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Atomic Weight of Silver, history






The value originally accepted by Berzelius for the atomic weight of silver was about four times the modern number; at a later period he changed it to about twice the modern atomic weight, 215.94 = 2×107.97 (O = 16). In 1808 Dalton preferred the value Ag = 100 (O = 7); in 1817 Meinecke selected Ag = 108; and in 1826 Gmelin gave the same value for the equivalent of the metal.

In the section on the atomic weight of sodium mention is made of the close relationship between the atomic weight of this element and the atomic weights of silver, potassium, chlorine, bromine, and iodine. In the sections on the atomic weights of sodium and potassium an outline of the general principles underlying the methods adopted by the earlier investigators in determining the atomic weights of these elements is given, and the experimental results of a number of researches are summarized. Here it will suffice to describe the determination of ratios not previously mentioned. As with sodium and potassium, the experimental work naturally falls into two main divisions
  1. that of the early investigators, and
  2. that carried out under the extremely accurate conditions required of modern atomic-weight research.
For silver sulphate Stas found the precentage of silver to be 69.203±0.0012, and Struve's result was 69.230±0.004. According to Clarke, the weighted mean is

Ag2SO4: 2Ag = 100: 69.205±0.0011.

Computation of the atomic weight of silver from the data cited involves a knowledge of the mean values for the ratios KCl: 30, KBr: 30, KI: 30, NaCl: 30, Ag: KCl, Ag: KBr, Ag: KI, and Ag: NaCl, as determined by the early workers. These values are given in the sections on the atomic weights of sodium and potassium. The two early determinations of the ratios RX: AgX lack accuracy, and are omitted.

Despite the agreement of these results, they are now known to be very erroneous, although the value Ag = 107.93 was given in the International Atomic Weight Committee's table down to 1908. The probability of the correct result being nearer to 107.89 or 107.88 was inferred by Guye in 1905 as a necessary consequence of changing the number representing the atomic weight of nitrogen from 14.04 to the modern value 14.01. Recent research has verified the accuracy of this assumption.

In 1907 Richards and Forbes effected the synthesis of silver nitrate from silver and nitric acid. Minute precautions to ensure the purity of the silver nitrate were taken, and the mean of six experimental results differing only by one unit in the third decimal place gave

Ag: NO3 = 100: 57.479,

allowance being made for traces of water, ammonium nitrate, and occluded air in the silver nitrate. Taking 14.008 as the atomic weight of nitrogen, determined directly with respect to oxygen, the ratio gives Ag = 107.879.

This value was confirmed in 1909, the work of Richards, Kothner, and Tiede establishing the ratio

AgCl: NH4Cl = 100: 37.3217.

This result is best utilized in conjunction with the ratio last cited and that of Richards and Wells:

Ag: AgCl = 100: 132.867.

Taking oxygen as the standard, and assuming with Morley that H = 1.00762, there are three equations for deducing by simple algebra the atomic weights of silver, chlorine, and nitrogen, the result being Ag = 107.880. Accepting Noyes's value H = 1.0078, then Ag = 107.878. The modern value for the atomic weight of silver is based mainly on these results.

The old value for silver was derived from numerous ratios. Some of them have been redetermined in the light of modern knowledge, examples being KClO3: KCl, KCl: Ag, and KCl: AgCl. The first of these values was measured bv Stabler and Meyer in 1911, and the others by Richards and Stahler in 1907:

  1. KCl: KClO3 = 100: 164.382;
  2. AgCl: KCl = 100: 52.0118;
  3. Ag: KCl = 100: 69.1073.


Hence,

(Ag/AgCl)×(AgCl/KCl) = Ag/KCl = 100/69.1064

In conjunction with that numbered (3), this ratio gives the mean value Ag:KCl = 100: 69.1069.. Then

(3O/KCl)×(KCl/Ag) = 3O/Ag = 0.64382 X 0.691069 = 0.444924 = 48:107.884;

or

Ag = 107.884.

A similar method of calculation can be applied to the results of Richards and Willards's analyses of lithium chloride and lithium perchlorate, as indicated in connexion with the atomic weight of lithium; it gives Ag = 107.871.

Another calculation of the atomic weight of silver can be made from modern determinations of the ratios 2Ag: I2O5 and Ag:I:



The iodine-pentoxide ratio was very carefully determined by Baxter and Tilley in 1909 to be

I2O5: 2Ag = 100: 64.6230,

and the composition of silver iodide was found by Baxter 2 in 1910 to be

Ag:I = 100:117.6601.

Combination of the two ratios gives the value Ag = 107.864.

In 1913 Scheuer dissolved silver in sulphuric acid, collected and weighed the sulphur dioxide evolved, and dried, fused, and weighed the silver sulphate:

2Ag + 2H2SO4 = Ag2SO4 + SO2 + 2H2O.

The weights are in the ratios 2Ag: Ag2SO4: SO2, and the atomic weight of silver is readily calculated, since



The mean of five experimental results gave Ag = 107.884.

The modern values for the atomic weight of silver vary between 107.864 and 107.884. The current table of the International Committee on Atomic Weights gives Ag = 107.88, but possibly 107.87 is a better approximation to the true value, as has been pointed out by Guye. As an essential factor in the calculation of many other atomic weights, the atomic weight of silver is of fundamental importance.


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