Determination of Calcium and Magnesium in Foodstuffs Simultaneous Removal of Iron and Phosphate as Interfering Ions by Ion Exchange W. E. SCHILZ and G. N. KRYNAUW Department o f lnorganic and Analytical Chemistry, University o f Pretoria, Pretoria, Transvaal, Sooth A f r i c a
umns were eluted with 4-V hydrochloric acid. follon-ed by water. This eluate was evaporated to dryness to remove excess hydrochloric acid, the residue was dissolved in water. and the calcium was determined with EDTA, hlurexide being used as indicator.
.4titration method using ethqlenediamine tetraacetate is described for the routine determination of calcium and magnesium in foodstuffs such as bread, enriching mixtures, and grains, in which iron and phosphate occur in interfering amounts. Murexide and Eriochrome Black T are used as indicators for calcium and magnesium, respectively. The iron is converted into an oxalato complex and is removed with the phosphate by means of a cation exchange resin in the hydrogen form in a small column. The calcium and magnesium are eluted with hydrochloric acid and determined in the eluate; the end points are completely satisfactory. The method is simple and much less time-consuming than the oxalate and oxinate methods, and results compare favorably with those obtained by the classical methods.
T
HE determination of calcium and magnesium by disodium
ethylenediamine tetraacetate [(ethylene dinitri1o)tetraacetate, EDTA] provides a rapid and simple method for routine analysis. This method, however, is subject to interference from and the elimination of ions present in interfering many ions (4), amounts becomes a necessity for efficient determinations. The foodstuffs analyzed contained mainly orthophosphate and iron in varying amounts with traces of copper, cobalt, zinc, and manganese as interfering ions. The interference from copper, cobalt, and zinc can be eliminated effectively by the addition of potassium cyanide ( 1 , 3, 4). The manganese remains in solution and is titrated with the EDTA ( 1 )but the error is negligible (9). Methods for the separation of orthophosphate by means of ion exchangers have been described (2,6-12). In the present inveetigation Amberlite IR-112, a medium capacity cation exchange resin of the sulfonic acid type, proved successful in the acid range investigated-pH 2 to 5. The removal of iron was effected simultaneously with the phosphate by converting the iron into an anion complex. Although the elimination of the interference from iron using potassium cyanide and hydroxylamine hydrochloride has been described ( 1 , 4),this method was found to be unsatisfactory, owing to an undesirable color which formed in the solution. The color of the indicator could hardly be discerned, making the determination of the end point practically impossible. The possibility of achieving a quantitative separation of iron from the other metals, among them calcium and magnesium, by percolation of oxalate solutions of the metals through cation exchangers, has been demonstrated by Djurfeldt, Hansen, and Samuelson (6). Following this line of investigation, experiments were carried out with artificial solutions containing calcium and varying amounts of iron. B number of solutions were made up containing 8 mg. of calcium and from 4 to 10 mg. of iron per portion. Each portion was boiled with 1 ml. of hydrogen peroxide to ensure the oxidation of all the iron. Volumes of 5, 10, and 15 ml. of a saturated oxalic acid solution were added to each combination of calcium and iron, and then boiled for a few minutes. After cooling, the solutions were percolated through the cation exchanger, and the resin columns were washed with 0.2M oxalic acid, followed by 0.015N hydrochloric acid to displace the oxalic acid. The col-
The results showed that &-here5- and 10-nil. portions of the saturated oxalic acid solution were added, insufficient complexing agent was present to remove the iron quantitatively even in the smallest amounts present and no satisfactory end point could be obtained. With the 15-ml. portions, however, a practically quantitative separation was effected in all cases and the end point was satisfactory, complete recovery of the cslcium being obtained, as can be seen from Table I. No definite dimensions could be laid dowi for the resin column, as varying amounts of cations accompanied an amount of calcium giving a convenient titer. For rapid determinations the column must be as small as possible, however, within certain limits of safety that ensure the quantitative adsorption of calcium and magnesium. A number of columns with rePiu beds 0.8 cm. in diameter and 10 em. in height were constructed. Since smaller particle size increases the breakthrough capacity of a resin column, the flow rate was adjusted to approximately 2 to 3 ml. per sq. em. per minute by grinding the commercially available resin and using the fraction that passed through the 50-mesh (U. S. number) sieve but was retained by the 120-mesh sieve. Recovery experiments revealed that up to 4 meq. of cations could be percolated through the columns a t a flow rate of 2 to 3 ml. per sq. cm. per minut'e and be adsorbed quantitatively (Table 11). This amount was considered satisfactory for the conditions of use.
Table I. Recovery of Calcium afterIQuarititative Removal of Iron Present, hIg.
Fe 4 6 8 10
Table 11.
Ca
Ca Determined, AI!&
8.00
8.02 s.01 1.98 7.98
8.00 8.00 8.00
Range of Quantitative Recovery of Calcium Percolated through Resin Column
Percolated through Column, RIg. 5.00 10.00 20.00 30.00 40.00 60.00 80.00 100.00
Recovered, AIg. 5.00 10.00 19.9: 30.01 39.98 60.01 80.05 94.50, 93.97 4.95, 10.00, 20.00, 30.03, 39.94, 60.05. 79.98,
The above quantities and dimensions were found to be eatisfactory in the case of the resin used--4mberlite IR-112-and the particular particle size mentioned. When another resin or particle
1759
ANALYTICAL CHEMISTRY
1760 size is used, it may be necessary to adjust the volumes of oxalic acid and 0.015N hydrochloric stipulated in the procedure given below. Analyses on the foostuffs were also carried out by the oxalate and oxinate methods. The results obtained by these methods are compared with those by the direct titration method with EDTA in Tables 111, IV, and V.
Table 111. Calcium a n d Magnesium Content of Different &Ia terials (Determined by gravimetric methods against volumetric method using EDTA as titrating agent. Tabulated results are mean of a number o determinations.) Magnesium Content/Ng. Sample, Mg. Oxalate EDTA Difference Oxinate EDTA Difference
solution into a conical flask of 250-mi. capacity. Add 1 nil. of sodium hydroxide solution and approximately 0.15 gram of Murexide indicator, the latter from a small measure made t o this capacity, Titrate with EDTA solution until the color changes from pink to a definite mauve, using a blank for comparison. Repeat until duplicate values are obtained. Calculate x, where 1 ml. of EDTA solution =
WITHERIOCHROYE BLACKT. Transfer by means of a pipet 10 ml. of the standard calcium chloride solution into a conical flask of 250-ml. capacity. Add 1 ml. of the ammonia-ammonium chloride buffer and 6 drops of Eriochrome Black T indicator. Titrate with EDTA solution until the color changes to a clear blue. Repeat until duplicate values are obtained. Calculate y, where 1 ml. of EDTA solution = y equivalents
Calcium Content/Sample, Material Bread 8 Bread 10 Enriching mixture Soybean meal Corn meal
Table IV.
5.30 5.92 4.11 3.57 0.48
5.35 5,99 4.03 3.58 0.48
SO.05 +0.07 -0.08
+O.Ol 0.00
Enriching mixture
Table V.
Bread 8 Bread 10 Enriching mixture Soybean meal Corn meal
+0.05 +0.03 -0.02 -0.07 -0.06
hfagnesium Mg./Sample' Oxinate EDTA
0.011
Coefficient of Variation (Per Cent) Calcium Oxalate EDTA
Material
2.31 2.30 0.24 2.45 3.34
Standard Deviation from Mean Value Calcium, Mg./Sample Oxalate EDTA
Material
2.26 2.27 0.26 2.52 3.40
1.32 1.44 0.27 0.53 5.42
0.19 0.13 0.22 0.39 3.34
Magnesium Oxinate EDTA 2.92 2.29 9.62 3.26 2.03
0.35 0.96 0.83 0.41 0.30
PROCEDURE FOR ROUTINE ANALYSIS
Reagents. EDTA Solution. Dissolve 4 grams of disodium ethylenediamine tetraacetate in water, add 0.2 gram of magnesium chloride crystals, and dilute to 1 liter. Allow to stand overnight and then filter the solution. Standard Calcium Chloride Solution. Dissolve 1.0000 gram of calcium carbonate, previously dried for 3 hours at 105" C., in the minimum quantity of dilute hydrochloric acid, transfer to a standard 1-liter flask, and fill up to the mark with water. Murexide Indicator. Grind together in a mortar and mix thoroughly 0.1 gram of ammonium purpurate and 100 grams of dry analytical grade sodium chloride. Eriochrome Black T Indicator. Dissolve 0.5 gram of Eriochrome Black T-241 Kith 4.5 grams of hydroxylamine hydrochloride in 100 ml. of ethyl alcohol. Sodium hydroxide solution, 4N. Ammonia-Ammonium Chloride Buffer. Add 67.5 grams of ammonium chloride to 570 ml. of concentrated ammonium hydroxide and dilute to 1 liter. Hydrochloric acid, 4N and 0.015AY. Oxalic acid, saturated and 0.2M. Hydrogen peroxide, 30 per cent by weight. Potassium cyanide, analytical grade. Hydroxylamine hydrochloride, analytical grade. ilmberlite IR-112 Resin. Prepare according to the usual method (IO), using the fraction passing through the 50-mesh (U. S. number) sieve but retained by the 120-mesh sieve. Apparatus. Resin columns, of standard type (IO). Standardization of EDTA solution. WITHMUREXIDE.Transfer by means of a pipet 10 ml. of the standard calcium chloride
x equivalents
METHOD
Dissolve the ash of a sample that will give a convenient titer and evaporate to dryness with hydrochloric acid, in 25 to 30 ml. of water and heat on the steam bath until the volume is 15 to 20 ml. Procedure. For materials containing iron with or without orthophosphate in interfering amounts: Transfer the solution of the salts obtained above into a 150-ml. flask. Add 1 ml. of hydrogen peroxide, cover with a watch glass, and heat on a hot plate. When the solution boils, add 15 ml. of saturated oxalic acid solution and heat for an additional 5 minutes. Cool and transfer to the cation exchange column. Wash the column with two separate 15-ml. portions of 0.2M oxalic acid solution followed by two 10-ml. portions of 0.015&V hydrochloric acid. Reject the effluents. Place a 250-ml. conical flask under the column and elute the calcium and magnesium with one 15-ml. portion of 4-V hydrochloric acid, follon-ed by 20 ml. of water. When approximately half of the water portion has passed through the column, place the flask on a hot plate and evaporate to dryness. Dissolve the residue in approximately 50 ml. of water. DETERXINATION OF CALCIUM.Add 0.2 gram of hydroxylamine hydrochloride to the solution, shake to dissolve, and then add 0.25 gram of potassium cyanide. Add 1 ml. of sodium hydroxide solution and a proximately 0.15 gram of Murexide indicator. Titrate with E 6 T A solution until the color changes from pink to a definite mauve, using a blank for comparison. Carry out the titration preferably against a background of artificial light (fluorescent light). zn x 20.04 x 1000 x 100 C a + + (mg. per 100 grams) = 'If where x = number of equivalents per ml. of EDTA solution n = titer (ml.) Jf = weight of sample DETERMINATION OF ~ ~ A G N E S I U Y ..Add 0.2 gram of hydroxylamine hydrochloride to the solution, shake to dissolve, and then add 0.25 gram of potassium cyanide. Add 1 ml. of ammoniaammonium chloride buffer and 6 drops of Eriochrome Black T indicator. Titrate with EDTA solution until the color changes to a clear blue. Mg++ (mg. per 100 grams) =
( y m - z n ) 12.16 x 1000
x
100
11
where y = number of equivalents per ml. of EDTA solution (Eriochrome Black T as indicator, m = Ca'+ Mg++ titer, ml. x = number of equivalents per nil. of E D T h solution (Murexide as indicator) n = C a + + titer, ml. M = weight of sample
+
ACKNOWLEDGMENT
G. N. Krynauw received financial assistance from the South African Nutritional Research Institute of the Council for Scientific and Industrial Research, Pretoria and Agricura Laboratories, Ltd., Silverton, Transvaal. LITERATURE CITED
(1) Botha, G. R., Webb, >hI., !I.J . Inst. Water Engrs. 6 , 459-63 (1952). (2) Brunisholz, G . , Genton, M., Piattner, E.. Hela. Chim. Acta 36, 782 (1953).
V O L U M E 28, NO. 11, N O V E M B E R 1 9 5 6
1761
(3) Cheng, K. L., Kurtz, T., Bray, R.R., ANAL.C H E M . 1640-1 ~~, (1952). (4) Diehl, H.,
Goete, C. A., Hach C . C., J . A m , Water Works
Assoc. 42, 80 (1950). (5)
Djurfeldt, R.,Hansen, J., Samuelson, O., Svensk. Kem. Tidskr.
59, 13-8 (1947). ( 6 ) Gehrke, C. W., Affsprung, H. E., Lee, Y . C., ANAL.CHEM.26, 1944 (1954).
(7) Hahn, R.,Backer, C., Backer, R.,A d . Chim. Acta 9 , 223-5 (1953). (8) Jenness* R.1 A N A L . CHEW 251 966-8 (1953). (9) Mason, A. C., Analyst 77, 529-33 (1952).
(10) Samuelson, O., "Ion Exchangers in Analytical Chemistry," Wiley, Xew York, 1953. (11) Samuelson, O., Szensk. Kem. Tidskr. 52, 241 (1940). (12) Ibid., 54, 124 (1942). RECEIVED for review December 7, 1955. Accepted June 18, 1956.
Determination of Traces of Mercury in Mercury Ore Ash by Catalytic Action of Mercuric Ions SMILJKO ASPERGER and DUSANKA PAVLOVIC lnstitute o f Inorganic, Analytical, a n d Physical Chemistry, Faculty o f Pharmacy, University o f Z a g r e b , Croatia, Yugoslavia
The determination of traces of mercury in burnt mercury ore was carried out on the basis of the cataly-tic action of mercuric ions on the reaction between potassium ferrocyanide and nitrosobenzene, in which a violet complex [Fe(CN)j(CsHjKO)]---is formed. In the range where the mercury in the ash amounted to 0.0024 to 0.0097q~the relative error of the analyses varied between 6 and Z q , and the standard deviation was approximately 0.00015.
T
HE determination of small amounts of mercury in burnt ore that remains in mercury mines serves as a control of the smelting process in industry. The well burnt ore usually contains only a few thousandths of 1 % of mercury. Such small amounts of mercury cannot be accurately determined by the classical Eschka method (3),which is accurate only when the quantity of mercury in the ore is greater than 0.1%. On the other hand, the temperature used in the Eschka method to expel mercury from the ash proved to be insufficient because of the adsorption of mercury on the ash. It appears that the other more sensitive methods for determination of mercury have not been applied to this case. Proceeding with earlier experiments on the determination of small amounts of mercury ( I ) , the authors used the catalytic action of mercuric ions on the reaction of potassium ferrocyanide and nitrosobenzene in aqueous solutions to determine traces of mercury in mercury ore residue. The reaction proceeds according to Equations 1, 2, and 3 (2): H20 Fe(CK)e----
$
[Fe(CS)j(H20)I--H?O
+
CS-
[Fe(CS)s(HzO)]---
+ CsHjKO
+ H20
-+
HCY
+ CY-
(1)
[Fe(CN)j(CsHjNO)]--violet (2)
+ OH-
(3)
The concentration of the violet reaction product [Fe(CN)j(C6HsNO)]- - - at a fixed reaction time depends on the concentration of mercuric ions present in the solution. By measuring the absorbance of the violet complex at 528 mp, as little as 2 y of mercury could be determined. The ore residue was heated to 1200' C. and the mercury vapors liberated were mixed with bromine vapors according to the procedure of Moldawskij (4,5 ) , and absorbed in bromine water, and the mercuric bromide was determined by an earlier procedure (1). EXPERI>IESTA L
The ash (0.2 to 0.6 gram) v a s heated for 1.5 hours at 1200" C. and a stream of fresh air was passed through the furnace. The
apparatus for oxidizing and absorbing mercury vapors xith bromine vapors (4,5 ) was attached to the furnace. After the liberation of mercury n7as completed, the glass tubes that connected the furnace and the absorption vessel were rinsed with bromine water, vhich was added to the bromine water of the absorption cell. The absorption solution containing traces of mercuric ions was then treated for the determination of traces of mercury in the atmosphere ( 1 ) . The concentration of the mercuric ions v a s calculated from the equation lOgioCEg++= 1.112 logmEso
- 4.427
(4)
where E30 is the absorbance of the violet reaction product at 30 minutes at 528 mp and a path length of 1 em. ( 1 ) . The table shows the results of the analysis of a sample under different conditions of burning. Temp.,
c.
Time of Heating, hlin.
Mercury Found in Ash, %
10
900
15 60 90
1000 1100
90 180 240 90
1200
90
1050
0,0025
0.0029 0.0031 0.0035 0.0041 0,0043 0,0044
0.0051 0.0051
Increase of temperature above 1200" C. and prolonged heating (over 90 minutes) caused an increase of the acidity of the otherwise slightly acid absorption solution to p H 2 or less. This is not desirable, because more sodium hydroxide would be needed for the subsequent adjustment of the p H to 3.5 before addition of potassium ferrocyanide, as required by the earlier procedure ( 1 ) . This would cause an increase of the otherwise negligible negative salt effect on the catalyzed reaction, and hence slightly lower results in the analyses. The reaction shows a negative salt effect, as the charges of ferrocyanide and mercury ions are opposite in sign. Fifty analyses on ash from the Idria mercury mine in Yugoslavia were carried out on samples containing 0.0024 to 0.0097% of mercury. The standard deviation was approximately 0.00015 in this whole range and the relative error varied between approximately 6% for the ash of low mercury content and 2% for the ash of higher mercury content. LITERATURE CITED
(1) ASperger, S., AIurati. I.,AXAL.CHEM. 26, 543 (1954). (2) hhperger, S., PavloviC,D., J . Chem. SOC.1955, 1449. (3) Holloway, G. T., Analysf 31, 66 (1906). (4) RIoldawskij, B. L., Zhur. Priklad. Khim. 3 , 9 5 5 (1930). Cucuel, F., Ber. deut. Chem. Ges. 71,550 (1938). (5) Stock, -4., RECEIVED for review June 2 , 1956.
Accepted June 16, 1956.
