Successive Determinations of Calcium and Magnesium in Drinking Water by Complexometric, Potentiometric Digital Titration to Two Equivalence Points T. F. Christiansen,’ J. E. Busch, and S. C. Krogh Radiometer A/S, 72 Emdrupvej, DK-2400 Copenhagen N V, Denmark
Calcium and magnesium are titrated using an indicator eiectrode sensitive to the calcium ion and EDTA as the titrant in a solution containing 3,4-dihydroxybenzoic acld or acetyiacetone. in thls solution, the ratio between EDTA’s conditional stability constants for calcium and magnesium Is increased so that two pronounced inflection points are obtained on the titration curve. The Inflection points are determined in a stepwise, computer-controlled titration. it is possible to determine calcium and calcium magnesium in drinking water with a relative standard devlation of less than 0.5%. The absolute accuracy is better than 0.6% for calcium determinatlons, and better than 0.8% for calclum magnesium determinatlons. A magnesium content constituting more than 4 % of the calcium content can be determined. A determination of calcium and magnesium can be performed in 3 min.
+
+
Hitherto, there has been no convenient method for determining calcium and magnesium separately in a single titration. This may be due to the absence of a) a suitable ligand for which the ratio between the stability constants for calcium and magnesium is appropriately large, b) a suitable indicator system that, throughoui the course of the titration, is capable of measuring the changes occurring in’the activity of one of the ions involved, and c) a suitable system for detecting the equivalence points. Determinations of calcium and magnesium are usually performed as two titrations. Calcium can be determined separately, either by masking the magnesium as Mg(0H)z a t p H 12-13 ( I , 21,and subsequently titrating the calcium with EDTA, or by using EGTA as a selective titrant for calcium (3) a t a p H value greater than 9. As the indicator system, a color indicator, viz., Murexide or Calcon ( I ) , or potentiometric indication with a calcium-ion electrode (2), a silver electrode ( 3 ) , or a mercury electrode ( 4 )may be used. Calcium magnesium can be titrated a t p H 9-10 with EDTA or DCTA ( 1 , 5 )as titrant. Here, too, the indicator system may be a color indicator, e.g., Eriochrome Black T ( I ) , a fluorometric indicator DHNA (5) or potentiometric indication with a calcium-ion electrode or a cupric-ion electrode (6). Alternatively, calcium and magnesium can be determined successively by titrating calcium with EGTA and then titrating magnesium with DCTA, using Phthalein Complexon as a color indicator (7). It is theoretically possible to obtain titration curves with inflection points for both calcium and magnesium. Figure 1 shows such curves resulting from a titration of a mixture of M calcium and M magnesium with ligands that have ~ L ) the stability constant for varying ratios ( K C ~ L I K Mbetween calcium and the stability constant for magnesium. An ideal calcium-ion electrode was used as the indicator electrode. L and Curve 2 represents EDTA for which K c ~ L I K M=~lo2, ~ In both curve 5 represents EGTA for which K c a ~ / K h l g lo5. cases, a secondary inflection is indicated a t the equivalence points of calcium and magnesium, respectively. I t is further evident from Figure 1 that if, as assumed, Kca is lolo, then the
+
-
ratio K c ~ L I K Mmust ~ L be 103-104 in order that two distinct inflections might be obtained. In practice, however, it has not been possible to find a ligand that meets the above-mentioned conditions. On the other hand, if EDTA is used as titrant, and if the nature of the supporting electrolyte in the titration solution is such that it slightly binds the magnesium, but not the calcium-i.e , the magnesium activity is ‘/lo to l/100 of the concentration-then the ratio between EDTA’s conditional stability constants ( 8 ) , K’C~LIK’M~?, will be 109-104, and titration curves with dual inflections similar to those seen in Figure 1 can be obtained. 3,4-Dihydroxybenzoic acid and acetylacetone (9) are capable of forming the above-mentioned weak bonds with magnesium and a t least 100 times as weak bonds with calcium. An ideal calcium-ion electrode must have a detection limit for calcium activities of Such an electrode has not yet been developed, and currently the lowest possible detectior limit is 10-s,5 (IO).According to Figure 1 it will be possible to obtain two inflections with the latter type of electrode if K ’ c ~ L I K ’ Mis~ L 103. I t is not feasible to perform calcium-magnesium titrations as a titration directly to two end-point potentials, since the potentials in the equivalence points will vary with the concentration of calcium and magnesium as well as with the rati:) of calcium to magnesium. The equivalence points can be determined by plotting the potential E vs. ml of titrant CC)IIsumed and assuming that the equivalence points are identical with the inflection points on the titration curve. Alternatively, dEld(m1) can be plotted against ml of titrant consumed, inasmuch as the maximum values of dEld(m1) are equivalent to the inflection points. Both of the above-mentioned methods are time-consuming and consequently not very attractive ior use in routine analyses. On the other hand, the inflectiorI points can be rapidly determined in a stepwise titration by utilizing a digital technique in which corresponding, digitized
10-91
a . s
10-7
, 0
0.1
0,2 0.3
DL
0.5
0.6
0.7
/
08
0.9
1.0
1.1
1.2
3
Fraction titmkd
Figure 1. Titration curves computedfor M Ca plus M Mg titrated with a variety of ligands having various ratios KcaLIKMgL as indicated on the curves. K c a = ~ 10’O ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
1051
5 sec. D E L A Y REm I N I T I A L pCo
I E;""""';
inflection point Ca t Mg
2 START DOSING
A N D P R I N T RESULT
0
NO
100 150 10' M EDTA (PI1
200
Figure 3. Digital titration curve for approx. M Ca plus in 0.01 M 3,44ihydroxybenzoicacid at pH 9.7
STOP R O U T I N E
I
50
250
M Mg
f
Figure 2. Flow chart of the digital titration sequence
values of the electrode signal, E, and the volume of titrant used, V, are fed to a microcomputer. The microcomputer has three principal functions: to check the stability of the electrode signal and ultimately accept it; to calculate and control the titrant delivery volume needed at each step in the titration; and to establish whether inflection points occur and determine the exact location of those detected. DIGITAL TITRATION
A flow chart of the digital titration sequence is shown in Figure 2. Read pCa. The electrode signal will not be stable immediately after a titrant increment has been delivered since it is influenced by the mixing speed and reaction rate in the solution and by the response of the electrode. The computer therefore calculates the mean value of five samplings (corresponding to 0.5 s). On the basis of the difference found between two consecutively calculated mean values, it then investigates whether the electrode drift is less than 16 counts/sec (resolution: 1000 counts/pCa) and subsequently accepts the electrode signal as the mean value of 10 samplings. In the event that the stability criterion is not satisfied after 45 s, the electrode signal will be accepted directly, although still as the mean value of 10 samplings. Selection of the mean value reduces the influence of noise and transients on the electrode signal. New Dosage. The computer calculates the slope of the titration curve--lAE,,,-ll/AVn,,-1 = An,,-1-and uses the result to determine the next titrant increment-AVn+l,naccording to the formula: a
AVn+l,n = &,n-1
+ b + (An,n-l - A n - l , n - 2 )
+ c counts (1)
The formula has been worked out empirically for S-shaped potentiometric titration curves in such a manner that the changes occurring in the electrode signal across the flat portion of the titration curve will, on the whole, be constant for each new addition of titrant. The formula also presupposes that the titrant increments in the vicinity of an inflection point cor1052
ANALYTICAL CHEMISTRY, VOL. 48,
NO. 7, JUNE 1976
responding to an equivalence point will be relatively small in order that the exact location of this point can be calculated. Where abrupt changes occur in the slope of the curve prior to an equivalence point, the last term in the denominator will ensure a rapid reduction in the titrant delivery. The term is excluded from the formula when negative, Le., when the slope of the curve decreases again after an equivalence point is passed. This provides for small titrant increments so that it will be possible to detect two close-lying equivalence points. The constants a, b, and c are adapted to the digital resolution of the electrode signal and the titrant increments (2500 counts for a full buret, 2.5 ml or 0.25 ml). Constant a (300) ensures the large titrant increments that are desirable over the flat portion of the titration curve (10-15 increments per component when the titration involves the use of half a buret of titrant). Constant b (0.8) ensures maximum titrant increments of 380 counts when the slope of the curve is small. Constant c ( 5 ) ensures minimum titrant increments in the vicinity of the inflection points, which is desirable in order to obtain good resolution of the calculated, derived parameters and, hence, an accurate determination of the location of the equivalence points. A digital titration curve is shown in Figure 3. The titration commences with two predetermined additions of titrant (start dosing: 100 counts), and the following increments are then calculated on the basis of Equation 1.These increments will be smallest where the curve is steepest, Le., in the vicinity of the inflection point. Analyze Data. The computer furthermore calculates the 2nd order difference coefficient 2(An,n-1
- An-1,n-2)
(2) (AVn,n-I + AVn-1,n-2) and where the coefficient changes sign from positive to negative, an inflection point corresponding to an equivalence point on the titration curve will be found. Compute Data. The exact location of the inflection point is determined by linear interpolation according to the formula Yn,n-:!
Vinfl= Vn
=
- AVn,n-I - AVn-1,n-z I~n~n-21
J~n,n-zI
+ I ~n-l,n-31
(3)
Thus, this computer-controlled titration technique uses very few points in the vicinity of the inflection point to determine its location, which makes it possible to employ a microcomputer with a limited storage capacity as only the numerical values needed to calculate the location of a possible inflection point are stored. Since only that portion of the curve that brackets the inflection point is used in the calculation, any errors introduced as a result of dilution or asymmetrical titration curves will be diminished. Furthermore, the computer-controlled titration technique makes it possible to detect equivalence points that are barely discernible on a conventional, recorded curve, thereby paving the way for the use of titration methods which hitherto have lacked all appeal because the equivalence points were difficult to locate. In addition, it is possible to detect equivalence points which, with respect to titrant consumption, are very closely placed. An added advantage is that the titration is performed in an identical manner each time and therefore provides highly reproducible results.
'J 'OI
Y
8
\lo-% 7
8
9
\
, \6 10
I \&
11
I 12
PH
Figure 4. Computed conditional stability constants for the Ca-EDTA complex and the Mg-EDTA complex as a function of pH: Pure solution (1,2); lo-* M 3,4-dihydroxybenzoic acid (3, 4); lo-' M 3,4dihydroxybenzoic acid (5,6)
EXPERIMENTAL Apparatus. An F2112Ca Calcium Selectrode (IO), a K401 Saturated Calomel Electrode, and a GK2301C Combined p H Electrode (Radiometer A/S) were used. T h e calcium determinations were performed with a PHM64 p H Meter with digital output operating as a divalent ion-meter (resolution: 0.001 pCa). The p H measurements were accomplished with a PHM63 p H Meter. The titration curves were recorded on an R T S Automatic Recording Titration System consisting of a PHM64 p H Meter, an REC61 Recorder fitted with an REA160 Titrigraph Module, a TTT6O Titrator (control unit), and an ABUl3 Autoburette with digital output. Digital titration DTS was performed by means of the PRSlO Alpha Printer with built-in microcomputer and interface cards in double inflection mode, a PHM64 pH meter and the above mentioned ABU13 Autoburette. The titrations were performed either separately in the TTA6O Titration Assembly or in series on the ATSl Autopipetting Titration Station. All instruments are fronl Radiometer A/S, Denmark. Reagents. A 0.1000 M CaC12 stock solution was prepared by dissolving 10.009 g dried CaCQ3 (E. Merck) in 10%excess of hydrochloric acid. CO2 was boiled out and the solution diluted to 1000 ml. The 0.100 M MgS04 stock solution was prepared from MgSO4 (E. Merck). These stock solutions were used for preparing the calcium and magnesium solutions. A 0.100 M EDTA stock solution was prepared from EDTA supplied by E. Merck. Then 0.4 mol KOH was added to each 1000 ml of the solution to prevent appreciable p H changes during titration. T h e acetylacetone and the 3,4-dihydroxybenzoic acid were both analytical-grade products from Fluka. Procedures. a. Calcium a n d Magnesium without Content of&carbonate. T h e titration is performed with 0.1 M EDTA in a 250-11 buret. T o a 10-ml sample containing max. 2.3 mmol calcium magnesium is added 10 ml of reagent consisting of 0.02 M 3,4-dihydroxybenzoic acid plus 0.03 M glycine (pH buffer, pH 9.7). The calcium-ion electrode is used as indicator electrode. The titration is carried out in the TTA6O Titration Assembly, either as a recorded titration (RTS) or as a digital titration (DTS). If the sample contains more than 2.3 mmol calcium + magnesium, it must be suitably diluted. b. Calcium a n d Magnesium with Content ojBicarbonate. The titration is performed wih 0.02 M EDTA in a 2.5-ml buret. T o a 10-ml sample containing max. 4.5 mmol calcium + magnesium is added 10 ml of reagent consisting of 0.02 acetylacetone plus 0.04 M TRIS buffer a t p H 8.5. The calcium-ion electrode is used as indicator electrode. The titration is carried out in the TTA6O Titration Assembly, either as a recorded titration (RTS) or as a digital titration (DTS). If the sample contains more than 4.5 mmol calcium + magnesium, it must be suitably diluted.
+
RESULTS AND DISCUSSION Calculation of the Conditional Stability Constants (8). In order to obtain a titration curve with two pronounced inflection points for mixed solutions of calcium and magnesium, using a calcium-ion electrode as the indicator electrode, the following conditions must be satisfied. The conditional sta-
11
Ca
1
10 -
3L 7
I 6
9
10
11
12
PH
Figure 5. Computed conditional stability constants for the Ca-EDTA complex and the Mg-EDTA complex as a function of pH: Pure solution (1, 2); 1 0-2 M acetylacetone (1, 3); lo-' M acetylacetone (1, 4)
bility constant of the titrant ligand with respect to calcium (Kc~~L must , ) lie in the range 10s-lO1o. The ratio between the conditional stability constants, K C a ~ ~ / K M gmust ' ~ , be approx. lo3.This ratio can be controlled by pH and the presence of a weakly binding ligand A (H,A-in this case: 3,4-dihydroxybenzoic acid or acetylacetone) according to the equation: (4)
where, if reaction with A is assumed to be the only possible side reaction for the metal ion,
(5) where PMeA, is the cumulative stability constant for the complex MeA,. Furthermore, by varying the pH, it is possible t o change the concentration of the free ligand A according to the equation:
In Figure 4, the conditional stability constants for the calcium-magnesium-EDTA complex are plotted as a function of p H in a background of 10-1 M and M 3,4-dihydroxybenzoic acid as well as in a solution not containing the weakly binding ligand. The curves are calculated on the basis of Equations 4-6. Figure 5 shows the corresponding curves for acetylacetone. M dihydroxybenzoic acid, KCa,L,/KMg,L, will be In ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
1053
Y
1
"1
klflection pant
Ca*Mg
7 1 7I flection point
1
6-
B 5 -
I
1
200
290
I 0
50
150
too
10 M EDTA
Figure 6. Titration curve for 10 ml
lull
Flgure 7. Tltration curve for 10 ml
M Ca plus
is added: (1) 10 ml 0.02 M 3,4dihydroxybenzoic acid pH 9.7; (2)10 ml 0.03 M glycine, pH 9.7
+
M Mg to which 0.03 M glycine,
greater than lo3 when the pH value is greater than 8.9, and the ratio will increase with increasing pH, since PHA = 11.7. In M acetylacetone, K C ~ , L , / K Mequals ~ ~ , ( lo3at pH 8.4. Above pH 9 only negligible changes will occur in the ratio, since PHA = 8.8. In order to verify the calculated results of Figures 4 and 5, pH-pCa recordings were made on calcium-magnesium solutions partially titrated with EDTA in a background of ligand A. At varying pH values, the pCa changes in the vicinity of the inflection point for calcium were as expected from the calculated conditional stability constants, although the magnesium binding to 3,4-dihydroxybenzoic acid appears to be weaker than could be expected from the calculated values. On the other hand, pCa changes in the vicinity of the inflection point for magnesium are decreased, owing to the detection limit of the calcium electrode at pCa = 8.5. The optimum conditions for titrations in 3,4-dihydroxybenzoic acid and in acetylacetone can be estimated from the calculated curves for the conditional stability constants (Figures 4 and 5). On the other hand, the experimental pH-pCa plots make allowance both for the nonideal behavior of the indicator electrode and for the uncertainty inherent in the stability constants used in the calculations, and these curves will consequently provide more reliable estimates. However, neither of the methods takes the dynamics of the indicator electrode during the titration process into account, and the ultimate choice of suitable conditions for the titration must therefore be based on reproducibility investigations performed in various solutions for which conditions as regards titration are more or less similar to those estimated. For 3,4-dihydroxybenzoic acid was selected a concentration of low2M in which the ligand was present in excess. The optimum pH value was found to be pH 9.7. A titration curve recorded under these conditions is shown in Figure 6 together with a second curve recorded at the same pH value but without addition of 3,4-dihydroxybenzoic acid. Acetylacetone is less suitable at pH 9.7 inasmuch as the concentration called for to obtain K c ~ ~ L ~ / K=Mlo3 ~ ~must L , be so low (3 X M) that the concentration of the free ligand will change substantially during titration when the sample contains large quantities of magnesium. For acetylacetone, a low pH value (see later) of pH 8.5 was selected. The optimum concentration was found to be lo-* M. A titration curve recorded under these conditions is shown in Figure 7 together with a second curve recorded at the same pH value but without addition of acetylacetone. 1054
1O'M EDTA lpl)
ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
M
is added: (1) 10 mlO.02 M acetylacetone ml 0.04 M TRIS, pH 8.5
Ca plus
M Mg to which
+ 0.04 M TRIS, pH 8.5; (2) 10
Table I. Variations in Titrant Consumption for the Magnesium Determination a t Various Ratios of Mg/Ca and Constant Magnesium Concentration of approx. 1 mmol/l. Titrant used, ~ 1 0 . M 1 EDTA Mg/Ca 5 2 1
94.3 97.3 97.8
0.66
97.8
Table 11. Variations in Titrant Consumption for the Calcium Determination a t Various Ratios of Ca/Mg and Constant Calcium Concentration of approx. 1 mmol/l. Ca/Mg
Titrant used, ~ 1 0 . M 1 EDTA
m
104.0
20 4 1 0.7
104.6
105.6 105.5 105.4
Applicability of the Methods. In the following examples, the calcium-magnesium determinations were carried out by means of automatic digital titration. Initially, the titrations were performed according to procedure a. Initially, the magnesium content in the sample was maintained at a constant level, whereas the calcium content was varied (Table I). It will be seen that the detected magnesium is independent within 0.5% of the calcium content when this amounts to more than 50% of the magnesium content. In samples with a lower calcium content, the values obtained for the magnesium content were too small. Subsequently, the calcium content in the sample was maintained a t a constant level, whereas the magnesium content was varied (Table 11). It will be seen that the detected calcium is independent within 1%of the magnesium content when this amounts to more than 5% of the calcium content in the sample. Furthermore, it is seen that the calcium determination must be corrected for a blind value when magnesium is present in the sample. The detection limit for magnesium in the presence of calcium depends on the digital resolution of the buret and the pH meter, which in this case was 0.1 ~1 (for a 250-11buret) and
Table 111. Linearity Investigations with Addition of 2.5 X M Ca2+plus 2.5 X M Mg2+a Background Determiconcn, M Ca nation
Concn range M X lo3
Slope regression line
Correlation coefficient
Ca C a + Mg Ca M Ca Mg
1.1-1.75 1.2-2.4 0.3-0.95 0.4-1.65
2.567 5.007 2.564
0.99983 0.99997 0.99988 0.99996
10-3M
+
2X
5.030
No precautions were taken to ensure absolutely identical concentrations of calcium and magnesium. a
0.001 pCa, respectively. In 10 ml of M Ca2+titrated with 0.1 M EDTA, it is possible to obtain two inflection points when the magnesium content is a t least 4% of the calcium content, corresponding to a concentration of 4 X M Mg2+ and a titrant consumption of 4.0 ~ 1 . Linearity investigations were carried out by adding various quantities of a stock solution (2.5 X M Ca2+ 2.5 X M Mg2+)to some 10-ml samples containing M Ca2+ and 2X M Ca2+, respectively. Regression lines were computed both for calcium and for calcium magnesium (Table 111).The regression lines were determined on the basis of eight points in the concentration ranges stated. The slope of lines of identical types is the same, irrespective of the background concentration. The correlation coefficients are all greater than 0.9998, which indicates a high degree of linearity. The mean standard deviation is 0.0013 X lod3 molfl. for the molfl. for the calcium determinations, and 0.0009 X calcium magnesium determinations. The standard deviation is not dependent on the content of calcium and magnesium in the sample. Drinking water contains bicarbonate, which a t a p H value of 9.7 involves a risk of calcium carbonate being precipitated before the calcium is bound to EDTA in the course of the titration. In digital titrations, where the electrode potential is read after each addition of titrant, it is necessary that equilibrium be attained in the solution before the data are collected. Redissolution of calcium carbonate is a slow process during which false data may be collected before equilibrium is established. I t is consequently essential that precipitation of calcium carbonate should not occur before or during the titration. This requirement is best satisfied by not exceeding the concentration given by the solubility product for calcium carbonate (9). In the case of a normal water sample which contains 3 X mol calcium per liter and 7 X mol total COz per liter, and which is diluted to double its volume, it is necessary to titrate a t p H 7.5 in order not to exceed the solubility product. However, it has been observed that even when the solubility product is exceeded by a factor of 10, there is no risk of any perceptible precipitation of calcium carbonate if the titration of the sample is completed within 1 hour. Nevertheless, it is advisable t o perform the titration as soon as the sample has been prepared. In order to obtain titration curves with clearly defined inflection points, it was decided to titrate the drinking water samples a t p H 8.5 in a background of M acetylacetone according to procedure b. Absolute Accuracy and Reproducibility. Calibration of the calcium-magnesium titration can be carried out as a two-point calibration (linear interpolation between two calibration points) or as a one-point calibration (linear interpolation between one calibration point and 0.0). A two-point calibration makes it possible to correct for a blind value in calcium determinations.
+
+
+
Table IV. Titration of Calcium-Magnesium Mixtures with Two-Point Calibration, Ca, Ca + Mg: 3.00,4.00, and 0.500,0.800 mmol/l, and One-Point Calibration, Ca, Ca Mg: 3.00,4.00 mmol/l.
+
New electrodea Sam- Taken Found ple mmol/l. 2-pt No. Ca mmol/l. 1
1.000
1.001
2 3 4
2.000 0.500 2.000
1.997 0.499 2.000
1-month old electrodeb
Found I -pt mmol/l.
Found 2-pt mmolA.
Std dev, mmol/l.
0.994 1.993 0.490 1.996
1.003 2.004
0.010 0.006
0.503
0.003 0.004
2.005 Mean std dev
0.006
Ca + Mg 1 2
3 4
2.012 2.487
2.000
2.500 1.000
1.008
2.200
2.199
2.012 2.489 1.009 2.200
0.002 0.004 0.007 0.004 0.004
2.010
2.490 1.005
2.189 Mean std dev Calibration lines, Ca: New electrode: y = 2.007~- 0.024 mmol/l. Old electrode: y = 2.008~- 0.022 mmol/l. Calibration lines,,Ca + Mg: New electrode: y = 2.012~- 0.007 mmol/l. Old electrode: y = 2.010~- 0.003 mmol/l. x denotes ml titrant 0.02 M EDTA, sample volume 10 ml, background concentration acetylacetone 0.01 M, pH 8.5. a = mean of two samples, b = mean of 4 samples.
In Table IV is given a collocation of the absolute accuracy obtained in two-point and one-point calibrations. In the case of calcium determinations based on one-point calibrations, it is imperative that the calibration point lies within a factor 3 of the sample to ensure that the error will be less than 1%. This is evident, too, from the constant in the equation for the calcium calibration line a t the bottom of the table. In calcium magnesium determinations, one-point and two-point calibrations yield almost identical results on account of the low value of the constant in the calibration equation. The absolute accuracy (based on two-point calibrations) in the investigated range is better than 0.6% for calcium determinations, and better than 0.8% for calcium magnesium determinations. The response characteristic of ion-exchanger electrodes may change as they grow older. Investigations were therefore made to ascertain whether the absolute accuracy and the calibration equation were similarly influenced by aging. The one-month old electrode had been used for making approx. 500 titrations. As it appears from Table IV, there is no significant difference in the values obtained in the calciumand the calcium magnesium determinations, and also the calibration equations are practically identical. The standard deviations are calculated on the basis of four determinations performed in the separate samples; the absolute value is independent of the magnesium and calcium content. Under the prevailing conditions, the content of calcium and of calcium magnesium must be greater than 0.5 mmol/l. in order t o achieve a standard deviation that is less than 1%. The detection limit for magnesium relative to calcium was found to be 4% in 0.01 M acetylacetone a t p H 8.5 when the M. calcium concentration was In samples of drinking water (Table V) collected from eight different water works in Copenhagen and environs, calcium and calcium magnesium were determined by two-point
+
+
+
+
+
ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
1055
Table V. Determination of Calcium and Calcium + Magnesium in Drinking Water. Titrant 0.1 M EDTAD Ca mmol/l. Ca + Mg mmol/l. Sample volume, mmol/l.
5 nil* found
Std dev
2 mlc found
5mlb found
Std dev
3.222 4.016 0.014 2.035 2.844 0.008 3 2.875 2.875 3.451 0.005 4 2.438 2.429 2.928 0.007 5 2.422 2.407 2.725 0.004 6 2.336 0.006 2.350 2.568 0.008 7 1.899 0.009 1.932 2.728 0.005 b 3.350 0.004 3.347 4.196 0.007 I'wck-point calibration performed as in Table IV. b 4 determinations. Mean of 2 determinations. 1
0
L
3.205 2 019
0.004 0.008 0.007 0.007 0.003
('
2 mlc
found 4.054 2.872 3.475 2.947 2.732 2.540 2.71, 4.198 Mean of
calibrations on 5-ml and 2-ml samples diluted to identical total volumes. Only in a single instance did the deviation between the 5-mi samples and the 2-ml samples exceed 1%. The mean standard deviation is 0.25% for the calcium determinations (5-ml samples, 4 measurements on each) and 0.23% for the calcium magnesium determinations. Interferences. In photometric titrations, trace elements of metal ions are often a problem because they form colored complexes with the indicators (7). In a potentiometric titration with a calcium-ion electrode, trace elements of other metals cause no problems, provided they are present in
+
quantities that lie below the detection limit of the analysis. Greater quantities of Fe3+ and A13+ will be effectively masked in 0.01 M acetylacetone (pH 8.5). Fez+,which often occurs in drinking water, will not be effectively masked by acetylacetone, but the transformation of the Fez+-acetylacetone complex into a Fez+-EDTA complex is so slow (rate constant = lo3 mol-' min-l) that the interference will be less than 1% for a Fez+ content up to 0.5 mmol/l. and a calcium and magnesium content of 1 mmol/l. ACKNOWLEDGMENT The authors express their appreciation to Ewa Dogonowski for valuable assistance with the experimental work, and to Henrik Malmvig and 0.J. Jensen for stimulating discussions. LITERATURE CITED (1) G. P. Hildebrand and C. N. Reilley, Anal. Chem., 29, 258 (1957). (2)7.P. Hadjiiannou and D. S. Papastathopoulos, Talanta, 17, 399 (1970). (3) I. E. Lichtenstein, Elia Coppola, and D. A. Aikens, Anal. Chem.. 44, 1681 (1972). (4) A . E. Martin and C. N. Reilley, Anal. Chem., 31, 992 (1959). (5) R. L. Clernents, J. I. Read, and G. A. Sergeant, Ana/yst(London), 96, 656 (1971). (6) Marco Mascini. Anal. Chim. Acta, 56, 316 (1971). (7) Hisakuni Sat0 and Kaso Mornoki, Anal. Chem., 44, 1778 (1972). (8) Anders Ringborn, "Chemical Analysis", Vol. XVI, "Complexation in Analytical Chemistry", interscience Publishers, New York/London, 1963. (9) Arthur Martell and L. G. Sillen, "Stability Constants of Metal-Ion Cornplexes", The Chemical Society, Burlington House W. 1, London 1964. (10) J. RGiEka, E. H. Hansen, and J. C. Tjell, Anal. Chim. Acta, 67, 155 (1973).
RECEIVEDfor review October 14, 1975. Accepted February 20,1976. Our thanks are due to Radiometer A/S whose support made this work possible.
Determination of Selenium(1V) by Anodic Stripping Voltammetry in Flow System with Ion Exchange Separation Richard W. Andrews' and Dennis C. Johnson* Department of Chemistry, Iowa State University, Ames, lawa 500 1 1
Selenium in several NBS Standard Reference Materials and unknown samples was determined by liquid chromatography with IRA-200 cation-exchange resin. Detection of Se(lV) In the chromatographic effluent was by anodic strlpping voltammetry at a tubular Au electrode. Analytical results were excellent except when Si02 was present. The detection limlt of the method was approximately 0.6 ng of Se(lV) in a 0.160-ml sample ( 4 ppb).
The authors recently reported the results of a study of the voltammetric deposition and stripping of Se(1V) at a rotating gold-disk electrode (RAuDE) in 0.1 M HC104 ( I ) . Three distinct stripping peaks are obtained following the deposition of the equivalent of several monolayers of Se. This evidence was interpreted to be the result of the formation of three distinct activity states of the deposited Se: a monolayer of Se contacting the Au surface which is oxidized at 0.8 V vs. SCE, bulk Se which is oxidized at 0.6 V, and a Au-Se intermetallic compound of unknown stoichiometry which is oxidized at 1.0 Present address, Department of Chemistry, University of Alabama in Birmingham, Birmingham, Ala. 35294. 1056
ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
V. When the quantity of deposited Se does not exceed the equivalent of a monolayer, a single stripping peak, which is resolved from the anodic wave for the formation of gold oxide, is obtained. This stripping peak is suitable for the determination of Se(IV) by anodic stripping voltammetry (ASV). A detection limit of approximately 0.04 ppb Se(1V) in 0.1 M HC104 was reported by the authors for the determination of Se(1V) by linear scan ASV a t a RAuDE following a 10-min deposition period ( I ). The ASV procedure was applied to the determination of Se in NBS Standard Reference Material 1577 (bovine liver). The average of five determinations was 1.12 & 0.03 ppm Se whereas the certificate value is 1.10 f 0.10 ppm. Interference from codeposited As(III), Hg(II), Sb(III), Cd(II),and Pb(I1) in the determination of Se(1V) by ASV is severe. Separation of Se(1V) from these metal ions in conjunction with the electrochemical determination of the procedure is necessary if the method is to be applicable to complex samples containing these species. Several ion-exchange separations for complex samples containing Se(1V) have been reported utilizing cation exchange resins (2-4), anion exchange resins (5, 6),or a combination of both (2, 7). Nelson, Murase, and Kraus reported
