Silver nitrate, the source of all silver preparations

At the beginning of all silver compounds is silver nitrate, a colourless, easily crystallizing salt that is soluble in large quantities, has amazing properties and is used in many areas. Although the salt is quite easy to extract from the metal in the laboratory, there are many hurdles to overcome in the industrial production of large quantities in high quality.

Silver nitrate provides good services in the ...

• Use of galvanic baths with and without electricity
• Preparation of other silver salts
• Production of certain organic compounds
• Production of photo emulsions and phototropic glasses
• Separation of alkanes.

The properties of silver nitrate (Fig. 1) have been extensively studied so that they are available for a wide range of applications if required.

History of silver nitrate

As early as the 13th century, the church teacher Albertus Magnus (around 1200-1280) documented the ability of nitric acid to separate gold and silver by dissolving the silver, as gold does not dissolve in nitric acid. Magnus also noted that the resulting silver nitrate solution can blacken the skin (Fig. 2). The discoloration of the bright white silver crystals into black when impurities were added earned them the name "infernal stone" early on, which the Romans called "lapis infernalis".

Around the 1880s, the Leipzig gynaecologist Dr. Carl Siegmund Franz Credé (1819-1892) used a 1% silver nitrate solution on infants to treat eye inflammation, thus significantly reducing the rate of eye diseases in newborns.

But people have been using silver in various forms for thousands of years to treat illnesses and fight infections. There are even records of the use and application of silver from ancient Egypt. Writings by the Greek physician Hippocrates of Kos (around 460-370 BC) and the mathematician and philosopher Pythagoras of Samos (570-510 BC) also document that the antibiotic, preservative and regenerative properties of silver were known. Even before the invention of chemical medicines, it was known that bacteria and other disease-causing pathogens could not survive in the presence of silver. In the Middle Ages, royal and wealthy families stored water, wine and food in silver containers to prevent spoilage by microorganisms.

Chinese emperors ate with silver chopsticks, and not just for prestige. Those who could afford it used silver plates and cutlery to eat and drank from silver cups. In those days - even before ice towers, ice cellars and other non-electric cooling systems were invented - it was common to throw a silver coin into a milk container or hang pieces of silver in water tanks. Silver cutlery and silver serving platters still adorn elegant, festive tables today [1].

The use of silver for medical purposes inevitably gave way to the development of synthetic pharmaceuticals, especially with the rediscovery of penicillin in 1928.

But even today, textile fabrics and plasters still contain impregnations that release silver ions so that sweat decomposition and wound dressings do not have to be constantly changed. Masks during the coronavirus epidemic were also treated with silver to prevent the spread of the virus.

In 1843, the head of the Frankfurt mint, Friedrich Ernst Roessler (1813-1883), set up a precious metal refinery in the mint building at the request of the city. Roessler also had a chemical-technical laboratory installed not far from the mint on the Main, where silver nitrate was also produced. In 1866, his two eldest sons Hector and Heinrich Roessler, both chemists by training, took over the refinery, which appeared on the precious metals market in 1873 as the listed Deutsche Gold- und Silberscheideanstalt (Degussa) in Frankfurt am Main [2]Fig. 3: Structure and crystal habitus of silver nitrate (template for a Degussa AG company brochure after W. Hasenpusch, 1979)

After the Second World War, the company rebuilt its destroyed production capacities, but had to relocate the silver nitrate production and the other refinery operations to Hanau on the old site of the former Wolfgang powder factory due to a lack of space. At the end of the 20th century, Hanau and its competitor W. C. Heraeus were among the largest precious metal refineries in the world. The Deutsche Gold- und Silber-Scheideanstalt (Degussa) provided the market with around 10% of the total precious metal demand, producing around 2000 tons of silver, 200 tons of gold and 20 tons of platinum group metals annually.

From the old vat crystallization in the Frankfurt production halls, Degussa's technicians in Hanau developed a modern silver nitrate production facility with a crystallization plant and spiral balancing dryer. From the relatively coarse crystals with traces of nitric acid, a fine-grained, acid-free _AgNO3 product was created, which customers first had to get used to. The company advertised this compound with an attractive brochure (Fig. 3).

Competitors included the Paris-based company Comptoir Lyon-Alemand, the British Johnson Matthey Group and the US Engelhard Corporation.

Production of silver nitrate

Anyone wishing to obtain silver nitrate in Germany today often has to rely on distributors who obtain the preparation in the desired specification from various manufacturers. The general purity requirement (Table 1) does not usually take into account the numerous requirements that a film manufacturer needs. These include, above all, the absence of increased radioactivity, which cannot be neglected in a number of silver mines, but also platinum group metals, which can have a considerable influence on the sensitivity of silver emulsions.

Silver refineries can use silver with a relatively high content of foreign metals, while small businesses often use silver that has been pre-purified by electroplating. In these cases, the silver is dissolved with semi-concentrated nitric acid according to the following equation

3 Ag + 4 _HNO3 → 3 _AgNO3 + NO + 2 _H2O<1>

In laboratories, production methods based on silver oxide <2>, silver with hydrogen peroxide <3> or silver carbonate <4> are also common:

_Ag2O+ 2 _HNO3 → 2 _AgNO3 + _H2O<2>

2 Ag + _H2O2 + 2 _HNO3 → 2 _AgNO3 + 2 _H2O<3>

_Ag2CO3+ 2 _HNO3 → 2 _AgNO3 +_CO2 + _H2O<4>

The advantage of these laboratory methods is the avoidance of nitrogen oxide emissions, which are difficult and time-consuming to largely eliminate in a cascade of scrubbers (Fig. 4). While nitrogen dioxide reacts relatively well with water, this is not the case with nitrogen monoxide. However, _NO2 repeatedly disproportionates to nitric acid with the release of NO:

2 NO + _O2 → 2 NO2_<5>

3 _NO2 + _H2O→ 2 _HNO3 + NO <6>

The advantage of nitrogen oxide scrubbers is that the escaping nitrogen oxides can largely be reused as nitric acid. According to old Japanese patents, pure oxygen is blown into the dissolving reactor to support this.

When using hydrogen peroxide, it should be borne in mind that the aqueous solutions are mixed with 400 to 800 ppm phosphate for stabilization, which can result in yellowish silver phosphate turbidity.

Silver separators also use the silver nitrate process to separate gold and platinum group metals without having to go through electrolysis. If the molten silver is not poured into ingots but into water to form surface-rich "spatter granules", the nitric acid is able to completely separate the gold from the silver, as in analytical docimasia and as early as the 15th century in Venice [3].

The filtrate of the crude silver nitrate is evaporated until it melts at 210 °C and then heated further to around 320 °C. This decomposes all the nitrates. This decomposes all the nitrates of the non-ferrous metals and the nitro complexes of the platinum group metals, which settle out quantitatively as hydrolyzate after the melt has been passed into water. Only lead(IV) nitrate, which has a decomposition temperature of around 440 °C, does not fully participate in this operation. This is the main reason why a further purification stage of the silver nitrate solution is necessary.

Of the trace precipitations known in analytics, the "manganese dioxide method" has proven to be particularly effective for purifying silver nitrate solutions [3]. Manganese dioxide, _MnO2, and lead(IV) oxide, _PbO2, form the same tetragonal lattice of the rutile type (_TiO2) (Fig. 5) and are practically insoluble in aqueous solutions with less than 0.1 mg/L. Traces of lead(IV) oxide are quantitatively incorporated by the manganese dioxide. The formation of manganese dioxide can occur from the dissolved nitrogen oxides or by adding a reducing agent that does not affect the silver nitrate quality, such as hydrogen peroxide.

The entire process of silver nitrate production must also take into account the separation of gold and platinum group metals in the cost unit calculation, which makes silver nitrate production appear in a much more favorable light.

The end product is also often characterized by a different crystal habit at the various companies (Fig. 6, 7 and picture on p. 309). The dried silver nitrate crystallizates or fused granules are available on the market in different specifications.

Important silver nitrate suppliers in Germany:

• Sigma-Aldrich Chemie GmbH, Taufkirchen
• alquera, Madrid/Spain
• Penta, Prague/Czech Republic
• PanReac AppliChem, Darmstadt
• S3 Handel UG, Bad Oeynhausen
• Cfm Oskar Tropitzsch GmbH, Marktredwitz
• AnalytiChem GmbH, Duisburg
• Tropag Oscar H. Ritter Nachf. GmbH, Hamburg
• A.M.P.E.R.E. Deutschland GmbH, Dietzenbach
• Todini Deutschland GmbH, Essen
• C. Roth GmbH + Co KG, Karlsruhe

Properties of silver nitrate and its analogous compounds

Silver nitrate dissolves very well in water at 2,160 g/L at 20 °C (Fig. 8), but also sufficiently well in organic solvents such as pyridine, dimethyl sulphoxide (DMSO), acetonitrile, aniline and benzonitrile, less well in ethanol, ethyl acetate, acetone or acetic acid and very poorly in benzene.

Due to the different solubility, many anhydrous silver compounds can also be produced according to the principle of "reciprocal salt pairs".

From an acetonitrile solution, which is capable of dissolving 1.118 g of silver nitrate at 25 °C, silver sulphide complexes of the type [R-S-Ag-S-R]- initially precipitate from mercapto compounds, which then convert into the extremely poorly soluble, black silver sulphide, _Ag2S. Also a possibility for many organic syntheses!

Fig. 8: Solubility of silver nitrate in water at different temperatures

Fig. 9: Melting points of Ag[element(V)]O3 compounds in relation to molecular weight

Since both the density and the refractive index of pure silver nitrate have been determined very accurately, these are also very good parameters for determining the concentration of solutions, which is particularly advantageous in operational processes in order to maintain control quickly and without loss.

Although melting points of the type Ag[Element(V)]_O3 largely harmonize with the data of the silver nitrate, the silver metaphosphate deviates significantly (Fig. 9). The same applies to the densities, which range up to the silver bismuthate with a density of 7.8 ^g/cm3 at room temperature. In addition to the metaphosphate, the silver iodate also deviates from the largely linear relationship between density and molecular weight (Fig. 10). If silver ruthenate(V) is included, the graph shows a density of 6.0 ^g/cm3 for this compound. The same applies to silver metavanadate(V), for which a density of 5.0 ^g/cm3 can be estimated.

The melting point and density of M(I)nitrates of the _MNO3 type differ significantly from those of silver nitrate in terms of their molecular weights (Fig. 11).

Compared to sodium nitrate, the activity coefficient of silver nitrate decreases more rapidly and more extensively with concentration (Fig. 12).

Fig. 10: Density of the compound types AgX(V)O3 in relation to molecular weight

Fig. 11: Melting points and densities of metal(I) nitrates

Fig. 12: Activity coefficient of silver nitrate solutions at 25 °C

The redox potential of silver salts increases almost linearly with the logarithm of their solubility (Fig. 13) and reaches its highest value of 0.8 volts for silver nitrate. Silver nitrate was also once used to determine the amount of current in coulombs: The amount of current required to deposit one mole of silver, 108 g, is 1 coulomb, abbreviated C (formerly Cb).

Use of silver nitrate

Due to the relatively low price of silver as a precious metal, it has found a wide range of applications: from the photographic industry to electroplating, mirror production, batteries and applications in industry, hygiene and medicine, to name but a few. Smaller quantities of silver nitrate of up to 6 kg increase linearly with the current silver metal price. In January 2024, the silver price was EUR 700/kg, while the price of silver nitrate, which itself contains only 63.5% silver, is EUR 2,000/kg (Fig. 14).

Photo materials

A large proportion of silver nitrate is used with high purity requirements in the production of "photoemulsions", a gelatine gel in which the silver nitrate is converted into the halogen compounds, mainly silver bromide. The light sensitivity decreases from silver iodide to silver chloride. This crystal suspension is marketed on coated carrier materials such as glass or films made of acetyl cellulose, polyethylene terephthalic acid ester, PET and others. Since chlorinated polyalkanes are still used as adhesion promoters between the emulsion and the film, high-quality film recycling, for example of PET, is significantly more difficult.

Used, overlaid and archived photographic material still contains 0.5 to 3.0% silver by weight, so that recycling via incineration, calcination or enzymatic washing and other metallurgical processes back to silver nitrate is worthwhile.

Around 24% of global silver production is still used as silver nitrate in the photographic industry [5]. With 26,000 tons of global silver production in 2023 [6], that would be 6,240 tons of silver. In the 1980s, the proportion of silver that went into the photographic industry was still 40 % by weight.

Electroplating

Electroplating is the electrochemical deposition of metallic coatings on objects.

The coating material, such as tin, nickel, gold or silver, as well as plastics coated with conductive paint, and the substrate are suspended in an electrolytic bath in which the coating material is applied to the positive pole and the substrate to the negative pole of a direct current electrical circuit. Jewelry and cutlery in particular are basic materials for galvanic silver plating. Electroplating with silver increases the solderability of wires. This is particularly important for automated processes. Avoiding contact corrosion or the formation of localized elements also increases the fatigue strength of crimp connections [7].

Silver plating is an electroplating process used to electrochemically coat base metals with a more or less thin layer of silver so that both parts form an inseparable whole. Plating can be carried out on one or both sides. This is called

single or double plating. Copper or nickel silver is most frequently plated with silver. The embossed abbreviation EPNS, which stands for electroplated nickel silver, is often found on cutlery or tea sets.

Electroplating is usually used instead of fine silver in the production of cheap versions of cutlery, tea and coffee services, table decorations and all kinds of flat and hollowware for the home. The first silver-plated products came from factories in Sheffield, England, in the mid-19th century [5].

Both silver cyanide, AgCN, and potassium silver cyanide, K[Ag(CN₎₂], are mainly used for preparing and re-sharpening electroplating silver baths or pre-treatment electrolytes in electroplating shops.

These silver cyanides, direct derivatives of silver nitrate, are characterized by very high purity and good flowability. The 80.5% silver cyanide is a white, powdery product that does not clump. When mixed with a sufficient amount of potassium cyanide, it is highly soluble in water,

AgCN + KCN → K[Ag(CN₎₂] <7>

while the pure silver cyanide is extremely poorly soluble with a solubility of 0.22 mg/L water at 20 °C. The 54.2% potassium silver cyanide forms colorless crystals that dissolve well in water at 143 g/L at 20 °C even without the addition of potassium cyanide [8].

The silver coating is suitable for a wide variety of jewelry such as watches, chains or rings. Matt silver baths deposit white fine silver layers, which are suitable for technical and decorative silver plating due to their exceptional depth of dispersion.

Cyanide-free silver baths offer significant environmental and safety benefits. Nevertheless, the purity of the silver deposits is almost 100 %. The coated surfaces have a very white silver tone and are suitable for both decorative and technical purposes.

Through anodic oxidation, the silver nitrate can be oxidized to _Ag7NO11 to form octahedral crystals. Thermal decomposition produces silver(II) oxide.

Silver can also be deposited from non-aqueous solutions, for example from acetonitrile, in which silver salts dissolve well as a complex of the type [Ag(_NC-CH3)2^]+.

Electroless silver plating is known from mirror coating or Christmas tree baubles. They are based on the use of mild reducing agents such as dextrose etc.

Other silver nitrate downstream products

Silver nitrate can be oxidized with potassium peroxodisulfate into silver(II) compounds, or more precisely: into silver(I,III) cations:

2 Ag(I)_NO3 + _K2S2O8 → Ag(II)(_NO3)2 +

Ag(II)_SO4 + _K2SO4<8>

With the addition of potassium hydroxide solution, silver(II) oxide is formed, which is used in organic chemistry to convert benzyl halides directly into dibenzyl ether.

Ag(II)O also serves as a surface catalyst in the epoxidation of alkenes [9]. Divalent silver oxide is also used in rechargeable batteries and button cells.

As early as the 19th century, the chemist Eduard Linnemann (1841-1886), who was born in Frankfurt/Main, recognized that alkyl iodides and silver acetate can be used to produce alkyl acetic acid esters [10]:

R-I + AgOAc → R-OAc + AgI<9>

Colloidal silver is mainly used for disinfection and sterilization. It is precipitated by mechanical grinding in colloid mills, electrolytically using various methods and by chemical reduction from diluted silver salt solutions in the presence of protective colloids, whereby ultrasonic support is also advantageous [11].

Other uses of silver nitrate

Silver nitrate is known as a detection reagent for halides (chloride, bromide and iodide) as well as pseudo-halides due to poorly soluble precipitates. The pseudo-halides include:

• hydrocyanic acid, HCN
• isocyanic acid, HNCO
• fulminic acid, HCNO
• Isothiocyanic acid, HNCS
• Thiocyanic acid, HSCN
• Selenocyanic acid, HSeCN
• Nitric acid, _HN3
• and their salts.

Silver nitrate is also used for the determination of aldehydes and proteins. For example, the aldehyde groups of reducing sugars such as glucose and lactose can be detected, for example in the "Tollensprobe" of the precipitating silver levels.

In protein biochemistry, silver nitrate is used in the course of staining to identify proteins that have been separated in a polyacrylamide gel, for example.

In histology, silver nitrate is used to stain tissue sections, for example in the Golgi-Cox method.

In forensic dactyloscopy, a silver nitrate-methanol solution is used to visualize fingerprints. With a limited circle of perpetrators, the perpetrators can also be unmasked by their black fingers using prepared locks, banknotes and other objects.

In medicine, silver nitrate is used as an antiseptic and astringent as a 0.5% solution for local treatment and as a cautery in the form of a "Höllenstein cautery pen" against skin growths, ulcers and warts.

Since 1881, a 1% silver nitrate solution has been dripped into the eyes of newborn babies to prevent gonorrheal eye infections (gonococcal infections). This "Credé prophylaxis" could be replaced by less painful, irritating and toxic substances [12]. In Germany, this prophylaxis was required by law as part of the preventive medical check-up until 1986, after which it was only recommended.

In the complex analysis of triglycerides, silver nitrate is used to separate the triglycerides by means of "argentation chromatography" [13].

Unsaturated hydrocarbons, n- and iso-paraffins can be separated using silver nitrate silica gel column chromatography [14].

Another application is in homeopathy. Globules in very high dilutions of silver nitrate are used to treat irritable bowel syndrome or anxiety.

Dangers when handling silver nitrate

Finally, the special and dangerous side reactions associated with silver nitrate must always be pointed out:

As with gold and mercury solutions, ammonia reacts as a gas or in solutions with silver nitrate at high pH values, as is also the rule with the "Tollens reagent" in analytical chemistry, over time to form the silver nitride, _Ag3N. The silver nitride is sufficiently technically described (cubic crystal structure; M = 337.61 g/mol; D = 8.91 ^g/cm3). However, there are no safety figures and classifications. The compound detonates when exposed to flash light and still undiminished at -190 °C [15].

The two precursors, the silver amide, _AgNH2 (M = 123.89 g/mol) and the silver imide, _Ag2NH(M = 230.75 g/mol), have not yet been classified in terms of their hazardousness.

Ammoniacal solutions should be processed quickly and disposed of immediately after use, i.e. reduced to silver. Silver reduced with dextrose or ascorbic acid can be remelted and used again as silver nitrate.

Laboratory bottles with diluted silver nitrate solutions must not come into contact with ammonium gas, which can easily escape from the corresponding containers or during experiments, as otherwise explosive silver compounds will form on the caps of the silver nitrate bottles.

Literature
[1] https://vitalpfoten.de/silber
[2] https://history.evonik.com/de/gesellschaften/degussa
[3] Fester, G.: "Die Entwickling der Chemischen Technik" (1923), reprint, BoD-Books on Demand (2020)
[4] Koch, O. G. and G. A. Koch-Dedic: "Handbuch der Spurenanalyse", Springer-Verl., 1600 p., 2nd edition (2013) 360
[5] https://www.mineralienatlas.de/lexikon/index.php/Mineralienportrait/Silber
[6] https://de.statista.com/statistik/daten/studie/37059/umfrage/produktion-von-silber-weltweit-nach-laendern
[7] https://www.leoni-wire-products-solutions.com/de/materialien/materialien-fuer-die-galvanisierung/silber/silbergalvanik/
[8] https://www.silbercyanid.de/Silbercyanid
[9] https://de.wikipedia.org/wiki/Silber(I,III)-oxide
[10] https://de.wikipedia.org/wiki/Silberacetat
[11] https://de.wikipedia.org/wiki/Kolloidales_Silber
[12] https://de.wikipedia.org/wiki/Crede-Prophylaxe
[13] Beyer, E. M.: "Potent inhibitor of ethylene action in plants"; Plant Physiology. 58/3 (1976) 268-271
[14] https://onlinelibrary.wiley.com/doi/abs/10.1002/lipi.19760780702
[15] https://de.wikipedia.org/wiki/Silberazid