Atomic Weight of Copper, history

The accepted value for the atomic weight of copper, 63.57, is in good accord with the periodic system, the properties of the metal and its compounds being functions of an atomic weight of this order belonging to an element of the fifth horizontal row of Group I. of this system. The atomic heat of copper at ordinary temperatures is 5.9 to 6.0, slightly less than the mean value 6.4 for the solid elements, but sufficiently in harmony with the law of Dulong and Petit. The isomorphism of the element with silver and gold, and that of the cuprous compounds with the derivatives of univalent silver, furnish additional evidence in favour of the atomic weight adopted.

Cryoscopic determinations of the molecular weight of copper in solution in other molten metals indicate the monatomic nature of its molecule.

Both chemical and physico-chemical methods have been employed in determining the atomic weight of copper. A summary is appended of the values obtained by both types of process, expressed in terms of the modern notation O = 16. The antecedent data employed in the recalculation from the experimental results are:

Ag = 107.880; C = 12.003; Na = 22.996; Ba = 137.37; H = 1.00762; S = 32.065; Br = 79.916.

In 1814 Wollaston calculated the equivalent of copper to be four times that of oxygen, or Cu = 64.

The early researches of Berzelius, Erdmann and Marchand, Dumas, and Millon and Commaille, involved either syntheses or analyses of cupric oxide, the values found for the atomic weight of copper being 63.30, 63.46, 63.5, and 63.13. Dumas also made syntheses of cuprous sulphide, but gave no details concerning his experiments. His result did not differ much from the modern value, and was adopted for many years.

Hampe made two series of experiments in 1874. In the first, copper was converted into the basic nitrate, and ignited in a current of oxygen; in the second, cupric sulphate was dehydrated at 250° to 260° C., and the copper estimated electrolytically. His results were:

Cu:O = 79.8347:20.1653, whence Cu = 63.34;
CuSO₄: Cu = 100: 39.725, whence Cu = 63.31.

In 1883 Baubigny dehydrated cupric sulphate by heating it to 440° C., ignited weighed portions of the salt at a temperature corresponding with the melting-point of gold, and weighed the residual cupric oxide. He found the ratio

CuO: SO₃ = 49.810: 50.190, whence Cu = 63.46.

The researches of Richards on the atomic weight of copper, based on chemical methods only, were carried out between 1887 and 1891. In his first experiments, silver was precipitated by addition of electrolytic copper to excess of an ice-cold solution of silver nitrate. The results were:

1. (6 experiments) Cu: 2Ag = 100: 339.408, whence Cu = 63.569.**
2. (5 experiments) Cu: 2Ag = 100: 339.404, whence Cu = 63.570**

In the first series, the silver was dried at 150° C., and a correction was applied for the trace of moisture retained; in the second series, the silver was dried at incipient red heat.

The composition of cupric bromide was next investigated, the salt being prepared by oxidizing electrolytic copper and dissolving the oxide in hydrobromic acid. Weighed portions of the solution were electrolyzed to determine the copper, and other portions were precipitated with silver nitrate to estimate the bromine. Four series of determinations of the ratio 2AgBr: Cu were made, the first three being of a preliminary character. In the more accurate fourth series, the cupric bromide was titrated against silver, and the silver bromide collected and weighed. The results obtained were:

1. 2AgBr: Cu = 100: 16.927, whence Cu = 63.576.*
2. 2AgBr: Cu = 100: 16.919, whence Cu = 63.546.*
3. 2AgBr: Cu = 100: 16.922, whence Cu = 63.558.*
4. 2AgBr: Cu = 100: 16.927, whence Cu = 63.576.**
   Cu: 2Ag = 100: 339.392, whence Cu = 63.573.**

The remainder of Richards's work on copper by chemical methods concerned the composition of cupric oxide and cupric sulphate, and was undertaken principally with the object of investigating the nature and magnitude of the errors of earlier workers. Cupric oxide prepared from the nitrate by ignition was found to contain occluded gases, mainly nitrogen. The impossibility of avoiding partial decomposition in the preparation of cupric sulphate free from water by dehydration of the pentahydrate was also demonstrated. At 260° C. the salt retains more than 0.1 per cent, of water, and at 400° C. 0.042 per cent. These errors vitiated the results of most of the early workers on the atomic weight of copper, and explain the low values found by Hampe.

In the investigation of Richards they were evaluated, and the necessary corrections applied. Both the water of crystallization and the copper in the pentahydrate of pure copper sulphate were determined. In some of the experiments, after electrolytic deposition of the copper, the sulphuric acid produced in the electrolysis was titrated with pure sodium carbonate, and the sodium sulphate formed was fused and weighed. The results were:

CuSO₄,5H₂O: Cu = 100: 25.451, whence Cu = 63.548.*
CuSO₄: Cu = 100: 39.832, whence Cu = 63.597.
Cu: Na₂CO₃ = 100: 166.838, whence Cu = 63.532.*
Cu: Na₂SO₄ = 100: 223.525, whence Cu = 63.553.*

In one experiment the sulphate was precipitated with barium chloride, and the barium sulphate weighed; but the result is of little value.

Richards also carried out three syntheses of cupric sulphate from the oxide, and two from the metal, deriving the ratios

CuSO₄: CuO = 100: 49.838, whence Cu = 63.548.*
CuSO₄: Cu = 100: 39.7835, whence Cu = 63.468.*

Five analyses of cupric oxide by reduction in hydrogen furnished the ratio

Cu: O = 79.900: 20.100, whence Cu = 63.602.

In 1906 Murmann examined the composition of cupric oxide, but his results are of doubtful value. Copper oxide formed by heating the metal in oxygen was reduced by hydrogen, and values for the atomic weight varying between 63.513 and 64.397 were obtained. Brauner regards the reduction-method as conducive of high values, but from the results of three oxidations he has calculated that Cu = 63.53.

In 1913 de Coninck and Ducelliez converted metallic copper into the nitrate, and ignited the salt to oxide, their value being Cu = 63.55.

This method consists in determining the electrochemical equivalent of copper with respect to that of silver. It involves passing the same quantity of electricity through solutions of cupric sulphate and silver nitrate, and weighing the metal deposited on the cathodes. The early researches of

1. Rayleigh and Mrs. Sidgwick,
2. Gray,
3. Shaw, and
4. Vanni gave the results

1. Cu:2Ag = 100:340.561, whence Cu = 63.354
2. Cu:2Ag = 100:340.935, whence Cu = 63.285
3. Cu:2Ag = 100:339.953, whence Cu = 63.468
   Cu:2Ag = 100:339.983, whence Cu = 63.462
4. Cu:2Ag = 100:340.406, whence Cu = 63.383


During an elaborative investigation of the electrochemical equivalent of copper between the years 1899 and 1902, Richards, Collins, and Heimrod identified two sources of error


1. Metallic copper slowly dissolves in an acid solution of cupric sulphate, the losses in weights of the electrodes when immersed in relatively large volumes of solution being approximately proportional to their surface-areas.
2. Plates of copper covered by neutral solutions of cupric sulphate always gain weight, becoming coated with a film of cuprous oxide.
The influence of the first factor was diminished by working at a low temperature in an atmosphere of hydrogen. The employment of two copper coulometers in series fitted with cathodes of different surface-areas, and the determination of the slight difference between the weights of the copper deposits, made it possible to extrapolate for the increase in weight of a cathode of zero-area, and thus to eliminate the solution-error.

The second difficulty was avoided by electrolyzing slightly acidic solutions of cupric sulphate.

A series of seventeen experiments gave the ratio

Cu: 2Ag = 100: 339.615.

Subsequent investigation by Richards and Heimrod of the silver coulometer employed, proved the " silver " in the deposits to be too heavy by 0.059 per cent. The corrected ratio is

Cu: 2Ag = 100: 339.415, whence Cu = 63.568.**

The mean of four somewhat discordant results obtained by Gallo in 1905 gave the ratio

Cu: 2Ag = 100: 339.483, whence Cu = 63.555.

In 1912 Pecheux found Cu = 63.43.

In a very careful redetermination of the electrochemical equivalent of copper by Shrimpton in 1914, the errors indicated by Richards, Collins, and Heimrod were determined, and due allowance made. The resulting ratio was

Cu: 2Ag = 100: 339.443, whence Cu = 63.563.

The numerous ratios established by Richards and his collaborators leave little doubt as to the atomic weight of copper. The five ratios considered most reliable by Richards are indicated by two asterisks (**), other ratios of a confirmatory nature being denoted by a single asterisk (*). Since the results with two asterisks vary only between Cu = 63.568 and Cu = 63.576, the atomic weight of copper must approximate closely to

Cu = 63.57,

the value given in the current table of the International Committee on Atomic Weights. The results obtained by Gallo, and by de Coninck and Ducelliez, accord reasonably with this number; and those of Shrimpton are in good agreement.

Calculating the foregoing results on the basis Ag = 107.883, Brauner has arrived at the value Cu = 63.56 from ratios considered by him most trustworthy.