Determination of Calcium Content and Total Hardnes's of W a t e r Nephelometric-Photometric Procedure ABRAHAM SAIFER', CAPT. SNC,
AND
FRANKLIN D. CLARK, SGT.
A r m y of the United States, Seventh M e d i c a l Laboratory
A rapid photometric-nephelometric procedure for the determination of calcium in water is described. A solution of potassium oleate in Duponol reacts with ammoniacal solutions of calcium to form a colloidal suspension of calcium oleate whose turbidity i s proportional to the concentration of the calcium present. The degree of turbidity i s measured in a photoelectric colorimeter with a suitable filter.
The method has an average error of ~ 4 over % the range of 0.004 to 0.28 my. of calcium in 10 ml. of solution. O n l y a few ions commonly present in water interfere; their maximum permissible concentrations are given. The method can b e used in conjunction with a previously published methcd for the determination of magnesium in water to determine total hardness.
T
and total hardness in known and unknown water samples and are presented in this paper.
HE Third .Irmy Medical Laboratory was faced with the problem of doing chemical analyses on water samples from Engineer Water Points whose locations and sources of supply varied almost daily with military necessity. I n addition, chemical analyses for calcium, magnesium, and total hardness were performed on water samples from a new type of ion-exchange unit Khich vras being tested for field operation. Since this entailed running large numbers of these determinations, it was thought desirable to attempt to develop a rapid photometric procedure for calcium which could compare favorably in accuracy with the more complicated and time-consuming gravimetric and volumetric procedures. This method could then be used in conjunction with an already published procedure for the photometric determination of magnesium in xyster, and the standard formula used for the calculation of total hardness. Although other investigators such as Gregoire and co-workers (1, 2) have used potassium oleate as a reagent for the nephelometric determination of calcium, it was recognized that magnesium was an interfering substance. Gregoire and Sola (3)made use of this fact to determine magnesium with the potassium oleate reagent. Romeo and Gambordella (6) also used this reagent for the simultaneous determination of calcium and magnesium in xater. I n the attempt to modify the reagent recommended by the latter authors, it was found experimentally that the emulsifying agent known as Duponol stabilized the colloidal suspension over long periods of time and that its presence prevented the formation of magnesium oleate and thus made the reaction specific for calcium. The reaction between calcium ion and the potassium olenteDuponol reagent was then investigated to determine such factors as the optimum spectral region for measurement, optimum concentration of reagents, effect of time of standing, pH, and temperature on the colloidal suspension, and effect of various concentrations of other ions on the reaction. The optimum concentration of Duponol and the ratio of potassium oleate to Duponol were determined experimentally for a fixed amount of calcium standard (0.40 mg. of calcium) in a 10ml. volume. The variation of temperature from 15' to 25' C. did not affect the transmission values and these transmission values remained constant a t room temperature after a 30-minute time interval. The optimum spectral region for making the photometric measurements was found to be about 420 mp. The colloidal suspension was most stable in slightly ammoniacal solution and the addition of ammonia to a sample and the filtration of any precipitate formed, previous to the addition of the potassium oleate-Duponol reagent, served to remove many of the interfering ions. These experimental criteria ;-ere then applied to the solution of the problem of the photometric determination of calcium Home address, 3082 Brighton 13th St., Brooklyn, N. Y.
PRINCIPLE OF THE M E T H O D
Potassium oleate reagent in Duponol solution reacts with calcium salts in ammoniacal solution to give a white colloidal suspension of calcium oleate. The degree of turbidity is proportional to the amount of calcium present over the range from 0.01 to 0.70 mg. of calcium carbonate (0.004 to 0.28 mg. of calcium) in 10 ml. of solution and is measured in a photoelectric colorimeter with a suitable filter. Most of the common ions, when present in quantities usually found in treated or untreated waters, have little or no effect on the reaction. REAGENTS AND APPARATUS
~ O ~ a s s I u nOLEATE f REAGEKT.This reagent is prepared according to the procedure of Romeo and Gambordella (6) : Shake 7.05 grams of oleic acid (Eastman Kodak) with a solution of 1.60 grams of potassium hydroxide in 5 ml. of distilled water. Transfer the emulsion by means of 50 ml. of 70y0 alcohol to a flask. Reflux the mixture for 1 hour and dilute with distilled water t o 250 ml. in a volumetric flask. DUPONOL SOLUTION.Prepare a 3% solution in distilled water. Duponol P.C. is an emulsifying reagent sold by the Dyestuffs Department of E. I. du Pont de Kemours and Co., Inc., Wilmington, Del. POTASSIUM OLEATE-DUPONOL REAGENT. T O each 100 d. Of Duponol solution add 20 ml. of potassium oleate reagent. Allow t o stand 12 hours or longer and filter off, or remove by centrifugation, any sediment formed. This reagent is stable at room temperature but will come out of solution a t lower temperatures. It can be brought back into solution by warming in an incubatoi a t 37" C. CALCIUM STANDARD SOLUTION.Dissolve 0.5 gram of pure calcite (calcium carbonate) in a 500-ml. Erlenmeyer flask with a little dilute hydrochloric acid, being careful to avoid spattering. Add about 200 ml. of distilled water and boil for a few minutes to drive off the carbon dioxide. Cool to room temperature and transfer t o a 500-ml. volumetric flask. Neutralize with ammonium hydroxide. Make up t o volume with carbon dioxide-free distilled water. Store in a glass-stoppered bottle. This standard should be checked for exact calcium content using a gravimetric or permanganate titration procedure. 1.00 ml. = 1.00 mg. of calcium carbonate = 0.40 mg. of calcium. DILUTE CALCIUMSTANDARD SOLUTION.Dilute the above standard solution 1to 10 in a volumetric flask with distilled water. 1.00 ml. = 0.10 mg. of calcium carbonate = 0.04 mg. of calcium. An Evelyn photoelectric colorimeter with selected test tubes graduated a t 5 and 10 ml. was used in this investigation. A spectral transmittance curve was prepared with a Coleman spectrophotometer, and a wave length of 420 m r corresponding to Corning filter No. 511-3 mm. was found to be most suitable for thls determination. EXPERIMENTAL
STUDIES OF REAGENTCONCENTRATIONS. The following studies were undertaken to determine the effect of varying the
INDUSTRIAL AND ENGINEERING CHEMISTRY
758
concentration of Duponol and proportion of potassium oleate reagent to Duponol solution on the turbidity of a solution having a constant calcium content.
Effect of Variation of Duponol Concentration. The effect of varying the Duponol Concentration was determined using 1, 3, 5, 7, and 10% solutions of Duponol. The 3% Duponol concentration was chosen as the most suitable as it gave stable readings after a 20-minute'time interval, did not inhibit the formation of the calcium oleate as did the higher concentrations of Duponol, and the turbidity appeared more in the nature of a true colloidal suspension than that given by other concentrations of Duponol. Effect of Variation of Potassium Oleate Reagent. To 10.0-ml. aliquots of 3% Duponol solution ryere added, respectively, 0.2, 0.6, 1.0, 1.4, and 2.0 ml. of the potassium oleate reagent and thc contents mixed by inversion. i
I
Q)
u
5 lo' Figure 1,
0:s
0:s 110 1:2 1:4 1:6 1:8 POTASSIUM OLEATE REAGENT-ml.
Effect of Variation
2:j
of Potassium O l e a t e Reagent
Vol. 17, No. 12
PREPARATION OF STANDARD CURVE. Quantities of calcium standard solution ranging from 0.1 to 0.7 mg. of calcium carbonate, or 0.04 to 0.28 mg. of calcium, were pipetted in duplicate into the graduated colorimeter tubes, 0.05 ml. of concentrated ammonium hydroxide w m added to each tube, and distilled water added to the 5-ml. mark. The contents were mixed by shaking and 5 ml. of the potassium oleate-Duponol reagent were added with a 5-ml. volumetric pipet. The contents were immediately mixed by inverting each tube several times and the tubes placed in a water bath a t 20 C. Transmission values %'ere read in the photoelectric colorimeter a t a 30-minute time interval against a reagent blank set a t 100%. The curve obtained is shown in Figure 1. PROCEDURE FOR CNK~VOWN SAMPLES.Into a 50-ml. volumetric flask 45 ml. of the unknown mater sample were measured. The sample was neutralized with concentrated ammonium hydroxide until just alkaline to litmus and 1 ml. of the ammonium hydroxide added in excess. The solution was mixed! diluted to the 50-ml. mark with distilled water, and mixed again. It was allowed to stand for 15 minutes or longer. If a precipitate forms, the sample was filtered through a good grade of filter paper. A 2.00-ml. and a 5.00-ml. aliquot were pipetted into graduated colorimeter tubes and the first tube was diluted to the 5.0-ml. mark. For the blank tube 0.05 ml. of concentrated ammonium hydroxide and distilled water were added to the 5-ml. mark. Five milliliters of the potassium oleate-Duponol reagent were added to each tube from a volumetric pipet, and the solution was mixed several times by inversion and read in the photoelectric colorimeter after a 30-minute waiting period, the blank tube being set at 100% transmission. The calcium content is read from the standard curve and the value obtained multiplied by 1.11 to give the actual calcium content of the sample. Where more accurate results are desired the tubes may be placed in a 20 O C. watcr bath during the waiting period. O
Reedings taken alter 30-minute waiting period
One milliliter of dilute calcium standard (0.04 mg. of calcium) and 0.05 ml. of concentrated ammonium hydroxide were added to each of five colorimeter tubes, graduated a t 5 and 10 ml., and distilled water was added to the 5-ml. mark. The contents of the tube were mixed by shaking and 5 ml. of the various potassium oleate-Duponol reagents were added to each tube with a 5-ml. volumetric pipet. The contents of each tube were immediately mixed by inverting the tube several times. The blank tube containing 5 ml. of distilled water and 5 ml. of the 3% Duponol-2.0 ml. .of potassium oleate reagent was set a t 100% transmission. The other tubes were then read a t various time intervals and the results are given in Figure 1 for the 30-minute interval. Gince relatively stable values were obtained after 20 minutes' standing with mixtures containing 1.4 ml. or more of potassium oleate reagent per 10 ml. of the 3% Duponol solution and since there is but little change in transmission values between the 1.4ml. value and the 2.0-ml. value, the latter was chosen for the pre aration of the combined reagent. of Variation of Temperature. For each of the five temperatures shown in Figure 2, 0.20 and 0.50 ml. of the dilute calcium standard (0.08 and 0.20 mg. of calcium) were pipetted in duplicate into the graduated colorimeter tubes, 0.05 ml. of concentrated ammonium hydroxide was added t o each tube, and the contents were diluted to the 5-ml. mark and mixed. The tubes were then placed in a water bath maintained a t the proper temperature, and after a 30-minute time interval 5 ml. of the potassium oleate-Duponol reagent adjusted t o the same temperature were added to each tube with a 5-ml. volumetric pipet. The contents were immediately mixed by inverting several times and the tubes replaced in the bath. The transmission values were read a t various time intervals in the photoelectric colorimeter against a reagent blank set a t 100% transmission and the results obtained are shown in Figure 2 for the 30-minute time interval. These results indicate that the temperature has little or no effect between 15 and 25 C. but beyond this range a marked increase in transmission values occurs, these values increasing with the temperature. While control of the temperature is not essential for routine analysis, more consistent values have been obtained by maintaining the temperature a t about 20' C. E$Pct of Variation of AZkalznity. It has been found experimentally that concentrations of ammonium hydroxide greater than 0.5%, after the final dilution to 10 ml. in the colorimeter tube, have an inhibitory effect on the formation of the colloidal suspension. If the concentration of the ammonium hydroxide is kept below this critical level, variation in the concentration of the ammonium hydroxide has no effect on the transmission values obtained.
k-
501
2o
'
I
Figure 9.
1'0 20 3 0 4'0 5'0 TEMPERATUREPC. Effect of Variation of Temperature
8ffect
Redlngs taken alter 30-minute waiting period
This procedure will determine the calcium content of water samples in the range of 8 to 350 p.p.m. of calcium carbonate. A 1.00-ml. aliquot extends the range of the method t o 700 p.p.m. of calcium carbonate, and samples below 8 p.p.m. of calcium carbonate can be determined after the concentration of a suitable aliquot. DETERMINATION OF CALCIUM CONTEXT OF KNOTXSOLUTIONS. The calcium content of known solutions was determined in duplicate as given above under the Procedure for Unknowns and the results obtained are given in Table I.
O
Table Ssrnule
I.
Determination of Known Samples Calcium Calcium Carbonate Carbonate Present Found Deviation P.p.m.
P.p.m.
%
ANALYTICAL EDITION
December, 1945
759
EFFECTOF IONS COMMOXLY PRESENT IN ~ ~ A T E R .An investigation was made of the effect of various ions which may be found in treated or untreated water with the purpose of determining their maximum allowable concentration (Table 11’). DISCUSSION
In the preparation of the standard curve, transmission values were read at various time intervals for 24 hours; little or no change in these values took place after a 30-minute waiting period. The spectrophotometric curve obtained by this procedure givcs a straight line with amounts of calcium from 0.004 to 0.12 mp. Beyond this value the curve deviates rather sharply but the results obtained by this procedurc have been found to be highly reproducible for values up to 0.28 mg. of calcium even with new batches of the potassium oleatc-Duponol reagent.
Table
2o
0.04
0.08
Figure 3.
0.12 0.16 0.20 CALCIUM-mg.
0.24
0.28
Ion
g+’ ++ hln + +
Spectrophotometric Curve for Calcium
M g++ zn +
A+-+ Sr+r I’b +
DETERMIXATION OF THE CALCIUM CONTENTOF UNKNOWN WATERSAMPLES. The calcium content of the unknown water samples was determined in duplicate with potassium oleateDuponol reagent following the procedure given above. A volume of the water sample was then taken so that its totb.1 calcium content was about 50 mg. and its calcium content was run in duplicate by both gravimetric and permanganate titration procedures as given by Kolthoff and Sandell (4). The results obtained are given in Table 11. DETERMINATION OF CALCIVM, MAGNESIUM, AKD TOTAL HARDNESS IN KNOWN WATERSAMPLES. The magnesium content of the water sample was determined photometrically by the method of Ludwig and Johnson (5) which uses Titan yellow as the colorimetric reagent (Table 111). The only modification in their procedure was that a solution of 0.05% of the Titan yellow in 2% Duponol was used in the determination, thus eliminating the unstable starch solution. The total hardness was then calculated from the formula: Total hardness (as calcium carbonate), p.p.m. = 2.493 Ca 4.1115 Xg.
+
Table 11. Determination of Unknown Samples Potassium OleateGravimetric Potassium Deviation Permanganate from Duponol Procedure a s Procedure Average Procedure CaCzOr . Hz0
Sample
P.p.m. 50.5 412 290 195 67.9 41.4
P.p.m. 50.1
1 2 3
409 286 192 65.5 38.0
5 6
w
P.p.m. 506 409 269 192 64.0 39.8
-0.69 -0.49 -1.38 -1.04 -0.76 -6.40
Table 111.
Determination of Known Samples for Calcium, M a g nesium, and Total Hardness Calcium Nagnesium Total Hardness Content Content soap Sample Known Found IiIiown Found Calculated Found method P.p.m. 1
2 3
104.5 43.6 20.9
P . P . ~ . P.p.m. 102 101 42.9 41.9 20.4 20.4
60.0
.
15 0 10.0
P.P.~. j6.3 59 0 17.6 17.0
12.4 12.6
P.p.m. CaC03
P.p.m. CaCOa
507
494
504 493
170
177
I75
P.p.m. CaC03
164 93.4
102
90 90
+
AS+++
Bar+ a
IV. Interference of Various Ions
Added as Cu(NOs)% FeCls MnCh Mg(NOs) z Zn(N0s)z AlCls Sr(N0a)s Pb(N0s)z AsaOs BaClz
R I aximum Permissible Concentration, P.p.m. 50 50
Other Ion, P.p.nl. Cdcium, P.p.m. 0,13 0.13
0.5 2.0
2005 10005 5005 200 100 2000 10 500”
1.0 0.5
0.25 0.5 0.03 1.0
Interference due t o this ion not investigated beyond concentration given.
The results given in Tables I and I1 indicate that the method is subject to maximum errors up to about 10% in the lower range of values but that this error decreases sharply for the higher calcium values. The average error for these determinations is approximately *4y0 and the results obtained compare favorably with the gravimetric Qrtitration procedures. The method is particularly useful for carrying out large numbers of determinations, as in routine or control analysis of water samples, and where great aocuracy is not the prime consideration. The interference of ions other than those given in Table IV was also studied: Ag+, Hg++, B i t + + , Cdt+, Sb+++, S n f f , Xi+ ++, Co++, K a t , K + , KH,+, co3--,SO4--, SO,--, SzO,--, Cr04-, B407--, C4H40s--, HP04--, F-, czo4--, AsOe-, HAS04--, C1-, Br-, I-, CK-, Fe(CN)a-‘, Fe(CN)e---, CNS-, S--,KO2-, NO3-, C1H3O2-, C103-, SiO,--. No anions below concentrations of 500 p.p.m. and few cations in concentrations less than 100 p.p.m. interfere with the procedure. Large amounts of magnesium, which are a source of difficulty in the oxalate procedure, have no effect in this procedure. Even when these ions are present in relatively,large amounts and the amount of turbidity formed by a given concentration of calcium is either increased or decreased by the presence of these ions the method may still be used if the standard curve is made up in the presence of these ions. Such a procedure is nom in progress for the photomctiic dctermination of calcium in biological fluids. LITERATURE CITED
(1) Gregoire, A, J. SOC.Chem. I n d . , 4 2 , 4 2 7 A (1923). (2) Gregoire, A., Carpiaux, E., Larose, E., and Sola, T., BUZZ. BOC. chim. Belg., 32, 131 (1923). (3) Gregoire, A., and Sola, T., Ibid., 32, 131 (1923).
(4) Kolthoff, I. hl., and Sandell, E. B., “Textbook of Quantitative Inorganic Analysis”, New York, Macmillan Co., 1943. ( 6 ) Ludwig, E. E., and Johnson, C. R., IND.ENG.CHEM.,t l n - . ~ ~ED., .. 14. 8 9 5 (1942).
(6)
Romeo, A., and Gambordella, V., Chim. i n d . (1941).
agr.
bid.. 17, 471-6
