Flame Photometric Determination of Calcium in Sea Water and Marine Organisms TSAIHWA J. CHOW Department
of
and
THOMAS G. THOMPSON
Oceanography, University o f Washington, Seattle, Wash.
The purpose of this investigation was to develop an accurate and convenient flame photometric procedure for analyzing calcium in sea water and marine organisms. Factors which affect the intensity of the light emitted by calcium were determined. The major constituents of sea water showed no band-width interference. The chloride, magnesium, potassium, and sulfate ions showed a negative radiation interference, while the sodium ions gave a positive radiation interference for the calcium determination. The “internal standards” technique was used for eliminating the effect of the interfering constituents. Sea water samples were collected at various depths down to 5000 meters in the Gulf of Alaska and were analyzed for calcium. A calcium-chlorinity ratio of 0.546 zk 0.002, which agreed with the previous reported values, was obtained when the calcium was expressed in milligram-atoms. The applicability of the method for the analysis of marine organisms, marine sediments, and limestone was demonstrated.
T
HE calcium content of sea water has been conventionally
determined by precipitating it as calcium oxalate under carefully controlled conditions and subsequent titration with potassium permanganate (3)or ignition to calcium oxide (3, 5 ) . The alkaline earth metals in river waters (7) and strontium in sea water ( I ) have been determined by means of the flame photometer. The purpose of the present investigation was to develop a convenient flame photometric procedure for the direct determination of calcium in sea water and in marine organisms. CHEMICALS AND EQUIPMENT
period of 15 minutes was allowed before measurements were made in order to attain the maximum operating stability of the instrument. The atomizer-burner was thoroughly rinsed with distilled water between each sample change. The reproducibility of the instrument was checked with calcium standard solutions a t the beginning and the end of each series of determinations. The emission intensity was read from the transmittance scale. Throughout the investigation the Beckman DU spectrophotometer setting for measuring the calcium emission intensity was as follows: W a r e length
.
Selectora Sensitivity Phototube Resistor Slit width Oxygen Hydrogen The selector may be set concentration.
BAND-WIDTH INTERFEREYCE
Studies were made to determine the extent of band-width interference that might be caused by the presence of some of the constituents of sea Tvater (chloride, sodium, sulfate, magnesium] potassium, and strontium). Individual standard solutions containing one of these ions whose concentration was equivalent t o that found in sea water were prepared. The emission intensity of those solutions was measured at the wave lengths of 422.7 and 418 mp. The net emission intensity obtained at the wave
50
All chemicals used in this investigation were of analytical grade and tested for traces of calcium. Stock solutions of calcium containing 20.0 mg.-atoms per liter were prepared by dissolving 2.002 grams of calcium carbonate in a limited volume of hydrochloric acid and diluting to 1liter. From such solutions, suitable aliquots Tvere diluted for the calcium standard solutions. Other standard solutions of sodium chloride, magnesium chloride, potassium chloride, ammonium chloride, and ammonium sulfate were prepared. Polyethylene containers were used for the storage of the standard solutions in order to avoid contamination from glassware. The intensity of the light emitted by calcium was measured with a Beckman DU spectrophotometer fitted with the multiplier phototube (Yo. 4300) and the modified flame attachments (KO. 9220). Tanks of hydrogen and oxygen were used as the fuel.
Calcium spectral line 4 2 2 . 7 mp Flame background 418 mp 1.0 Counterclockwise Multiplier phototube 22 megohms 0 . 0 1 mm. 15 lb. per sq. inch 4 lb. per sq. inch o n the 0.1 position for samples of low calcium
I
EXPERIMENTAL
The spectral line 422.7 mp was chosen as the -wave length for the calcium emission intensity measurement. Because of the continuous emission of the solvent, the flame background should be subtracted from all calcium emission readings. The background intensity a t the wave length of 418 mk was equal to that a t 422.7 mp and was unaffected by changing calcium concentration. The difference in reading obtained a t these two wave lengths was used as a measurement of the net light intensity emitted by calcium. The sensitivity knob of the Beckman DU spectrophotometer was operated a t the counterclockwise position. A slit width of 0.01 mm. w-as used in order to minimize the possible band-width interference from major constituents of sea water. A warm-up
CALCIUM mg.-atom / liter Figure 1. Calibration graphs of calcium in presence of various interfering ions A . S o d i u m chloride, 470 m g . - a t o m s per liter B . Distilled water C. P o t a s s i u m chloride, 10 m g . - a t o m s per liter D . A m m o n i u m chloride, 550 m g . - a t o m s per liter E. Magnesium chloride, 50 m g . - a t o m s per liter F. A m m o n i u m sulfate, 30 m g . - a t o m s per liter G. Sea water, 19Q/po chlorinity. c a l c i u m free
910
911
V O L U M E 27, NO. 6, J U N E 1 9 5 5 Table I.
Emission Intensity of Major Constituents of Sea Water
Constituents Chloride Sodium Magnesium Sulfate Calcium Potassium Strontium
Concentration, hlg.-Atom/ Liter
Net Reading a t 422.7 mp 0.00 0.14 0.00 0.00 34.0 0.00 0.00
550 500 50 30 10
10 0 1
length of 422.7 m r is given in Table I. Kone of these ions emitted any appreciable amount of light a t the wave length of 422.7 mp.
The calibration graphs were constructed by plotting the net emission intensities against calcium concentrations as shown in Figure 1. The intensity of the light emitted by calcium deviated from the linear at the high calcium concentrations. However, a linear relationship was obtained (Figure 2) between emission intensity and calcium concentrations less than 2.0 mg.-atoms per liter when the selector switch was set at the 0.1 position. The deviation from the linear line (Figure 1) demonstrated that the emission intensity was affected not only by the calcium concentration but also by the concentration of other constituents of the solution. The magnitude of deviation was dependent upon the nature of the interferences. Thus, calibration graphs would have to be frequently prepared for waters of varying chlorinities when making calcium analysis on sea water samples.
RADIATION INTERFEREZTCE
The effect of radiation interference on the calcium recovery because of the presence of the various major constituents of sea water was studied. The net emission intensity of the solutions containing known quantities of calcium together with possible radiation interfering ions v a s measured at the wave lengths of 422.7 and 418 mp, and the amount of calcium recovered in each case is given in Table 11. As shown in Table 11, the presence of magnesium and sulfate ions greatly suppressed the calcium recovery. The presence of chloride and potassium ions also gave a lower recovery of calcium, but t o a much lesser degree. The sodium ions caused calcium to emit more light and therefore a high calcium recovery. The strontium ions did not interfere with the calcium determination when present in small quantities approximating that found in sea Fater and most marine organisms. CALIBRATION GRAPHS O F CALCIUM
80
60
* k
U J 2
w
c
t 4c 2
P v)
4
I
w
2c
The emission intensities of calcium standard solutions ranging from 0.0 to 20.0 mg.-atoms per liter in the presence of various interfering ions were measured a t the wave length of 422.7 mp. (
Tahle 11. Radiation Interference on Calcium Determination constituents
Concentration, .\Ig.-Atoin.’ Liter
I 0.8
CALCIUM
Figure 2. Calcium. Mg.-Atom/Liter Added found-^-
I 0.4
I
I
12
I.6
D
mg.- a t o m l l i t e r
Calibration graph of calcium in distilled water
I n order to secure the most consistent results and to make the determination independent of the chemical and phi-sical conditions of the solutions being analyzed, the “internal standards” technique of flame photometry ( 1 ) was applied to the calcium determination. This was done as follows: Sulfate
0 .0
7.3 1.5 0 30 O‘
4.5 0 60 0 90.0
hIagnesium
0 0 13.8 27.5 55 o a 82.5 110.0 165.0
8 0 8 0 8 0 8 0 8.0 8 0
8 0 8.0 8.0 8.0 8.0 8 0 8 0 8.0
8 0 3 8 3 6
3 8 4 0 4 1 4 1
8 0 3.4 3.8 4.0 4.1 4.1 4.1
Potassium
Strontium
0.0 0 In 0.3
8 0 8 0
8.0
a Denotes approximate concentration in sea water.
8.0 8.0 8.0
Tn-enty ml. of the sea water sample to be analyzed was diluted to 1 liter. After thorough mixing, a series of six portions of this solution, each containing 5 ml., was taken. To one portion, 5 ml. of distilled water were added, while to the other five portions there was introduced 5 ml. of standard calcium chloride solution, cwh portion receiving a standard solution of different concentration of calcium. The emission intensity of these six portions was determined in the Beckman flame photometer in the manner outlined above. The emission intensity was plotted against the calcium concentration of the standards added.
X straight line, analogous’ to that shown in Figure 3, was obtained. The combined net intensity of the light was that emitted by calcium in the standard, plus the calcium in the unknown. If there Rere any interfering substances existing in the solution, they affected equally the emission of the light resulting from that of the standard and the unknown. The line intersecting the ordinate indicated the emission intensity of calcium in the unknown. ,4s the graph in Figure 3 was a straight line, y = n bx. The unknown calcium concentration, 5, was given b3- the ratio
+
ANALYTICAL CHEMISTRY
912 Table 111. Results of Analysis Testing Validity of Technique Calcium, Mg.-Atom/Liter Added Found 0.00 0.00 0.80 0.80 1.60 1.60
Medium Distilled water Synthetic sea water, Ca-free, 1.90jeo C1
0.00 0.20 0.79 1.97
0.00 0.20 0.80 2.00
Table I V were obtained on several samples of fresh water and on a series of samples of sea water taken a t varying depths down to 5000 meters in the Gulf of Alaska. The fresh water samples were analyzed directly, but 20 ml. of each sample of sea water was diluted to 1 liter with redistilled water. This dilution adjusted the calcium concentration t o such a range that the light intensity responded linearly with the concentration. It also avoided the formation of salt crystals on the tip of the atomizer-burner and greatly minimized the flame background.
Table IV.
Analysis of Water Samples Fresh Water Calcium, big.-Atom/Kg.
Location Tap water Seattle Lake Wasdington. Seattle Chase Lake, Washington Halls Lake, Washington
0 0 0 0
08
12 09 07
Sea Watera Depth, Meters 0 20 50 100 200 300 500 750 1000 1300
Temp., C. 13.80 11.26 5.65 5.68 5,15 4.67 4.15 3.59 3.14 2.62 2.00 1.77 1.62 1.54 1.52 1.52
Chlorinity, 8/00
Calcium, Alp.-Atom/ Kg. 9.85 9.90 9.90 10.1 10.2 10.3 10.4 10.3 10.4 10.4
Ca/Q//oa CI
Average Location, Pacific Ocean, Gulf of .4laska, -4ugust 1954.
0 546
---
The calcium-chlorinity ratio found for the sea waters was CALCIUM mg.-otom 1 liter Figure 3. Determination of calcium by “internal standards” technique
0.546 & 0.002, which is in good agreement with data previously reported (8, 5 ) but somewhat higher than the values given b y
Sverdrup and coworkers ( 4 ) . However, the values quoted by the latter are actually those of the former which had been arbitrarily corrected for possible presence of strontium, using the apparently incorrect assumption of Webb (6) that the relatively minute concentration of strontium was quantitatively precipitated as the oxalate. In Table V are given the results obtained from the analysis of various marine organisms, and on samples of a deep sea sediment, a globigerina ooze, and a limestone. The calcareous portion of the marine specimens was dried. Weighed samples (0.50 to 1.0 gram) of this dried material were ignited to 500” C. to destroy the organic matter. After cooling, the residues were treated with 10 ml. of water, and 12Y hydrochloric acid was added dropwise
of y to the slope of the line, b, when a = 0. Thus the concentration of calcium in the diluted unknown was read graphically from Figure 3 by doubling the emission intensity, Y , of the unknown because of the equal dilution of the portion with distilled water. From the point 2 Y , a line parallel with the abscissa was extended until it intersected the calibration graph. The point of intersection indicated the calcium concentration in the portion of the diluted Eample of the unknown. This observed concentration, when multiplied by 50, gives the actual concentration of calcium. The variation between analyses made on the same Sam& averaeed kO.1 me._ _ ~ _ _ ._____~__atom of calcium per kilogram of sea water. Table V. Calcium Content of Some hlarine Organisms The validity of this technique for elimiCoinmon Name Scientific Name Source of Samples Ca, % nation of interference from the major Calcareous alga Bossea orbioniana Xonterey, Calif. 2 9 . 3 zt 0 . 2 Calcareons alga Corallina gracilis llonterey, Calif. 26 2 zt 0 . 1 constituents of sea water was studied. Foraminifer Calcarina s p . Ifalik Atoll 31 6 i 0 . 2 Calcareous sponge Rhabdodernslla n u l t i n o i Monterey, Calif. 29 6 i 0 . 2 Known quantities of calcium were added Coral Porites s p . Ifalik -4toll 3 7 4 i 0 2 Chiton M o p a l i a muscosa Friday Harbor, Wash. 3 7 . 7 i. 0 . 2 t o synthetic calcium-free sea water and Oyster Crassostrea virginica Fort Stark, N. H . 3 3 . 7 zt 0 . 2 Alonterey. Calif. 39.2 f0 . 2 Haliotis cracherodiz the calcium concentrations of the resultAbalone Oliua litterata Marc0 Island, Fla. 3 9 . 6 i0 . 4 Snail ing solutions were determined. The reBarnacle B a l a n u s eburneus .Illigator Harbor, Fla. 36.6 & 0 2 Cancer borealis Fort Stark, N. H. 24 o zt 0 . 2 Crab sults are given in Table 111. Asterias vulgaris hlarblehead Neck, Mass. 20 1 i 0 . 1 Starfish I
-
ANALYSIS OF SAMPLES
The results of analyses given in Table I V and Table V demonstrate the applicability of the method. The data shown in
Brittle starfish Sea urchin Deep sea sediments Globigerina ooze Limestone deposit a
O p h i u r a sarsi Hcteroccntrotus trigonarius
Friday Harbor, Wash. Ifalik Atoll Indian Oceann Pacific Ocean Roche Harbor, Wash.
Swedish deep sea expedition, 1947-48.
____
31 2 i 0 . 3 3 2 . 7 =k 0 . 1 29.9 f0 . 2 3 7 . 5 zt 0 . 3 38 1 z t O . 2
--
V O L U M E 27, N O . 6, J U N E 1 9 5 5
913
until the residue had dissolved. This solution was then diluted t o 1 liter, and after thorough mixing, a series of 5-ml. portions was taken for analysis using the method outlined above, 4CKNOWLEDGMEUT
The authors )?ish to acknowledge assistance received from the National Science Foundation LITERATURE CITED
(1) Chon-, T. J.. and T h o m p q o n , T. G.. ABL. 21 (1
