Chemical elements
  Iron
    History of Iron
    Mineralogy
    Isotopes
    Energy
    Production
    Application
    Physical Properties
      Allotropy
      Occlusion of Gases
      Absorption of Nascent Hydrogen
      Permeability to Gases
      Passivity of Iron
      Iron Powder
      Iron sponge
      Iron amalgam
      Colloidal Iron
      Pyrophoric Iron
      Catalyst
      Iron Ions
      Atomic Weight
    Chemical Properties
    Corrosion
    Iron Salts
    PDB 101m-1aeb
    PDB 1aed-1awd
    PDB 1awp-1beq
    PDB 1bes-1c53
    PDB 1c6o-1ci6
    PDB 1cie-1cry
    PDB 1csu-1dfx
    PDB 1dgb-1dry
    PDB 1ds1-1e08
    PDB 1e0z-1ehj
    PDB 1ehk-1f5o
    PDB 1f5p-1fnp
    PDB 1fnq-1fzi
    PDB 1g08-1gnl
    PDB 1gnt-1h43
    PDB 1h44-1hdb
    PDB 1hds-1i5u
    PDB 1i6d-1iwh
    PDB 1iwi-1jgx
    PDB 1jgy-1k2o
    PDB 1k2r-1kw6
    PDB 1kw8-1lj0
    PDB 1lj1-1m2m
    PDB 1m34-1mko
    PDB 1mkq-1mun
    PDB 1muy-1n9x
    PDB 1naz-1nx4
    PDB 1nx7-1ofe
    PDB 1off-1p3t
    PDB 1p3u-1pmb
    PDB 1po3-1qmq
    PDB 1qn0-1ra0
    PDB 1ra5-1rxg
    PDB 1ry5-1smi
    PDB 1smj-1t71
    PDB 1t85-1u8v
    PDB 1u9m-1uyu
    PDB 1uzr-1vxf
    PDB 1vxg-1wri
    PDB 1wtf-1xlq
    PDB 1xm8-1y4r
    PDB 1y4t-1ygd
    PDB 1yge-1z01
    PDB 1z02-2a9e
    PDB 2aa1-2azq
    PDB 2b0z-2boz
    PDB 2bpb-2ca3
    PDB 2ca4-2cz7
    PDB 2czs-2dyr
    PDB 2dys-2ewk
    PDB 2ewu-2fwl
    PDB 2fwt-2gl3
    PDB 2gln-2hhb
    PDB 2hhd-2ibn
    PDB 2ibz-2jb8
    PDB 2jbl-2mgh
    PDB 2mgi-2o01
    PDB 2o08-2ozy
    PDB 2p0b-2q0i
    PDB 2q0j-2r1h
    PDB 2r1k-2spm
    PDB 2spn-2vbd
    PDB 2vbp-2vzb
    PDB 2vzm-2wiv
    PDB 2wiy-2xj5
    PDB 2xj6-2ylj
    PDB 2yrs-2zon
    PDB 2zoo-3a17
    PDB 3a18-3aes
    PDB 3aet-3bnd
    PDB 3bne-3cir
    PDB 3ciu-3dax
    PDB 3dbg-3e1p
    PDB 3e1q-3eh4
    PDB 3eh5-3fll
    PDB 3fm1-3gas
    PDB 3gb4-3h57
    PDB 3h58-3hrw
    PDB 3hsn-3ir6
    PDB 3ir7-3k9y
    PDB 3k9z-3l4p
    PDB 3l61-3lxi
    PDB 3lyq-3mm8
    PDB 3mm9-3n62
    PDB 3n63-3nlo
    PDB 3nlp-3o0f
    PDB 3o0r-3p6o
    PDB 3p6p-3prq
    PDB 3prr-3sel
    PDB 3sik-3una
    PDB 3unc-4blc
    PDB 4cat-4erg
    PDB 4erm-4nse
    PDB 4pah-8cat
    PDB 8cpp-9nse

Atomic Weight of Iron





Approximate Atomic Weight of Iron

That the atomic weight of iron is approximately 56, and not a multiple or submultiple of this amount, is indicated by various considerations.
  1. The mean specific heat of iron is approximately 0.110. Assuming a mean atomic heat of 6.4, the atomic weight of iron, according to Dulong and Petit's Law, is approximately 58.
  2. As already indicated, iron closely resembles manganese in many of its chemical properties, and the three elements iron, cobalt, and nickel constitute very fitting intermediaries between manganese and copper. Reference to the Periodic Table shows that the only manner in which this relationship can be harmonised with the Periodic Law is to assume that the atomic weights of these three metals lie between 54.93 (the atomic weight of manganese) on the one hand and 63.57 (the atomic weight of copper) on the other.
  3. Ferric sulphate yields, with the sulphates of the alkali metals, a series of well-defined crystalline salts which are isomorphous with similar series yielded by aluminium sulphate. By the application of Mitscherlich's Law, therefore, analogous formulae are to be anticipated, so that the general formula for these iron alums is

    M2SO4.Fe2(SO4)3.24H2O.

    Analyses of these compounds indicate that the atomic weight of iron is 56.


Exact Combining Weight and Atomic Weight of Iron

The early determinations of the atomic weight of iron are of no present value, and little need be said concerning them. Most investigators chose to determine the composition of ferric oxide; Wackenroder, Svanberg and Norlin, Erdmann and Marchand, and Rivot worked by reducing the weighed oxide to metal in a stream of hydrogen, while Berzelius, Maumene, and also Svanberg and Norlin converted a known weight of iron into ferric oxide by oxidation.

The only other early determinations to be mentioned are Dumas' analyses of anhydrous ferrous and ferric chloride, in which the amount of silver required to combine with the chlorine was determined. and a few experiments by Winkler, in which a weighed amount of iron was dissolved in a solution of iodine in potassium iodide, the excess of iodine being determined by titration with sodium thiosulphate. The results were as follow (Cl = 35.457, I = 126.92, Ag = 107.880): -

2Ag:FeCl2:: 100.000: 58.866; Fe = 56.09
Ag:FeCl3:: 100.000: 50.244; Fe = 56.23
I2: Fe:: 100.00: 22.145; Fe = 56.21

Modern work on the atomic weight of iron begins in 1900 with Richards and Baxter's analyses of ferric oxide by reduction to the metal in a stream of hydrogen. Richards and Baxter found that the exact determination of this ratio is a matter of extreme difficulty, and regarded their experiments as preliminary in character. The mean result was as follows (O = 16.000): -

Fe2O3: 2Fe:: 100.000: 69.9576; Fe = 55.887

Three years later Baxter made another determination of the atomic weight of iron, analysing anhydrous ferrous bromide for the purpose. The salt was sublimed in a porcelain tube, which introduced a little sodium bromide into it; due allowance was made for this source of error, and the following results obtained (Ag = 107.880, Br = 79.916): -

FeBr2: 2AgBr:: 57.4195: 100.000; Fe = 55.833
FeBr2: 2Ag:: 99.960: 100.000; Fe = 55.842

At the time these experiments were made an erroneous value for the atomic weight of silver was in use, in consequence of which the value Fe = 55.87, in confirmation of that deduced from the oxide analyses, was deduced from the bromide analyses. With the establishment of the modern value for the atomic weight of silver, it accordingly became desirable to repeat the preceding work in order to determine the source and magnitude of the errors involved in it. Hence, in 1911, Baxter, Thorvaldson, and Cobb repeated the analyses of ferrous bromide, which they were able to obtain quite free from sodium bromide by utilising fused quartz apparatus in its preparation. Their preliminary analyses gave the following results (Ag = 107.880, Br = 79.916): -

FeBr2: 2Ag:: 99.9593: 100.0000; Fe = 55.840
FeBr2: 2AgBr:: 57.4221: 100.0000; Fe = 55.840

and the final experiments gave almost identical results: -

FeBr2: 2Ag:: 99.9583: 100.0000; Fe = 55.838
FeBr2: 2AgBr:: 57.4214: 100.0000; Fe = 55.838

The essential accuracy of Baxter's earlier work on the bromide was thus confirmed.

Baxter and Thorvaldson extended the preceding investigation by making a number of analyses of ferrous bromide prepared from meteoric iron. The results were as follow: -

FeBr2: 2Ag:: 99.9561: 100.0000; Fe = 55.834
FeBr2: 2AgBr:: 57.4191: 100.0000; Fe = 50.829

These results are a trifle lower than the preceding, but in each series of five experiments two are of doubtful value as the result of a modification of the method of analysis; excluding the doubtful analyses, the above two results become Fe = 55.837 and Fe = 55.835 respectively.

The preceding results leaving little doubt that Richards and Baxter's analyses of ferric oxide are affected by a slight, but, nevertheless, appreciable error, Baxter and Hoover undertook a thorough revision of this process. For full details of their work, which, though apparently quite simple, was, in reality, extremely difficult, the reader must be referred to the original memoir. The result was as follows (O = 16.000): -

Fe2O3: 2Fe:: 100.0000: 69.9427; Fe = 55.847

The preceding result requires slight correction. Baxter and Hoover found that at 1050° to 1100° C., the temperature at which they prepared and reduced their ferric oxide, there was a slight doubt as to the stability of the ferric oxide. Five grams of the oxide when ignited to constant weight in oxygen, lost in weight when ignited in air, the loss being one- fifth of a milligram. The subsequent researches of Sosman and Hostetter have shown that this loss in weight is due to the dissociation of ferric oxide into magnetic oxide and oxygen. Thus the higher weight is the correct weight of ferric oxide. Baxter and Hoover, however, chose the lower, regarding the higher weight as due to occluded oxygen. The necessary correction to the atomic weight of iron is — 0.007, and so the corrected value is Fe = 58.840. Of the twelve analyses on which this result is based, seven were made with terrestrial and five with meteoric iron, and no difference in the results was observed. The International Committee for 1920 give the value

Fe = 55.84.
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