titanium and neodymium fluoride, which are easily soluble in perchloric acid, a mixed solution of hydrochloric acid hydrofluoric acid nitric acid perchloric acid was selected as the digestion acid system for the experiment.
In addition, the acid consumption of the digestion acid system in the sample pretreatment methods used in some standard methods in Table 2 is 50-100 mL, resulting in approximately 500 mL of test waste liquid. In contrast, this method consumes 10-15 mL of acid (saving 85%), resulting in approximately 95 mL of waste liquid (reducing emissions by 81%). This method not only saves sample pretreatment time and material costs, but also greatly reduces experimental waste liquid.
2.3 Selection of acid dosage in the digestion acid system
The amount of acid used affects the reaction rate of digestion. When the amount of acid is too high, the reaction becomes too intense, causing the sample to be washed to the tank wall, resulting in incomplete dissolution of the sample and causing waste. Preliminary research has found that the reaction rate is more suitable when using a mixed solution of 33% hydrochloric acid solution and 25% hydrofluoric acid solution as the digestion acid system. Further select different dosages of 33% hydrochloric acid solution (2.0, 5.0, 8.0, 12.0 mL) and 25% hydrofluoric acid solution (1.0, 2.0, 3.0, 4.0 mL) for orthogonal experiments. The results showed that when the amounts of 33% hydrochloric acid solution and 25% hydrofluoric acid solution were 8.0 and 3.0 mL, respectively, the sample dissolved completely and there were no titanium shavings splashing on the tank wall. Nitric acid is mainly used to oxidize low valent titanium, making the digested sample solution clear and transparent, making it easy to observe whether the sample is completely dissolved. A 50% nitric acid solution is added dropwise to the digested solution. The experiment found that when the amount of 50% nitric acid solution added is 1.0mL, the low valent titanium in the sample is completely oxidized and the solution is clear and transparent.
In addition, 5 parts of high-purity titanium (with Nd mass fraction less than 0.001%) were added to an equal amount of 30mg · L − 1Nd single element standard solution. Under the same treatment conditions, the influence of the amount of perchloric acid added (0, 0.5, 1.0, 1.5, 2.0mL) on the dissolution effect was investigated. The results show that 0.5mL of perchloric acid can completely dissolve neodymium fluoride. In summary, the experiment selected 8.0mL of 33% hydrochloric acid solution, 3.0mL of 25% hydrofluoric acid solution, 1.0mL of 50% nitric acid solution, and 0.5mL of perchloric acid for the digestion acid system.
2.4 Selection of Heating Time and Target Temperature for Microwave Digestion
The heating time and target temperature of microwave digestion affect the digestion effect of the sample. In the gradient heating mode, the effects of heating time and target temperature on the digestion efficiency of the same sample (sponge titanium, titanium alloy) were experimentally investigated. The results are shown in Table 3.
Table 3: Effects of Heating Time and Target Temperature on Sample Digestion Efficiency

　The results show that the target temperature for the * stage is 60 ℃ with a heating time of 1 minute, and the target temperature for the second stage is 120 ℃ with a heating time of 8 minutes, which is the * heating program.
2.5 Working Curve and Detection Limit
According to the working conditions of the instrument, determine the matrix matching multi-element mixed standard solution series I or series II, with the mass concentration of the tested element as the horizontal axis and the corresponding response intensity as the vertical axis to draw the working curve. The obtained linear parameters are shown in Table 4.
Table 4 Linear Parameters and Detection Limits

　Under the working conditions of the instrument, the blank solution was continuously measured 11 times, and the standard deviation (s) of the measured values was calculated. The detection limit of the method was determined at 3 times the standard deviation (3s). The detection limits of each element are shown in Table 4.
2.6 Accuracy and Precision Testing
Determine the elemental content of four titanium alloy standard samples IARM345A, IARM300B, IARM178C, and IARM303B according to the experimental method, and compare the results with the standard method GB/T 4698 series (hereinafter referred to as the standard method). The titanium alloy standard sample IARM303B (Pd identification value of 0.13% ± 0.01%) was used to verify the accuracy of this method for determining Pd. The results showed that the measured value of this method was 0.13%, and the measured value of the standard method was 0.12%. The determination results of elements in other standard samples are shown in Table 5.
Table 5 Accuracy Test Results

　　Due to the absence of four elements such as Mg, W, Ru, and Nd in the standard samples of titanium alloys, three actual samples (TA12 titanium alloy, TA26 titanium alloy, and TC25 titanium alloy) containing these four elements were analyzed according to the experimental and standard methods to verify the accuracy of the method for determining these four elements. The results are shown in Table 6.
The results show that the measured values of this method are basically consistent with those of the standard method, indicating that the accuracy of the method is good. At the same time, according to the experimental method, the standard samples IARM345A and IARM178C of titanium alloy, as well as the actual samples TA12 titanium alloy and TC25 titanium alloy, were measured in parallel for 5 times. The RSD of the measured values was calculated, and the results showed that the RSD of each element measured was less than 4.0%, meeting the requirements for simultaneous determination of 19 elements in titanium and titanium alloys.
Table 6 Determination results of some elements in samples under different methods

　3. Experimental conclusion
Researchers used a mixed acid system consisting of 8.0mL of 33% hydrochloric acid solution, 3.0mL of 25% hydrofluoric acid solution, 1.0mL of 50% nitric acid solution, and 0.5mL of perchloric acid to digest titanium and titanium alloy samples using microwave digestion. ICP-AES was used to simultaneously determine the content of 19 elements in titanium and titanium alloys. Compared with the sample pretreatment method of the standard method, this method has the advantages of less acid consumption, reduced loss of tested elements, and less discharge of test waste liquid. However, the sample pretreatment time, including the pressure relief time, is still relatively long and needs further research.