The palladium gravimetric method, EDTA complexometric titration method, spectrophotometry and flame atomic absorption spectrometry, which are commonly used in palladium recovery and palladium product deep processing, are introduced.
1 dimethylglyoxime palladium gravimetric method
dimethylglyoxime palladium gravimetric method has been used as a standard method for the determination of palladium because of its specificity. As an organic precipitant, dimethylglyoxime has high selectivity, the molar mass of precipitated dimethylglyoxime palladium is large, and the percentage of palladium in the weighing form is small, which is beneficial to improve the accuracy of analysis and the amount of samples required is small. There are many available media for palladium precipitation, such as HCl, HNO3, H2SO4 and HClO4, and the acidity range of complete precipitation is relatively wide. The experiment shows that .2mol/LHCl medium is the most suitable precipitation medium. However, under this precipitation condition, a large number of Au ions are easy to be reduced and interfere with the determination. It can be reduced to metal with formic acid in advance, and then the palladium content can be determined after filtration. Pt ion is easy to precipitate with palladium in 7 ~ 8%
HCl, which affects the accuracy of palladium content determination.
1.1 principle
In acidic solution, palladium can form chelate precipitation with dimethylglyoxime, which is weighed after filtration, washing and drying. Palladium content can be calculated by the conversion factor between palladium and dimethylglyoxime.
1.2 experimental method
Accurately weigh a certain amount of samples (or a certain amount of test solution, containing about .1g of palladium), add 5mL of water, 2mL of hydrochloric acid, heat for dissolution, add 2mL of water for dilution, add 8mL of 1% butanedione oxime ethanol solution, keep the temperature at 6 ~ 7oC for 1h, and cool. Filtering with a sand core crucible with constant weight at 11 5℃, transferring the precipitate to the sand core crucible, washing the precipitate with dilute hydrochloric acid, washing the precipitate with hot distilled water until it is free of Cl-, drying at 11 5℃ to constant weight, and weighing.
1.3 Applicable analysis objects
The palladium dimethylglyoxime gravimetric method is suitable for the analysis of palladium in high-content samples, that is, the analysis of palladium-containing samples with a mass fraction higher than .1% (or with a palladium content higher than .1g/L in the test solution) and a low content of other impurities, such as raw materials for palladium deep processing, alloy materials, high-content palladium catalysts, palladium dichloride and palladium tetrachloride (II). For samples with complex composition, the procedures for eliminating interference when palladium is determined by gravimetric method are often complicated, the analysis time is prolonged and the analysis speed is reduced.
2 EDTA complexometric titration
2.1 analysis principle
EDTA complexometric titration of constant palladium can be divided into direct titration, back titration and indirect titration. There are many interfering components in EDTA direct titration, and the introduction of separation procedures complicates the determination process and brings new errors. At room temperature and pH 3.5 ~ 1., palladium and EDTA can react rapidly to form a 1∶1 complex, and the content of palladium can be determined by dropping back with Zn or Pb standard solution.
for the system with serious interference, in order to improve the selectivity of EDTA titration of Pd, indirect titration is adopted, that is, after back titration of excessive EDTA, a stripping agent is added to destroy Pd(Ⅱ)-EDTA, and then the released EDTA is titrated with Zn or Pb standard solution to obtain the content of Pd. This can greatly improve the selectivity of complexometric titration of palladium. Commonly used stripping agents are thiourea, thiocyanate, phenanthroline, dimethylglyoxime, DL- methylthiobutyric acid and so on.
2.2 analysis method
accurately weigh a certain amount of palladium-containing samples, dissolve them in 1mL of nitric acid, and adjust the pH to 5.5 with sodium acetate solution under constant stirring. Add a known excess of EDTA (.5
mol/L) and stir thoroughly. Add 5 drops of xylenol orange indicator, and use Zn2+ standard solution to drip back until it turns from yellow to purple.
For the sample with complex composition, after the standard solution of Zn2+ is dripped back to the end point, the conditions can be controlled, and the stripping agent is added to release the EDTA in the Pd(Ⅱ)-EDTA complex, and then the released EDTA is titrated with the standard solution of Zn2+ to obtain the palladium content.
2.3 Applicable analytical objects
Complexometric titration has high accuracy in the analysis of palladium in high-content samples. This method can be used to determine the content of palladium in raw materials, alloy materials, palladium dichloride, palladium tetrachloride (II) and palladium dichloride (II) products. For palladium-containing samples with high content of interfering components, corresponding measures can be taken to eliminate interference, and extraction separation method can be used to improve the selectivity of the method if necessary.
Because the stoichiometric ratio of palladium to EDTA is 1∶1, there must be enough samples to ensure a certain accuracy, so the complexometric titration method requires a large number of samples.
3 spectrophotometry
The commonly used chromogenic reagents for the determination of palladium by spectrophotometry are potassium iodide and pyridine azo reagents.
3.1 Analysis principle
The absorbance of the test solution is directly proportional to the concentration of metal ions in it. According to the absorbance value of the test solution, the concentration is found on the standard curve, so as to calculate the metal content in the sample. For example, KI and Pd(Ⅱ) form red coordination ions in acidic medium, which can be used for the determination of Pd. The ions such as Au, Fe, V contained in the sample are reduced to a low-valence state and do not absorb, so the palladium content can be quickly determined without separation.
3.2 analysis method
(1) Measuring the absorption curve of Pd2+
Transfer 3.mL
of 2μg/mL Pd standard solution with a pipette into a 25.mL volumetric flask, and add 2mL of potassium iodide solution, 2. ml (1+3) sulfuric acid solution and 2.5 ml
3. mol/in turn. With 1cm cuvette and reagent blank solution as reference solution, the change of absorbance with wavelength was measured on spectrophotometer in the range of 34~7nm, and the maximum absorption wavelength was determined.
(2) Drawing the standard curve
Transfer , 1., 2., 3., 4., 5. and 6.mL of palladium standard solution of 2μg/mL
with a pipette and put them in 25.mL colorimetric tubes respectively, and add 2 ml (1+3) sulfuric acid solution and 2. ml in turn. Use a 1cm cuvette, take the reagent blank solution without palladium as the reference solution, and measure the absorbance of each solution as the same as that of the sample.
(3) Determination of the sample
Place the palladium-containing sample in a beaker and add aqua regia to dissolve it to prepare palladium test solution. Put a certain amount of diluted test solution in a 25.mL volumetric flask, then add 2 ml (1+3) sulfuric acid solution, 2.5mL
3.mol/L potassium iodide solution and 2.mL.6% ascorbic acid solution in turn, and let it stand for 1min. Take reagent blank solution as reference solution, use 1cm cuvette, and measure absorbance at the measured maximum absorption wavelength.
3.3 Suitable analytical objects
are suitable for the determination of palladium in low-content palladium samples, such as palladium-containing catalyst analysis and palladium content analysis in low-content palladium-containing samples (liquids).
4 flame atomic absorption spectrometry
Weigh a certain amount of samples into a 15mL beaker, add aqua regia, cover with watch glasses, heat at low temperature, filter, and wash the filter residue with a small amount of water for 3-4 times. Add a little NaCl solution to the filtrate, drive away nitric acid with hydrochloric acid, evaporate and concentrate until it is nearly dry, add 5ml HCl (1+1), dissolve the residue at low temperature, move it into a 1mL volumetric flask, and dilute it to scale with water. The absorbance of Pd was measured at 244.8nm (content .5%) or 276.3nm (content 5%) on an atomic absorption spectrometer.
This method is simple, rapid, sensitive and accurate. It is widely used for the determination of trace palladium, such as palladium catalyst, low-content palladium alloy and palladium-containing waste (or waste liquid).