Basicity:
There are the different systems of describing basicity, all expressing in some way or the other the ratio of the amount of chromium Cr or chromic oxide, Cr₂O₃ to the amount of acid or basic radical contained in a given liquor.
The degree if basicity can be measured by either of the following three ways-
The degree if basicity can be measured by either of the following three ways-
- Percentage basicity
- Proctor basicity
- Freiberg basicity
ⅰ. Percentage basicity:
According to Schorlemmer percentage basicity is expressed as a percentage of total chromium.
Percentage basicity is often called as ％ Schorlemmer.
ⅱ. Proctor basicity:
Percentage basicity is often called as ％ Schorlemmer.
ⅱ. Proctor basicity:
Basicity on proctor scale is expressed by the number of parts of acid sulphate radical (SO₄) associated with 52 parts of chromium in the chrome liquor.
According to this system the basicity is expressed as the ratio Cr:SO₄.
To convert Schorlemmer value into proctor basicity the following formula is used-
Basicity(Proctor)= 144- (144× Scholemmer value)÷100
Basicity(Schorlemmer)= {( 144-Proctor)× 100}/144
ⅲ. Freiberg basicity:
According to this system the basicity is expressed as the ratio Cr:SO₄.
To convert Schorlemmer value into proctor basicity the following formula is used-
Basicity(Proctor)= 144- (144× Scholemmer value)÷100
Basicity(Schorlemmer)= {( 144-Proctor)× 100}/144
ⅲ. Freiberg basicity:
Basicity values on this scale is expressed in terms of 1∣12 ths. The extent to which the (SO₄) in normal chromic sulphate has been replaced by (OH) in the basic chromium sulphate.
According to Freiberg scale the normal chromic sulphate is represented by the formula Cr₈(SO₄)₁₂ which is its usual formula Cr₂(SO₄)₃ multiplied by 4.
Basicity(Freiberg)= Schorlemmer value( shown as twelfths)/8.32
An increase of 12 Freiberg basicity corresponds with the decrease of 12 of basicity number of the Proctor system and an increase of 8.32 Schorlemmer scale.
According to Freiberg scale the normal chromic sulphate is represented by the formula Cr₈(SO₄)₁₂ which is its usual formula Cr₂(SO₄)₃ multiplied by 4.
Basicity(Freiberg)= Schorlemmer value( shown as twelfths)/8.32
An increase of 12 Freiberg basicity corresponds with the decrease of 12 of basicity number of the Proctor system and an increase of 8.32 Schorlemmer scale.
Formula for basic chromium sulphate from basicity
⏩ Formula for 40% basic chromium sulphate.
If the valence of chromium was 100 then 45 (OH) groups would be attached to one chromium atom.
But the valence of chromium is three.
So, one chromium atom will be attached to (45 × 3)/100 =27/20 hydroxyl groups.
The chromium is now left with (3/1- 27/20) = 33/20 valence which should be satisfied with divalent (SO₄) group i.e., (33/(20×2))= 33/40 SO₄ group.
So the empirical formula of 45% basic chromium sulphate will be Cr(OH)₂₇/₄₀ (SO₄)₃₃/₄₀ or, Cr₄₀(OH)₅₄(SO₄)₃₃.
⏩ Formula for 40% basic chromium sulphate.
If the valence of chromium was 100 then 45 (OH) groups would be attached to one chromium atom.
But the valence of chromium is three.
So, one chromium atom will be attached to (45 × 3)/100 =12/10 hydroxyl groups.
The chromium is now left with (3/1- 12/10) = 18/10 valence which should be satisfied with divalent (SO₄) group i.e., (18/(10×2))= 18/20 SO₄ group.
So the empirical formula of 45% basic chromium sulphate will be Cr(OH)₁₂/₁₀ (SO₄)₁₈/₂₀ or, Cr₁₀(OH)₁₂(SO₄)₉.
If the valence of chromium was 100 then 45 (OH) groups would be attached to one chromium atom.
But the valence of chromium is three.
So, one chromium atom will be attached to (45 × 3)/100 =27/20 hydroxyl groups.
The chromium is now left with (3/1- 27/20) = 33/20 valence which should be satisfied with divalent (SO₄) group i.e., (33/(20×2))= 33/40 SO₄ group.
So the empirical formula of 45% basic chromium sulphate will be Cr(OH)₂₇/₄₀ (SO₄)₃₃/₄₀ or, Cr₄₀(OH)₅₄(SO₄)₃₃.
⏩ Formula for 40% basic chromium sulphate.
If the valence of chromium was 100 then 45 (OH) groups would be attached to one chromium atom.
But the valence of chromium is three.
So, one chromium atom will be attached to (45 × 3)/100 =12/10 hydroxyl groups.
The chromium is now left with (3/1- 12/10) = 18/10 valence which should be satisfied with divalent (SO₄) group i.e., (18/(10×2))= 18/20 SO₄ group.
So the empirical formula of 45% basic chromium sulphate will be Cr(OH)₁₂/₁₀ (SO₄)₁₈/₂₀ or, Cr₁₀(OH)₁₂(SO₄)₉.
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