A method for rapidly preparing palladium sulfate and palladium nitrate by mechanical activation

Abstract

The application discloses a kind of mechanical activation method for quickly preparing palladium sulfate and palladium nitrate, belongs to the field of palladium compound product preparation, comprising the following steps: (1) pretreatment: ceramic jar and ceramic ball are used as ball milling container and medium, ceramic ball diameter includes two specifications of 5mm and 10mm, and is mixed according to the mass ratio of 3:1;(2) raw material mixing: palladium powder with purity ≥99.95% and ammonium persulfate are mixed according to the mass ratio of 1: (2.36~2.57);(3) slurry preparation;(4) mechanical activation;(5) post-treatment: after filtering, washing treatment of ball milling product, according to the purification path of target product, palladium sulfate or palladium nitrate is selected.The application replaces heat energy with mechanical energy as reaction driving force by mechanical activation method, and stable and consistent products can be prepared in a short time at room temperature, and nitrogen oxide toxic gas generation is also eliminated.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of palladium compound product preparation, and particularly relates to a method for rapidly preparing palladium sulfate and palladium nitrate by mechanical activation. BACKGROUND

[0002] Palladium sulfate and palladium nitrate are both important palladium compounds. Among them, palladium nitrate has good solubility (especially in water and various solvents), and is one of the most commonly used and most convenient precursors in palladium catalyzed coupling reactions. Palladium sulfate has more advantages in catalytic reactions that avoid the oxidative interference of nitrate or require higher temperature stability (sulfate is more stable than nitrate), such as the classic Wacker oxidation process.

[0003] Since it is extremely difficult to directly dissolve palladium powder with dilute sulfuric acid, there are usually two paths for the preparation of palladium sulfate: path 1, palladium powder is dissolved with dilute sulfuric acid + H2O2, and the addition amount and addition speed of H2O2 need to be accurately controlled during the dissolution process. If the addition speed is too fast, it will cause instantaneous reaction to be violent, causing the solution to splash and causing the loss of palladium. In addition, the excess H2O2 needs to be decomposed by high-temperature heating, which on the one hand causes the valence state of palladium ions to change due to long-term repeated heating and evaporation, and even generates palladium oxide precipitate, resulting in complex valence state of palladium ions in the solution, reducing the stability and consistency of the product; on the other hand, the method of high-temperature heating cannot completely decompose the excess H2O2, greatly reducing the stability of the compound and the activity of the catalyst. Path 2, palladium powder is dissolved with sulfuric acid + nitric acid, and the solution needs to be chased with sulfuric acid. This method needs to be reacted at 150℃~200℃ for 8h~9h, and the long-term high-temperature condition also reduces the stability and consistency of the product, and the introduced NO3 - cannot be completely removed, greatly reducing the activity of the catalyst. In summary, the current wet preparation process of palladium sulfate needs to introduce a solubilizing agent, but the introduction of the solubilizing agent cannot be completely removed, which greatly reduces the activity of the catalyst, and the long-term reaction at high temperature causes the product to have poor stability and consistency. In addition, the preparation process has many steps and complex operation, and requires high skills of personnel.

[0004] Palladium nitrate is usually prepared by direct dissolution of nitric acid. This method is reacted at 120℃~150℃, and also has the problem of poor stability and consistency of the product due to long-term high-temperature heating. In addition, the palladium powder cannot be completely dissolved at one time, and needs to be repeatedly dissolved by replacing the palladium powder with new nitric acid, which consumes a lot of reagents, has a long production cycle, and is accompanied by the release of a large amount of nitrogen oxides (NO x ), which must rely on a waste gas treatment device, increasing the equipment investment and operating cost.

[0005] Therefore, the person skilled in the art provides a method for rapidly preparing palladium sulfate and palladium nitrate by mechanical activation method to solve the problems raised in the above background. SUMMARY

[0006] The purpose of the present application is to provide a method for rapidly preparing palladium sulfate and palladium nitrate by mechanical activation method, which uses mechanical energy instead of heat energy as the driving force of the reaction, and can produce stable and consistent products in a short time at room temperature, while eliminating the generation of toxic nitrogen oxide gas, to solve the problems raised in the above background.

[0007] To achieve the above purpose, the present application provides the following technical solutions:

[0008] A method for rapidly preparing palladium sulfate and palladium nitrate by mechanical activation method, comprising the following steps:

[0009] (1) Pretreatment: ceramic jar and ceramic balls are used as ball milling container and medium, ceramic balls with two specifications of 5mm and 10mm in diameter are mixed in a mass ratio of 3:1, and are cleaned with deionized water for 2-3 times before use, and are dried at 100°C for 2 hours and then cooled to room temperature;

[0010] (2) Raw material mixing: palladium powder with a purity of ≥99.95% and ammonium persulfate are mixed in a mass ratio of 1:(2.36-2.57), and 0.3%-0.8% of auxiliary grinding agent BN based on the mass of palladium powder is added, and the mixture is poured into the ceramic jar for preliminary mixing;

[0011] (3) Slurry preparation: analytical pure sulfuric acid with a concentration of 1-1.5mol / L is added to the mixture of step (2) in a liquid-solid ratio of (2-4):1, and ceramic balls are added after the uniform slurry is stirred, and the jar body is sealed;

[0012] (4) Mechanical activation: the planetary ball mill is operated at a speed of 350-450rpm for 3-4 hours, and is paused for 5 minutes every 30 minutes;

[0013] (5) Post-treatment: the ball milling product is filtered and washed, and then the purification path of palladium sulfate or palladium nitrate is selected according to the target product.

[0014] As a further scheme of the present application, the purification path of palladium sulfate in step (5) comprises:

[0015] (5a) After the ball milling product is filtered and washed, the filtrate is heated to 70-80°C, Ba(OH)2 is added to adjust the pH to 9-11 to remove NH4 + , and Pd(OH)2 and BaSO4 precipitates are obtained by filtration and washing;

[0016] (5b) adding dilute sulfuric acid to the precipitate to adjust the pH to 2-3, and then filtering the filtrate in a 50-80℃ water bath for 1-2 hours to obtain a pure palladium sulfate solution;

[0017] (5c) evaporating and crystallizing to obtain palladium sulfate crystals.

[0018] As a further scheme of the present application, the purification path of the palladium nitrate in step (5) comprises:

[0019] (5d) after filtering and washing the ball-milling product, the filtrate is heated to 50-60℃, 5mol / L ammonia water is added to adjust the pH to 9-11, and then filtering and washing to obtain a Pd(OH)2 precipitate;

[0020] (5e) adding dilute nitric acid to the precipitate until it is completely dissolved, and then filtering after being kept in a 50-80℃ water bath for 1-2 hours to obtain a pure Pd(NO3)2 solution;

[0021] (5f) evaporating and crystallizing to obtain Pd(NO3)2 crystals.

[0022] As a further scheme of the present application, the ammonium persulfate in step (2) can be replaced by sodium persulfate or potassium persulfate, and the number of washing times needs to be increased to reduce the content of impurity elements after the replacement.

[0023] As a further scheme of the present application, the auxiliary grinding agent BN in step (2) can be replaced by SiO2 or graphene, and the ball-milling time needs to be adjusted to 4-5 hours to compensate for the difference in grinding efficiency after the replacement.

[0024] As a further scheme of the present application, the key control parameters in the purification process comprise:

[0025] For the palladium sulfate path, the Ba(OH)2 addition end point is pH=9-11, and the dilute sulfuric acid dissolution end point is pH=2-3;

[0026] For the palladium nitrate path, the ammonia water addition end point is pH=9-11, and the dilute nitric acid dissolution end point is marked by complete dissolution of the precipitate;

[0027] The temperature deviation in the incubation stage is ≤±2℃, and the time deviation is ≤±10 minutes.

[0028] As a further scheme of the present application, the sulfuric acid is added in step (3) in three equal portions, and after each addition, the mixture is stirred for 5 minutes before continuing to add.

[0029] As a further scheme of the present application, the purity index of the target product after purification is:

[0030] The total amount of metal impurities in the palladium sulfate crystals is ≤8ppm;

[0031] The total amount of metal impurities in the palladium nitrate crystal is less than or equal to 4 ppm.

[0032] Compared with the prior art, the application has the following beneficial effects:

[0033] 1. The application can prepare palladium sulfate and palladium nitrate products at room temperature, the valence state of palladium ions does not change during the preparation process, the stability of the products is strong and the consistency is high, the generation of toxic nitrogen oxide gas is avoided, the pollution to the environment is reduced, and green preparation is realized.

[0034] 2. In the ball milling process of the application, mechanical energy is used instead of heat energy as the driving force of the reaction, heating is not required, and the energy consumption is reduced. The traditional method requires more than 12 hours to dissolve palladium powder, and the dissolution rate of palladium powder is less than 80%. The mechanical activation method shortens the preparation time to less than 5 hours, and the solubility of palladium powder is greater than 99.5%, and the reaction rate and solubility are greatly improved.

[0035] 3. The traditional method is prone to local overheating and large concentration gradient due to uneven reaction, thereby causing side reactions. In contrast, the mechanical activation method used in the application promotes uniform mixing of the system at room temperature through mechanical force, effectively avoiding the generation of by-products caused by excessive local concentration or temperature.

[0036] 4. The traditional method requires complex heating-stirring-refluxing devices, and manual monitoring of temperature and acid concentration, which is complicated to operate. The mechanical activation method realizes automatic reaction relying on the ball mill, which is suitable for batch production, and the equipment maintenance is simple (the material of the mill pot is ceramic or zirconia, which is acid-resistant and wear-resistant), and the operating cost is lower than that of the traditional heating reaction kettle. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 A flowchart of a mechanical activation method for rapidly preparing palladium sulfate and palladium nitrate. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the application will be described below in detail with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the application.

[0039] As described in the background art of the application, the existing method requires long-time high-temperature heating when preparing palladium sulfate and palladium nitrate. This process not only easily causes the precipitation of palladium sulfate and palladium nitrate solution during storage, affecting the stability and consistency of the products, but also generates a large amount of toxic nitrogen oxide gas, thereby limiting the feasibility and safety of actual application, and there are certain defects.

[0040] In order to solve the above defects, the application discloses a method for rapidly preparing palladium sulfate and palladium nitrate by mechanical activation method, which uses mechanical energy instead of heat energy as the driving force of reaction, and uses a ball mill to prepare products with strong stability and high consistency in a short time at room temperature, and at the same time, eliminates the generation of toxic nitrogen oxide gas.

[0041] The application will be described in detail below in combination with the drawings.

[0042] Please refer to Figure 1 In the embodiment of the application, a method for rapidly preparing palladium sulfate and palladium nitrate by mechanical activation method comprises the following steps: (1) pretreatment: ceramic jars and ceramic balls are used as the ball mill container and medium, the ceramic balls have two specifications of 5 mm and 10 mm in diameter, and are mixed in a mass ratio of 3:1, and are cleaned with deionized water 2-3 times before use, and are dried at 100°C for 2 hours and then cooled to room temperature; (2) raw material mixing: palladium powder with a purity of ≥99.95% and ammonium persulfate are mixed in a mass ratio of 1:(2.36-2.57), and 0.3%-0.8% of auxiliary grinding agent BN based on the mass of the palladium powder is added, and the mixture is poured into the ceramic jar for preliminary mixing; (3) slurry preparation: analytical pure sulfuric acid with a concentration of 1-1.5 mol / L is added to the mixture of step (2) in a liquid-solid ratio (2-4):1, and the mixture is stirred into a uniform slurry, and then ceramic balls are added and the jar is sealed; (4) mechanical activation: the slurry is ball milled in a planetary ball mill at a speed of 350-450 rpm for 3-4 hours, and is paused for 5 minutes every 30 minutes; compared with the traditional process, no toxic nitrogen oxide gas is generated after ball milling; (5) post-treatment: the ball-milled product is filtered and washed, and then a purification path of palladium sulfate or palladium nitrate is selected according to the target product. In the preparation process of the application, long-time high-temperature heating operation is avoided, the valence state of palladium ions in the prepared solution is uniform, the obtained palladium sulfate and palladium nitrate solution does not precipitate during storage, and the stability and consistency of the products are significantly improved. In addition, in the traditional process for preparing palladium sulfate and palladium nitrate, the subsequent impurity removal step will inevitably introduce nitrate, which will cause the generation of a large amount of toxic nitrogen oxide gas, and the application avoids the pollution of toxic gas to the environment and realizes green preparation.

[0043] In the embodiment, the purification path of palladium sulfate in step (5) comprises: (5a) after the ball-milled product is filtered and washed, the filtrate is heated to 70-80°C, Ba(OH)2 is added to adjust the pH to 9-11 to remove NH4 +, the filtrate is kept in a water bath at 50-80℃ for 1-2 hours to obtain a pure palladium sulfate solution; and (5c) palladium sulfate crystals are obtained by evaporation and crystallization. In the embodiment, the purification path of palladium nitrate in step (5) includes: (5d) after the ball-milled product is filtered and washed, the filtrate is heated to 50-60℃, 5 mol / L ammonia water is added to adjust the pH to 9-11, and Pd(OH)2precipitate is obtained by filtration and washing; (5e) dilute nitric acid is added to the precipitate until it is completely dissolved, and a pure Pd(NO3)2solution is obtained by filtration after keeping it in a water bath at 50-80℃ for 1-2 hours; and (5f) Pd(NO3)2crystals are obtained by evaporation and crystallization. In the application, the ball-milling process does not require heating, which reduces energy consumption. In the traditional method, the palladium powder is dissolved for more than 12 hours, and the dissolution rate of the palladium powder is less than 80%. The mechanical activation method shortens the preparation time to less than 5 hours, and the solubility of the palladium powder is greater than 99.5%, which greatly improves the reaction rate. Compared with the traditional method which produces toxic gases and requires complex tail gas treatment, the mechanical activation method eliminates the generation of toxic nitrogen oxides. In the traditional method, due to uneven reaction (local overheating and large concentration gradient), by-products are easily generated, which affects the stability and consistency of the product. The mechanical activation method effectively avoids the generation of by-products caused by local high concentration or high temperature through mechanical force to promote uniform mixing of the system at room temperature. The traditional method requires complex heating-stirring-refluxing devices and manual monitoring of temperature and acid concentration, which is complicated to operate. The mechanical activation method relies on the ball mill to realize automatic reaction, which is suitable for batch production, and the equipment maintenance is simple (the grinding tank is made of ceramic or zirconia, which is acid-resistant and wear-resistant), and the operating cost is lower than that of the traditional heating reaction kettle.

[0044] In the embodiment, ammonium persulfate in step (2) can be replaced by sodium persulfate or potassium persulfate, and the number of washing times needs to be increased to reduce the content of impurity elements after the replacement. The purity lower limit of the alternative scheme is ensured by additional washing conditions to avoid the virtualization of the protection scope caused by the replacement of raw materials.

[0045] In the embodiment, the auxiliary grinding agent BN in step (2) can be replaced by SiO2 or graphene, and the ball milling time needs to be adjusted to 4-5 hours after the replacement to compensate for the difference in grinding efficiency. The reaction efficiency is maintained by compensatory time extension to ensure that the palladium powder is completely oxidized (particle size ≤1 μm) under the alternative scheme.

[0046] In the embodiment, the key control parameters in the purification process include: for the palladium sulfate path, the Ba(OH)2addition end point pH=9-11, and the dilute sulfuric acid dissolution end point pH=2-3; for the palladium nitrate path, the ammonia water addition end point pH=9-11, and the dilute nitric acid dissolution end point is marked by complete dissolution of the precipitate; the temperature deviation in the incubation stage is ≤±2℃, and the time deviation is ≤±10 minutes.

[0047] In the present embodiment, the sulfuric acid in step (3) is added in three equal portions, and after each addition, the mixture is stirred for 5 minutes before adding the next portion. This protects the uniformity of the slurry and avoids the problem of palladium powder wrapping failure caused by local over-concentration.

[0048] In the present embodiment, the purity indicators after purification of the target product are: total metal impurities in the palladium sulfate crystal ≤8 ppm; total metal impurities in the palladium nitrate crystal ≤4 ppm.

[0049] It should be noted that the total reaction equation of the present application is:

[0050]

[0051] (1) Electrode potential difference;

[0052] Reduction half-reaction: persulfate is a strong oxidizing agent, and its reduction reaction is ;

[0053] Standard electrode potential ;

[0054] Oxidation half-reaction: Pd is oxidized to Pd 2+ , and the reaction is ;

[0055] Standard electrode potential ;

[0056] Total electromotive force .

[0057] (2) Gibbs free energy change (ΔG°);

[0058] ;

[0059] Where: n = 2 (number of transferred electrons );

[0060] F = 96485 C / mol (Faraday constant);

[0061] ;

[0062] ;

[0063] In summary: and both indicate that the reaction can proceed spontaneously.

[0064] The present application can prepare products with strong stability and high consistency in a short time at room temperature by using mechanical energy instead of heat energy as the driving force of the reaction. In addition, compared with the traditional method of producing nitrogen oxide toxic gas, which requires complex tail gas treatment, the mechanical activation method eliminates the generation of toxic gas, reduces the pollution of toxic gas to the environment, and realizes green preparation. The ball milling process of the present application does not need heating, which reduces the energy consumption. The traditional method dissolves palladium powder, and the dissolution time is more than 12h, and the dissolution rate of palladium powder is less than 80%. The mechanical activation method shortens the preparation time to less than 5h, and the solubility of palladium powder is greater than 99.5%, and the reaction rate and dissolution efficiency are significantly improved. The traditional method is easy to produce by-products due to uneven reaction (local overheating, large concentration gradient), which affects the stability and consistency of the product, while the mechanical activation method adopted by the present application promotes the uniform mixing of the system at room temperature through mechanical force, avoiding the generation of by-products caused by local over-concentration or overheating. The traditional method needs complex heating-stirring-refluxing devices, and needs manual monitoring of temperature and acid concentration, which is complicated to operate; while the mechanical activation method realizes automatic reaction relying on the ball mill, which is suitable for batch production; and the equipment maintenance is simple (the material of the mill pot is ceramic or zirconia, which is acid-resistant and wear-resistant), and the operation cost is lower than that of the traditional heating reactor. Among them, the specific advantages of the mechanical activation method are as follows:

[0065] As a new material synthesis technology, mechanical activation method has shown significant advantages in the preparation of palladium sulfate and palladium nitrate, especially in solving the core pain points of traditional process (such as low dissolution efficiency, many side reactions, difficult impurity control, and great environmental pressure). The following is a multi-dimensional comparison and analysis of its core advantages:

[0066] 1. High reaction efficiency and low energy consumption preparation;

[0067] Mechanical activation can significantly refine palladium powder particles and introduce lattice defects through high-frequency mechanical force generated by ball milling and vibration milling equipment. The particle size of palladium powder can be reduced from microns to nanometers during ball milling, and the specific surface area can be increased by 5-10 times, forming a sponge-like porous structure that greatly increases the reaction contact area. This physical activation improves the chemical activity of palladium, increases the reaction rate by 3-5 times, shortens the reaction time, and increases the conversion rate of palladium to nearly 100%, reducing the loss of noble metals due to residue and improving production efficiency.

[0068] 2. Completely solve the problem of waste gas;

[0069] During the preparation of palladium sulfate and palladium nitrate by mechanical activation method, no nitrogen oxide toxic gas is produced, which eliminates pollution and safety hazards from the source. The operation is extremely safe, and no complex and expensive tail gas absorption device is needed, which is friendly to the environment.

[0070] 3. The product has better stability and consistency;

[0071] The mechanical activation method adopted in the application is based on a ball milling device, so that the reaction process is carried out at normal temperature, and problems such as precipitation of palladium sulfate or palladium nitrate solution caused by long-time high-temperature heating in the traditional process are effectively avoided, and the stability and consistency of the product are significantly improved.

[0072] Obviously, those skilled in the art can make various modifications and variations to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalent technologies, the present application also intends to include these modifications and variations.

[0073] The above is only the preferred specific embodiment of the present application, but the protection scope of the present application is not limited to this. Any person skilled in the art can make equivalent substitutions or changes according to the technical scheme and the inventive concept of the present application within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A method for rapid preparation of palladium sulfate and palladium nitrate by mechanical activation method, characterized by, The method comprises the following steps: (1) Pretreatment: ceramic jar and ceramic balls are used as the ball milling container and medium, the ceramic balls have two specifications including 5 mm and 10 mm, and are mixed according to a mass ratio of 3:1, and are cleaned with deionized water for 2-3 times before use, and are dried at 100 DEG C for 2 hours and then cooled to room temperature; (2) Raw material mixing: palladium powder with a purity of greater than or equal to 99.95% and ammonium persulfate are mixed according to a mass ratio of 1:(2.36-2.57), and 0.3%-0.8% of auxiliary grinding agent BN of the mass of the palladium powder is added, and the mixture is poured into the ceramic jar and is preliminarily mixed; (3) Slurry preparation: analytical pure sulfuric acid with a concentration of 1-1.5 mol / L is added into the mixture of step (2) according to a liquid-solid ratio of (2-4):1, and the mixture is stirred to form a uniform slurry, and ceramic balls are added and the jar is sealed; (4) Mechanical activation: the jar is ball milled in a planetary ball mill at a rotating speed of 350-450 rpm for 3-4 hours, and is paused for 5 minutes every 30 minutes; (5) Post-treatment: after the ball milling product is filtered and washed, a purification path of palladium sulfate or palladium nitrate is selected according to the target product; The purification path of palladium nitrate in step (5) comprises: (5d) after the ball milling product is filtered and washed, the filtrate is heated to 50-60 DEG C, 5 mol / L ammonia water is added to adjust the pH to 9-11, and Pd(OH)2 precipitate is obtained after filtration and washing; (5e) dilute nitric acid is added to the precipitate until it is completely dissolved, the mixture is kept in a water bath at 50-80 DEG C for 1-2 hours, and then filtered to obtain pure Pd(NO3)2 solution; (5f) Pd(NO3)2 crystals are obtained by evaporation and crystallization.

2. A method for rapid preparation of palladium sulfate and palladium nitrate by mechanical activation as claimed in claim 1, wherein, The purification path of palladium sulfate in step (5) comprises: (5a) After the ball-milling product is filtered and washed, the filtrate is heated to 70-80°C, Ba(OH)2 is added to adjust the pH to 9-11 to remove NH4 + , and Pd(OH)2 and BaSO4 precipitates are obtained by filtration and washing; (5b) dilute sulfuric acid is added to the precipitate to adjust the pH to 2-3, the filtrate is kept in a water bath at 50-80 DEG C for 1-2 hours after filtration, and pure palladium sulfate solution is obtained; (5c) palladium sulfate crystals are obtained by evaporation and crystallization.

3. A method for rapid preparation of palladium sulfate and palladium nitrate by mechanical activation as claimed in claim 2, wherein, In step (2), the ammonium persulfate can be replaced by sodium persulfate or potassium persulfate, and the number of washing times needs to be increased to reduce the content of impurity elements after the replacement.

4. A method for rapid preparation of palladium sulfate and palladium nitrate by mechanical activation as claimed in claim 3, wherein, In step (2), the auxiliary grinding agent BN can be replaced by SiO2 or graphene, and the ball milling time needs to be adjusted to 4-5 hours to compensate for the difference in grinding efficiency after the replacement.

5. A method for rapid preparation of palladium sulfate and palladium nitrate by mechanical activation as claimed in claim 4, wherein, The key control parameters in the purification process comprise: For the palladium sulfate path, the Ba(OH)2 addition end point is pH=9-11, and the dilute sulfuric acid dissolution end point is pH=2-3; For the palladium nitrate path, the ammonia water addition end point is pH=9-11, and the dilute nitric acid dissolution end point is marked by complete dissolution of the precipitate; The temperature deviation in the heat preservation stage is less than or equal to ±2 DEG C, and the time deviation is less than or equal to ±10 minutes.

6. A method for rapid preparation of palladium sulfate and palladium nitrate by mechanical activation as claimed in claim 5, wherein, In step (3), the sulfuric acid is added in three equal portions, and is stirred for 5 minutes after each addition before continuing to add.

7. A method for rapid preparation of palladium sulfate and palladium nitrate by mechanical activation as claimed in claim 6, wherein, The purity index of the target product after purification comprises: The total amount of metal impurities in the palladium sulfate crystals is less than or equal to 8 ppm; The total amount of metal impurities in the palladium nitrate crystals is less than or equal to 4 ppm.

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