- Email: [email protected]
Calcium
CHAPTER 14 Calcium
Peridotites and dunites, the earliest rocks to crystallise, contain only small amounts of calcium which, with the precipitation of olivine and enstatite, tends to be concentrated in the remaining melt.
The succeeding stages of magmatic emplacement
involve the crystallisation of much of the monoclinic pyroxenes and the calcium-rich felspars, giving rocks containing a great deal more calcium and culminating in the gravity separation of anorthosites which may contain up to 20 per cent CaO.
The
residual magma is depleted in calcium and the succeeding rocks contain successively less.
Late stage granites contain only small amounts. Gravimetric Determination of Calcium
After the removal of iron, aluminium and other elements of the ammonia group, aiy calcium present in the filtrate can be precipitated as oxalate, accompanied by much of the small amount of strontium found in most silicate rocks.
In the classical
procedure for determining calcium, the first oxalate precipitate is redissolved in dilute hydrochloric acid and then re-precipitated from a smaller volume of solution. This gives a precipitate that is almost completely free from magnesium and manganese (1' that can be ignited to oxide in a platinum crucible. After ignition, calcium oxide tends to increase in weight by absorption of water and carbon dioxide.
This does not usually give rise to any serious error if cooled in a
desiccator and weighed without undue delay, but can be avoided by igniting the precipitate at a temperature of only 500 , converting the oxalate to carbonate, in which form it is weighed. Although it is customary to remove iron, aluminium, titanium and phosphorus before precipitating calcium this is not absolutely necessary as calcium oxalate can be precipitated quantitatively from a weakly acid solution containing citric or other organic acid preventing precipitation of the ammonia group elements.
Manganese
interferes and difficulties also occur if the rock material is rich in magnesium or titanium. In the classical scheme of rock analysis the ignited calcium oxalate precipitates were used for the determination of strontium, and a correction was then applied to obtain the "true" calcium content.
However, none of the chemical methods gives a perfect
-117-
118
Chemical Methods of Rock Analysis
separation and values for calcium are likely to be almost as much in error after the correction as before.
The most frequently used method for this separation was
based upon the solubility of calcium nitrate in concentrated nitric acid.
Strontium
nitrate is relatively insoluble and can be collected and weighed on a sintered glass or silica crucible and subtracted from the total weight of oxides.
This gravimetric
procedure for the determination of strontium (and hence correction of the gravimetric calcium value), has now been displaced by flame photometric and atomic absorption methods that do not require any extensive separation stages. As an alternative to the gravimetric measurement the calcium oxalate can be dissolved in dilute sulphuric acid and the liberated oxalic acid titrated with standard potassium permanganate solution (see below).
The small amount of strontium present
in silicate rocks and collected largely in the oxalate precipitate will also be counted as calcium. Method Reagents:
Ammonium oxalate wash solution, dissolve 1 g of reagent in 500 ml of water and make just alkaline to methyl red indicator. Potassium permanganate 0.1 N solution, standardise by titration with sodium oxalate or arsenious oxide.
Procedure.
Precipitate the calcium as oxalate as described in Chapter 3·
Collect the precipitate on a close-textured filter paper and wash with the appropriate wash solution as described.
Dissolve the calcium oxalate from the filter with a
small amount of hot dilute hydrochloric acid. to a temperature of 60-70
Add 100 ml of 3 N sulphuric acid, heat
and titrate with standard 0.1 N potassium permanganate
solution. 1 ml of 0.1 N potassium permanganate is equivalent to 2.80 mg calcium oxide, giving a titration of about 36 ml for a 1 g portion of silicate rock containing 10 per cent CaO.
Thus for anorthosites and similar rocks containing greater amounts of calcium,
0.5 g sample portions should be taken for the determination.
For carbonate rocks a
1 g sample weight should be used, and the rock solution diluted to volume in a 250-ml volumetric flask.
A 50-ml aliquot can then be taken for the precipitation of
calcium and subsequent titration as described below. Titrimetric Determination of Calcium with EDTA Ethylenediaminetetraacetic acid forms complexes with most metals and cannot be used
Calcium
119
for the titrimetric determination of calcium unless special precautions are taken to avoid interference from trivalent and other divalent elements.
In the analysis
of silicate rocks this interference is largely from iron, aluminium, manganese and magnesium.
Iron and aluminium can be precipitated with ammonia, but traces of aluminium
and some part of the manganese can then always be recovered in the filtrate.
Small
amounts of both calcium and magnesium are usually co-precipitated with the ammonia precipitate, but these can be recovered in a subsequent filtrate following re-precip itation with ammonia.
Alternatively iron and aluminium can be removed from the rock
solution by extraction of the complexes with 8-hydroxyquinoline into chloroform as (2) described by Cluley for the analysis of glass. The interference from any remaining traces of iron and, aluminium can be considerably reduced by adding triethanolamine. In order to titrate calcium in the presence of magnesium a pH of about 12 is used; at this pH magnesium is precipitated as hydroxide and does not seriously interfere. A detailed procedure for this determination is given in the chapter dealing with magnesium where it is combined with the titrimetric determination of calcium plus magnesium in order to obtain values for both elements. Although suitable for routine analysis this procedure, in common with many others that have been suggested, is subject to certain errors.
The end-point of the calcium
determination is particularly difficult to determine, especially in the presence of manganese or iron which affect the indicator, even with the addition of triethanolamine as complexing agent.
Only small amounts of ammonium salts can be tolerated, as these
prevent the complete precipitation of magnesium, which is then titrated with the calcium.
Certain indicators cannot be used in the presence of magnesium, although
these are undoubtedly the best for pure calcium solutions.
In the absence of
magnesium, as for example in certain limestones and marbles, acid alizarin black SN (mordant black 25, C.I. 21725)» with naphthol green B, end-points.
(3 k)
metalphthalein (phthalein complexone) screened
and methyl thymol blue
all give sharp, easily identified
A procedure for this determination is given below.
The difficulties that arise with indicators in the presence of manganese are particularly acute with high concentrations of this element. be oxidised and precipitated as MnO p with potassium bromate.
When so present it can (7)
An alternative approach to the determination of both calcium and magnesium in silicate rocks is that based upon ion-exchange separation from all other interfering elements and from eath other.
Once this separation has been made there is no difficulty in
determining calcium, as a somewhat lower pH of about 10-10.5 can be used with erichrome black T as indicator for both calcium and magnesium.
This procedure, devised by
120
Chemical Methods of Rock Analysis /ON
Abdullah and Riley,
takes several days for the complete separation, but most of
this time can be used for other determinations. Determination of Calcium in Carbonate Rocks (Low in Magnesium) In this procedure any calcium present in the acid-insoluble fraction is discarded, and only the soluble calcium titrated.
Up to about *f per cent MgO can be tolerated.
Acid-soluble iron, aluminium and other metals are not likely to be present in more than trace amounts, and these traces can be complexed by the addition of potassium cyanide and triethanolamine. Method Reagents;
EDTA 0.02 M standard solution, dissolve 7.*f g of the disodium salt of EDTA in 1 litre of water and standardise by titration using standard calcium solution. Triethanolamine-potassium cyanide (care - POISON) solution, dissolve 6.^ g of potassium cyanide in 60 ml of water and mix with kO ml of triethanolamine. Hydroxylamine hydrochloride solution, dissolve 10 g of the reagent in 100 ml of water. Sodium hydroxide solution, dissolve 30 g of the reagent in water and dilute to 100 ml. Acid alizarin black SN indicator, grind together 0.2 g of the reagent with 10 g of sodium chloride. Standard calcium solution, dissolve 0.500 g of pure, dry calcium carbonate in the minimum amount of dilute hydrochloric acid, transfer to a 500-ml volumetric flask and dilute to volume with water.
Procedure.
Accurately weigh approximately 0.5 g of the finely powdered limestone
rock into a *K)0-ml beaker of the "tall" or "conical" pattern, and moisten with water. Cover the beaker with a clock glass and add dilute perchloric acid down the side of the beaker, until all solid material has dissolved, avoiding an excess. Boil the solution to expel carbon dioxide, allow to cool and dilute with water to volume in a 500-ml volumetric flask.
If any residue remains, collect this on a filter paper, wash
with water and transfer the combined filtrate and washings to the 500-ml volumetric flask before dilution to volume with water.
Discard the residue.
Calcium
121
Pipette 50 ml of this limestone solution into a 250-ml conical flask, add 5 ml of hydroxylamine hydrochloride solution followed by 5 ml of the triethanolamine-cyanide solution (NB: use a measuring cylinder!), 10 ml of the sodium hydroxide solution and enough of the indicator to give a reasonably strong red-to-purple colour to the solution.
Titrate with standard EDTA solution until the indicator is pure blue with
no trace of a pink colour. Photometric Determination of Calcium Very few reagents are known that give colour reactions which are specific or even selective for the calcium ion.
One of the most interesting of these few is calcichrome,
believed to be cyclo-tris-7-(1-aso-8-naphthalene-3:6-disulphonic acid) used as an (9) indicator for the titration of calcium with EDTA. This reagent has been used for the photometric determination of calcium,
but does not appear to have been
applied to this determination in silicate or carbonate rocks, possibly because of interference from magnesium.
Murexide (ammonium purpurate) and glyoxal bis(2-hydroxy-
anil) have also been suggested as photometric reagents for calcium, but also do not appear to have been used for rock analysis. Leonard (11) has, however, used glyoxal bis(2-hydroxyanil) for determining calcium in magnesium carbonate and his method can probably be adapted for use with magnesites. Determination of Calcium by Flame Photometry The spectrum obtained when calcium salts are aspirated into a suitable flame is relatively simple, consisting of a resonance line at 422.7 nm and band systems with maxima at wavelengths of 5^4* 6θ6 and 622 nm.
There is further emission in the near
infrared, and a doublet at 393/97 nm due to calcium ions present in high temperature flames.
Sodium interferes with the determination of calcium by contributing to the
background emission at the wavelength of the resonance line, but this can be overcome by using a recording instrument and tracing the emission from about *f10 nm to kkO nm. At high concentration the alkali elements also interfere by reducing the calcium emission, but this type of effect is a more serious problem in the presence of iron, aluminium, sulphate and phosphate.
These elements form compounds with calcium,
particularly in low-temperature flames.
This interference can be completely
prevented by separating the calcium by precipitation as oxalate, as in the classical procedure.
As a double precipitation of the ammonia group elements is necessary,
this method is long and tedious.
An alternative rapid procedure is to add an excess
of each of the interfering elements to both the rock solution and the calcium standard
122
Chemical Methods of Rock Analysis
solutions.
The amounts added are such as to give limiting calcium suppression. This (12) for the determination of calcium in silicate
technique has been reported by Kramer rocks and minerals.
Determination of Calcium by Atomic Absorption Spectroacopy As with flame emission photometry, interference with the determination of calcium arises from the presence of aluminium, iron and other elements that form compounds with calcium in the flame.
This interference is very much less than that recorded
with emission photometry, and can be reduced still further by using a nitrous oxideacetylene flame. Under these conditions, the only serious interference with the determination in silicate rocks is from aluminium. potassium to the solution.
This can be overcome by adding
The determination of calcium by this technique is usually
combined with that of magnesium and one rock solution can be prepared for both determinations, see chapter dealing with magnesium. For a review of the mutual interferences between the atomic absorption determination (13) of calcium and some other elements, see Harrison and Ottaway. References 1.
JEFFERY P G and WILSON A D, Analyst (1959) 84, 663
2.
CLULEY H J, Analyst (195*0 79, %1
3.
BELCHER R, CLOSE R A and WEST T S, Chemist Analyst (1958) 47, 2
4.
BELCHER R, CLOSE R A and WEST T S, Talanta (1958) J_, 238
5.
TUCKER B M, J. Austr. Inst. Agr. Sei. (1955) Ü , 100
6.
KORBL J and PRIBIL R, Chem. and Ind. (1957) p.233
7.
BORISSOVA-PANGAROVA R and MITROPOLITSKA E, Dokl. Bolg. Akad. Nauk, (1977)
8.
ABDULLAH M I and RILEY J P, Anal. Chim. Acta (1965) 33, 391
30, 395 9.
CLOSE R A and WEST T S, Talanta (i960) 5.» 221
10.
HERRERO-LANCINA M and WEST T S, Analyt. Chem. (1963) ,35, 2131
11.
LEONARD M A, J. Pharm. Pharmacol. (1962) _l4 (suppl.) 63T
12.
KRAMER H, Anal. Chim. Acta (1957) V_, 521
13.
HARRISON A and OTTAWAY J M, Proc. Soc. Analyt. Chem. (1972) £, 205
Peridotites and dunites, the earliest rocks to crystallise, contain only small amounts of calcium which, with the precipitation of olivine and enstatite, tends to be concentrated in the remaining melt.
The succeeding stages of magmatic emplacement
involve the crystallisation of much of the monoclinic pyroxenes and the calcium-rich felspars, giving rocks containing a great deal more calcium and culminating in the gravity separation of anorthosites which may contain up to 20 per cent CaO.
The
residual magma is depleted in calcium and the succeeding rocks contain successively less.
Late stage granites contain only small amounts. Gravimetric Determination of Calcium
After the removal of iron, aluminium and other elements of the ammonia group, aiy calcium present in the filtrate can be precipitated as oxalate, accompanied by much of the small amount of strontium found in most silicate rocks.
In the classical
procedure for determining calcium, the first oxalate precipitate is redissolved in dilute hydrochloric acid and then re-precipitated from a smaller volume of solution. This gives a precipitate that is almost completely free from magnesium and manganese (1' that can be ignited to oxide in a platinum crucible. After ignition, calcium oxide tends to increase in weight by absorption of water and carbon dioxide.
This does not usually give rise to any serious error if cooled in a
desiccator and weighed without undue delay, but can be avoided by igniting the precipitate at a temperature of only 500 , converting the oxalate to carbonate, in which form it is weighed. Although it is customary to remove iron, aluminium, titanium and phosphorus before precipitating calcium this is not absolutely necessary as calcium oxalate can be precipitated quantitatively from a weakly acid solution containing citric or other organic acid preventing precipitation of the ammonia group elements.
Manganese
interferes and difficulties also occur if the rock material is rich in magnesium or titanium. In the classical scheme of rock analysis the ignited calcium oxalate precipitates were used for the determination of strontium, and a correction was then applied to obtain the "true" calcium content.
However, none of the chemical methods gives a perfect
-117-
118
Chemical Methods of Rock Analysis
separation and values for calcium are likely to be almost as much in error after the correction as before.
The most frequently used method for this separation was
based upon the solubility of calcium nitrate in concentrated nitric acid.
Strontium
nitrate is relatively insoluble and can be collected and weighed on a sintered glass or silica crucible and subtracted from the total weight of oxides.
This gravimetric
procedure for the determination of strontium (and hence correction of the gravimetric calcium value), has now been displaced by flame photometric and atomic absorption methods that do not require any extensive separation stages. As an alternative to the gravimetric measurement the calcium oxalate can be dissolved in dilute sulphuric acid and the liberated oxalic acid titrated with standard potassium permanganate solution (see below).
The small amount of strontium present
in silicate rocks and collected largely in the oxalate precipitate will also be counted as calcium. Method Reagents:
Ammonium oxalate wash solution, dissolve 1 g of reagent in 500 ml of water and make just alkaline to methyl red indicator. Potassium permanganate 0.1 N solution, standardise by titration with sodium oxalate or arsenious oxide.
Procedure.
Precipitate the calcium as oxalate as described in Chapter 3·
Collect the precipitate on a close-textured filter paper and wash with the appropriate wash solution as described.
Dissolve the calcium oxalate from the filter with a
small amount of hot dilute hydrochloric acid. to a temperature of 60-70
Add 100 ml of 3 N sulphuric acid, heat
and titrate with standard 0.1 N potassium permanganate
solution. 1 ml of 0.1 N potassium permanganate is equivalent to 2.80 mg calcium oxide, giving a titration of about 36 ml for a 1 g portion of silicate rock containing 10 per cent CaO.
Thus for anorthosites and similar rocks containing greater amounts of calcium,
0.5 g sample portions should be taken for the determination.
For carbonate rocks a
1 g sample weight should be used, and the rock solution diluted to volume in a 250-ml volumetric flask.
A 50-ml aliquot can then be taken for the precipitation of
calcium and subsequent titration as described below. Titrimetric Determination of Calcium with EDTA Ethylenediaminetetraacetic acid forms complexes with most metals and cannot be used
Calcium
119
for the titrimetric determination of calcium unless special precautions are taken to avoid interference from trivalent and other divalent elements.
In the analysis
of silicate rocks this interference is largely from iron, aluminium, manganese and magnesium.
Iron and aluminium can be precipitated with ammonia, but traces of aluminium
and some part of the manganese can then always be recovered in the filtrate.
Small
amounts of both calcium and magnesium are usually co-precipitated with the ammonia precipitate, but these can be recovered in a subsequent filtrate following re-precip itation with ammonia.
Alternatively iron and aluminium can be removed from the rock
solution by extraction of the complexes with 8-hydroxyquinoline into chloroform as (2) described by Cluley for the analysis of glass. The interference from any remaining traces of iron and, aluminium can be considerably reduced by adding triethanolamine. In order to titrate calcium in the presence of magnesium a pH of about 12 is used; at this pH magnesium is precipitated as hydroxide and does not seriously interfere. A detailed procedure for this determination is given in the chapter dealing with magnesium where it is combined with the titrimetric determination of calcium plus magnesium in order to obtain values for both elements. Although suitable for routine analysis this procedure, in common with many others that have been suggested, is subject to certain errors.
The end-point of the calcium
determination is particularly difficult to determine, especially in the presence of manganese or iron which affect the indicator, even with the addition of triethanolamine as complexing agent.
Only small amounts of ammonium salts can be tolerated, as these
prevent the complete precipitation of magnesium, which is then titrated with the calcium.
Certain indicators cannot be used in the presence of magnesium, although
these are undoubtedly the best for pure calcium solutions.
In the absence of
magnesium, as for example in certain limestones and marbles, acid alizarin black SN (mordant black 25, C.I. 21725)» with naphthol green B, end-points.
(3 k)
metalphthalein (phthalein complexone) screened
and methyl thymol blue
all give sharp, easily identified
A procedure for this determination is given below.
The difficulties that arise with indicators in the presence of manganese are particularly acute with high concentrations of this element. be oxidised and precipitated as MnO p with potassium bromate.
When so present it can (7)
An alternative approach to the determination of both calcium and magnesium in silicate rocks is that based upon ion-exchange separation from all other interfering elements and from eath other.
Once this separation has been made there is no difficulty in
determining calcium, as a somewhat lower pH of about 10-10.5 can be used with erichrome black T as indicator for both calcium and magnesium.
This procedure, devised by
120
Chemical Methods of Rock Analysis /ON
Abdullah and Riley,
takes several days for the complete separation, but most of
this time can be used for other determinations. Determination of Calcium in Carbonate Rocks (Low in Magnesium) In this procedure any calcium present in the acid-insoluble fraction is discarded, and only the soluble calcium titrated.
Up to about *f per cent MgO can be tolerated.
Acid-soluble iron, aluminium and other metals are not likely to be present in more than trace amounts, and these traces can be complexed by the addition of potassium cyanide and triethanolamine. Method Reagents;
EDTA 0.02 M standard solution, dissolve 7.*f g of the disodium salt of EDTA in 1 litre of water and standardise by titration using standard calcium solution. Triethanolamine-potassium cyanide (care - POISON) solution, dissolve 6.^ g of potassium cyanide in 60 ml of water and mix with kO ml of triethanolamine. Hydroxylamine hydrochloride solution, dissolve 10 g of the reagent in 100 ml of water. Sodium hydroxide solution, dissolve 30 g of the reagent in water and dilute to 100 ml. Acid alizarin black SN indicator, grind together 0.2 g of the reagent with 10 g of sodium chloride. Standard calcium solution, dissolve 0.500 g of pure, dry calcium carbonate in the minimum amount of dilute hydrochloric acid, transfer to a 500-ml volumetric flask and dilute to volume with water.
Procedure.
Accurately weigh approximately 0.5 g of the finely powdered limestone
rock into a *K)0-ml beaker of the "tall" or "conical" pattern, and moisten with water. Cover the beaker with a clock glass and add dilute perchloric acid down the side of the beaker, until all solid material has dissolved, avoiding an excess. Boil the solution to expel carbon dioxide, allow to cool and dilute with water to volume in a 500-ml volumetric flask.
If any residue remains, collect this on a filter paper, wash
with water and transfer the combined filtrate and washings to the 500-ml volumetric flask before dilution to volume with water.
Discard the residue.
Calcium
121
Pipette 50 ml of this limestone solution into a 250-ml conical flask, add 5 ml of hydroxylamine hydrochloride solution followed by 5 ml of the triethanolamine-cyanide solution (NB: use a measuring cylinder!), 10 ml of the sodium hydroxide solution and enough of the indicator to give a reasonably strong red-to-purple colour to the solution.
Titrate with standard EDTA solution until the indicator is pure blue with
no trace of a pink colour. Photometric Determination of Calcium Very few reagents are known that give colour reactions which are specific or even selective for the calcium ion.
One of the most interesting of these few is calcichrome,
believed to be cyclo-tris-7-(1-aso-8-naphthalene-3:6-disulphonic acid) used as an (9) indicator for the titration of calcium with EDTA. This reagent has been used for the photometric determination of calcium,
but does not appear to have been
applied to this determination in silicate or carbonate rocks, possibly because of interference from magnesium.
Murexide (ammonium purpurate) and glyoxal bis(2-hydroxy-
anil) have also been suggested as photometric reagents for calcium, but also do not appear to have been used for rock analysis. Leonard (11) has, however, used glyoxal bis(2-hydroxyanil) for determining calcium in magnesium carbonate and his method can probably be adapted for use with magnesites. Determination of Calcium by Flame Photometry The spectrum obtained when calcium salts are aspirated into a suitable flame is relatively simple, consisting of a resonance line at 422.7 nm and band systems with maxima at wavelengths of 5^4* 6θ6 and 622 nm.
There is further emission in the near
infrared, and a doublet at 393/97 nm due to calcium ions present in high temperature flames.
Sodium interferes with the determination of calcium by contributing to the
background emission at the wavelength of the resonance line, but this can be overcome by using a recording instrument and tracing the emission from about *f10 nm to kkO nm. At high concentration the alkali elements also interfere by reducing the calcium emission, but this type of effect is a more serious problem in the presence of iron, aluminium, sulphate and phosphate.
These elements form compounds with calcium,
particularly in low-temperature flames.
This interference can be completely
prevented by separating the calcium by precipitation as oxalate, as in the classical procedure.
As a double precipitation of the ammonia group elements is necessary,
this method is long and tedious.
An alternative rapid procedure is to add an excess
of each of the interfering elements to both the rock solution and the calcium standard
122
Chemical Methods of Rock Analysis
solutions.
The amounts added are such as to give limiting calcium suppression. This (12) for the determination of calcium in silicate
technique has been reported by Kramer rocks and minerals.
Determination of Calcium by Atomic Absorption Spectroacopy As with flame emission photometry, interference with the determination of calcium arises from the presence of aluminium, iron and other elements that form compounds with calcium in the flame.
This interference is very much less than that recorded
with emission photometry, and can be reduced still further by using a nitrous oxideacetylene flame. Under these conditions, the only serious interference with the determination in silicate rocks is from aluminium. potassium to the solution.
This can be overcome by adding
The determination of calcium by this technique is usually
combined with that of magnesium and one rock solution can be prepared for both determinations, see chapter dealing with magnesium. For a review of the mutual interferences between the atomic absorption determination (13) of calcium and some other elements, see Harrison and Ottaway. References 1.
JEFFERY P G and WILSON A D, Analyst (1959) 84, 663
2.
CLULEY H J, Analyst (195*0 79, %1
3.
BELCHER R, CLOSE R A and WEST T S, Chemist Analyst (1958) 47, 2
4.
BELCHER R, CLOSE R A and WEST T S, Talanta (1958) J_, 238
5.
TUCKER B M, J. Austr. Inst. Agr. Sei. (1955) Ü , 100
6.
KORBL J and PRIBIL R, Chem. and Ind. (1957) p.233
7.
BORISSOVA-PANGAROVA R and MITROPOLITSKA E, Dokl. Bolg. Akad. Nauk, (1977)
8.
ABDULLAH M I and RILEY J P, Anal. Chim. Acta (1965) 33, 391
30, 395 9.
CLOSE R A and WEST T S, Talanta (i960) 5.» 221
10.
HERRERO-LANCINA M and WEST T S, Analyt. Chem. (1963) ,35, 2131
11.
LEONARD M A, J. Pharm. Pharmacol. (1962) _l4 (suppl.) 63T
12.
KRAMER H, Anal. Chim. Acta (1957) V_, 521
13.
HARRISON A and OTTAWAY J M, Proc. Soc. Analyt. Chem. (1972) £, 205