In this paper, ascorbic acid (Vc) was used as a matrix modifier to establish a direct method for the determination of trace platinum in tailings by graphite furnace atomic absorption spectrometry. The mechanism of different matrix modifiers was discussed. The characteristic quality of this method is 1.3×10, and the detection limit is 1.9×10 linear. The recoveries of the sample are 100.7%-104.0%.
1 Introduction Atomic absorption method has been widely used to determine platinum in different materials. However, due to the low content of platinum in these materials and the complexity of the matrix, the sample processing and separation before the measurement are rich and important. The pretreatment process before the determination of platinum, such as foam adsorption, ion exchange fiber separation and enrichment, etc., makes the graphite furnace atomic absorption method, which has a relatively long measurement time, more complicated. The proposed matrix improvement technology has been widely used in eliminating or reducing matrix interference, improving the sensitivity of graphite furnace measurement, and the method is simple and easy to implement. In this paper, through the discussion of the properties of different matrix modifiers, a method for the determination of trace platinum in tailings sand by graphite furnace atomic absorption spectrometry with ascorbic acid (Vc) as matrix modifier was established. The method has the advantages of simple operation, small interference of coexisting ions, accurate measurement results and the like.
2 Experimental part 2.1 Instruments and reagents HitachiZeeman 180-70 polarized atomic absorption spectrometer, GA-3 graphite furnace with automatic sample introduction device and precision optical temperature control unit; 056 ​​desktop double pen recorder; platinum hollow cathode lamp (Beijing vacuum) Institute of Electronic Technology); domestic pyrolytic coated graphite tube Hitachi produces ordinary graphite tube (GT).
Platinum standard stock solution: accurately weigh 0.1000g of high-purity platinum, heat it with about 10mL aqua regia, transfer it to a 100mL volumetric flask with 5% aqua regia, and shake it to a constant volume. Is this solution 1.0mg? mL-1. Dilute to the desired concentration with 5% HCl; ascorbic acid: -1, ready to use.
The above reagents were of analytical grade, and the vessel used was made of quartz or polytetrafluoroethylene. The experimental water was deionized secondary quartz distilled water.
2.2 Instrument working conditions 10mA; slit 0.4nm; tube wall atomization mode; atomization stage stop argon gas; measurement method is peak area; injection volume 20LL. Graphite furnace working conditions are shown in Table 1.
Step drying 1 Drying 2 Ashing Atomization Depuration temperature (°C) Time (s) 2.3 Determination method In a 10 mL plastic quantitative tube, transfer a certain amount of platinum standard solution (or platinum test solution), add 1 mL of ascorbic acid solution, and dilute with water until Scale and shake. The absorbance was measured under the selected instrument operating conditions using the reagent blank as a reference.
3 Results and discussion 3.1 Influence of acidic medium When the platinum is determined by pyrolytic coating graphite tube, hydrochloric acid, nitric acid, aqua regia, sulfuric acid, perchloric acid and phosphoric acid within 5% do not affect the determination, so as to reduce the matrix and make platinum. Stable, this article uses 0.1mol / L nitric acid medium.
3.2 Matrix modifier effect comparison Platinum is a medium-high temperature element with a higher atomization temperature and lower measurement sensitivity. Using the instrument's recommended conditions, the absorbance is still not present near the highest atomization temperature (3000 ° C), so the application of the matrix modifier is mainly to reduce the atomization temperature and improve the sensitivity of the measurement. The effects of nine single and mixed matrix modifiers were compared using pyrolytic coated graphite tubes. Experiments have shown that the addition of cerium salts in a single modifier or in a mixed form improves the sensitivity of platinum to some extent. . The effect is consistent with the formation of refractory carbides in the literature, which prevents platinum from penetrating into the wall during atomization. The addition of palladium also increases the sensitivity of platinum. The addition of nickel causes a decrease in platinum sensitivity. It was found in the test that ascorbic acid as a matrix modifier has the best effect on lowering the atomization temperature of platinum and improving the sensitivity of platinum.
3.3 Mechanism of Action of Matrix Modifiers The atomization and ashing curves of platinum were determined using ascorbic acid as a matrix modifier, as shown in Figure 1. It can be seen from Fig. 1 that the addition of the base 1 - the ashing curve of platinum after the addition of Vc; 2 - the atomization curve of platinum after the addition of Vc; 3 - the atomization curve of platinum without Vc.
After the bulk modifier, platinum begins to lose when the ashing temperature is greater than 1600 ° C, and the platinum loss becomes larger as the temperature increases. However, the higher ashing temperature is not only advantageous for separating the substrate, but also for shortening the temperature rise time from the ashing temperature to the atomization temperature, which is advantageous for the measurement. In a comparative experiment without the addition of a matrix modifier, as the atomization temperature increased, the signal was always increased and no platform appeared, indicating that complete atomization had not yet been reached. After the addition of ascorbic acid, the atomization signal increased, and a platform appeared at 2700 ° C, indicating that the addition of ascorbic acid can reduce the atomization temperature of platinum, so that the atomization of platinum at 2700-2800 ° C tends to be complete.
Under the same conditions, the measurement sensitivity of the pyrolytic coated graphite tube was about 4 times higher than that of the ordinary graphite tube. Since platinum does not form carbides, the loss of sensitivity in the measurement of ordinary graphite tubes may be due to the porosity of the ordinary graphite tube wall, which causes the platinum permeation loss in the atomization stage, and the sensitivity of the ordinary graphite tube is improved after coating. This is also confirmed. It is believed that the atomization process of platinum belongs to the metal reduction type chemical reaction. The reaction process is: the addition of ascorbic acid in the spectral laboratory can reduce the atomization temperature of Pt, because ascorbic acid can maintain the graphite furnace in a strong reducing atmosphere in the atomization stage. It promotes the reduction of PtO to the ground state platinum atom, thereby improving the sensitivity of the measurement.
The inner surface of the ordinary graphite tube is loose and porous, has strong permeability, contains more carbon active centers, and has a strong reducing atmosphere, which is beneficial to the reduction of most elements, but the atomization temperature of platinum is relatively high, and ordinary graphite tubes are used. In the process of atomization, it is easy to lose due to infiltration. The pyrolysis coated graphite tube is used, and ascorbic acid is used as a matrix modifier, which not only reduces the permeation loss, but also has a strong reducing atmosphere to facilitate the reduction of platinum. Higher sensitivity.
3.4 The amount of matrix modifier The effect of the matrix modifier is related to its amount. A comparative experiment was carried out on different amounts of ascorbic acid, as shown in Fig. 2. The results showed that 4Lg of ascorbic acid matrix modifier was added to each measurement, and the effect was the best.
3.5 Atomic time selection experiments show that at the atomization temperature of 2800 ° C, in the case of adding a matrix modifier, the absorbance value has a maximum value and a plateau when the atomization time is greater than 5 s. Select the atomization time of 7s for better measurement precision.
3.6 Influence of coexisting ions Under the selected conditions, when ascorbic acid is used as a matrix modifier, the coexisting ions of the following content do not interfere with the determination: Na3.7 calibration curve and method characteristic quality, the detection limit is under the selected experimental conditions, The platinum standard series was prepared, and the calibration curve was determined and plotted by the GFAAS method. The platinum content is linear in the range of 2 to 500. The linear regression equation is A=0.0108 0.0025C, r=0.9984, and the detection quality is 1.9×103.8. The sample analysis and the spike recovery test pulverize and study the tailings sand. Evenly pass through a sieve of 80 mesh and dry to constant weight. Weigh the appropriate amount of sample in a Teflon beaker, first wet with a little water, then add 10mL of hydrofluoric acid, digest in a water bath of 60-80 ° C, add 15mL of new aqua regia, and heat to near dry. Add a small amount of 1mol/L nitric acid to dissolve the residue, transfer it to 50mL with pure water, add the scalar quantity and measure the recovery rate (%) of the volumetric flask, and make up the volume. Pipette 5.00mL according to the analytical method for platinum determination and spike recovery test, the results are shown in Table 2.
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