Determination of Inorganic Constituents in Sucrose Solutions ALLEN GEE, LOUIS P. DOMINGUES, National Bureau
VICTOR R. DEITZ o f Standards, Washington 25, D. C. and
Rapid procedures were investigated for the analysis of commercial sugars for potassium, sodium, calcium, magnesium, chloride, sulfate, phosphate, and silicate without ashing of the sugar. The metallic constituents, particularly the alkali metals, are conveniently determined in a 5" Brix solution by flame photometry. The hydrogen flame was better than the acetylene because of lower radiation interferences. The flame photometric determinations for calcium and magnesium agreed well with the values obtained by titration with ethylenediaminetetraacetic acid, but the advantage is with the latter in many instances. Chloride was satisfactorily determined by conductometric titration and sulfate by turbidimetric measurements on barium sulfate. Soluble silicate and inorganic phosphate were measured by the intensities of the molybdenum blue formed under two different sets of conditions.
T
HE purpose of this paper is to organize, under one title, rapid methods which do not involve ashing for determining the individual constituents in the presence of sucrose. Although the determination of total ash in cane sugars and sugar liquors is an important commercial function, the individual constituents in the ash are seldom determined. Ashing of the sugars is a time-consuming process. I n order to determine carbonated ash ( 3 )the sugar sample is carefully charred and then ignited to constant weight a t a dull red heat. I n the process a part of the inorganic matter is lost by volatilization and, a t the same time, carbonates not originally found in the liquor appear in the ash. I n the sulfated ash method the metallic constituents are recovered as sulfates together with phosphates and silicates after the organic matter is completely oxidized. Although the results are reproducible ( & 2 % of the amount present) for any given procedure, they do not represent the true amounts of inorganic materials present when the relative amounts of the constituents vary. A relative precision, equal to that for total ash determination (2%) for the analyses of the major constituents, is satisfactory and a loxer precision is tolerated for the minor ones. These methods include flame photometry for the alkali and alkaline earth metals; titration for calcium, magnesium, and chloride; nephelometry for sulfate; and colorimetry for phosphate and silicate. The suitability of any method was determined by one or more of these tests: satisfactory recovery of a quantity of the component to be determined when added to a reference lowpurity sugar. satisfactory determination of the constituents of known synthetic mixtures in deionized sucrose; and agreement nith results found by an independent method. MATERIALS
Reference Sugar. This sugar is a low-purity Jamaican raw cane sugar, having high color and low polarization, and was used in this laboratory in color removal tests. The samples for analyses were carefully subdivided from a large quantity of the master material, and the following pertinent characteristics were determined from 16 samples of this sugar. Moisture, %, Direct polarization, Invert, % PH Total ash, % - Ignited a t J J O O C. Attenuation index X = 560 m p , 60° Brix
037i.002 92 28 o 03 3 00 0 005 5 42 0.02 1 42 =t0 02
*++
3
oa
i. 0.06
A riffled sample of this reference sugar was dissolved in water to make a 25' Brix solution (by refractometer), and filtered through a coarse fritted-glass filter. Deionized Sucrose. Granulated sugar of 30' Brix was passed through a monobed composed of the strong anion exchanger, Amberlite IRA-400 and the cation exchanger IR-120. This solution was diluted to 25" Brix and stored in a refrigerator. Standard Solutions. Standard solutions are prepared by weighing appropriate amounts of salt for each of the following components: sodium as sodium chloride, potassium as potassium chloride, calcium as calcium carbonate (dissolved in hydrochloric acid), sulfur trioxide as potassium sulfate, phosphorus pentoxide as potassium phosphate, dibasic magnesium as magnesium sulfate heptahydrate, and silicon dioxide as sodium metasilicate, nonahydrate. The last two were weighed as the hydrate and the magnesium sulfate solution was further standardized by titration with ethylenediaminetetraacetic acid (EDTA, Versene). The silicon dioxide content of sodium d i c a t e was checked gravimetrically after acid dehydration. Ethylenediaminetetraacetic Acid Solution (10mM). Dissolve 4.0 grams of Versene, analytical grade (obtainable from Bersworth Chemical Co., Framingham, Mass.) and 20 mg. of magnesium sulfate, heptahydrate in 500 ml. of water, adjust the solution to p H 7 to 8 with ammonia, and dilute to 1 liter. Store in a polyethylene bottle. Standard Calcium Solution (8.00mM).Dissolve 0.400 gram calcium carbonate (analytical reagent, low in alkalies, ACS specifications) in a slight excess of hydrochloric acid and dilute to 500 ml. Eriochrome Black T Indicator. Dissolve 50 mg. of the dye in 20 ml. of triethanolamine, technical grade, purified (6). Murexide. Grind 0.10 gram of the dye with 20 grams of potassium chloride, analytical grade. Ammonia-Ammonium Chloride Buffer. Mix 10 volumes of concentrated ammonia with 9 parts of water and 1 part of concentrated hydrochloric acid. Acid Molybdate Reagent A. Dissolve 8.0 grams of ammonium molybdate, reagent grade (ACS specification), in warm water and dilute to 200 ml. In another bottle, dilute 61 ml. of concentrated sulfuric acid to 200 ml. Mix daily as needed equal volumes of the molybdate and acid solutions. If the blank in water is above 0.060 absorbance in a 1-em. cell, increase the sulfuric acid concentration in the stock solution until this condition is met. The properties of the ammonium molybdate solution change SlOTVly. Chlorostannous Acid (0.25%). Dilute 1part of a stock solution of 5 % stannous chloride in 6 N hydrochloric acid (stored with metallic tin, if desired) with 19 parts of water. This dilution must be done the same day the solution is used. The 5% solution is usable for about a week. Acid Molybdate Reagent B. Dissolve 4.0 grams of ammonium molybdate (reagent grade, ACS specification) in 190 ml. of water and add 4.0 ml. of concentrated sulfuric acid. Dilute a small portion with 15 parts of water and measure the pH. Add acid to the original solution to make the pH of the diluted portion between 1.85 and 1.90. Discard solution when any precipitate forms in storage. Barium Chloride Reagent. Barium chloride, dihydrate, reagent grade, was screened to 40 to 80 mesh. METHODS AND RESULTS
Potassium, Sodium, Calcium, and Magnesium by Flame Photometry. The Beckman DU spectrophotometer, with flame attachment KO.9200 and an acetylene burner, was employed. A 4000-megohm phototube load (actually two resistors in series) was added to the third position in the resistance box. A reading beyond the scale was covered by s ~ i t c h i n gto a smaller load resistor and multiplying the scale reading by the appropriate factor. This procedure gave reliable results when the relative values of the resistors were calibrated in the instrument on the same day. Both acetylene and hydrogen were used with the same burner, The capillary of the all-metal burner handled solutions containing as much as 15% of sucrose, although large amounts of gray incrustation, which would eventually clog the capillary, formed on
1488
ANALYTICAL CHEMISTRY
the tip of the burner a t the higher concentrations. The tendency of the capillary to clog can be reduced by aspiration of water between measurements. Even when distilled water was aspirated by the burner, the luminosity of the flame produced a background reading. This background was smaller with hydrogen than with acetylene and tended to become larger a t shorter wave lengths. It is appreciable in the calcium determination with the acetylene flame and the magnesium determination with the hydrogen flame. Except for the case of magnesium, it is not affected by sucrose concentration. The ratio of hydrogen to oxygen was controlled by keeping the pressure gage readings constant a t 8 and 10 pounds per square inch, respectively. Also, for acetylene and oxygen, constant gage readings of 2.5 and 10 pounds per square inch, respectively, were used. 120
z
g BO Lo
5 g - 4c 4W
= o
Ex Ref. 2. Ex R e f .
e 5.
I-
0
I
5 POTASSIUM, mM
I
IO
1 15
Figure 1. Emission of Potassium, Acetylene Flame Figure 1 gives a conventional plot of the emission of potassium with respect t o concentration with the acetylene flame. I n Figure 2, sucrose enhanced the potassium emission less than 4% and the extent depended on both the sucrose and potassium concentrations. The emission of sodium behaved similarly. On the other hand, the 554 mp oxide band of calcium was greatly enhanced by sucrose to over 3001, above 10" Brix, and the percentage enhancement was found t o be independent of calcium concentration. Because raw sugars contain a mixture of salts, the influence of the simultaneous presence of other salts normally encountered was investigated. The change in emission of a given metal in a four-component synthetic mixture, and that for the same concentration in pure sucrose but mithout the other salts, expressed as per cent enhancement, is compared in Table I for both the acetylene and hydrogen flames. The mixture contained G.2mL11 potassium, 0.5mJ.I sodium, 1.0rnM calcium, and 1.0mil.I magnesium. The enhancement in 5 " Brix sucrose n i t h the acetylene flame a t the salt concentrations listed is given in the last two columns of this table. Comparison between these two columns shows that with all the salts present, mainly potassium, the sodium and calcium emissions are enhanced to a greater degree than in sucrose alone. Qualitatively, the enhancement by other salts is in agreement with the reported radiation interferences (2, 12). With the hydrogen flame, similar enhancements occur, but these were much smaller in magnitude. Magnesium can be determined with this flame with limited precision. Although the spectral emission in the hydrogen flame is lower, the smaller background and radiation interferences recommended it as the choice for all subsequent work. The large radiation interferences which occurred with the magnesium emission parallel those of another study ( 4 ) . Figure 3 s h o w that pure sucrose, as well as other salts present in a raw sugar, produce both a change in slope and in intercept of the curves. While radiation interferences are large, their magnitudes become sensitive t o flame and instrument adjustments. Because there are organic substances in the raw sugars which cannot be duplicated by synthetic mixtures, an attempt was made to find whether these substances would cause additional interferences. A mixture was made with pure salts in deionized
Table I.
Radiation Interference in 5" Brix Sucrose
(Per cent enhancement of emission over t h a t of the aqueous salt solution) Hz Flame ClHz Flame Wave One FourOne Four" Length, Concn., comcomcomcomIon Mfi mM ponent ponents ponent ponents K 768 6.2 0.8 0.5 1.4 0.9 Na 589 0.5 1.0 7.2 3.3 35.0 Ca 554 1.0 5.0 11.0 27.0 84.0 Mg 371 1.0 84.0 166.0 , . .. 5
Mixture of 6.2m.M K , 0.5m.ll Na. l.OmZICa, 1.Om.M N g .
sucrose such that the emissions for potassium, sodium, calcium, and magnesium matched those for the reference sugar. For calcium and magnesium, the concentrations thus obtained checked with those determined independently by titration. Radiation interferences other than from sucrose and inorganic salts are therefore absent. The emissions for the alkali metals were followed when increments of the appropriate salts wereadded to two different concentrations of reference sugar. The good agreement of the potassium emissions, when various amounts of potassium chloride were added t o 2' and 5' Brix reference sugar with those in pure sucrose (connected by the curve in Figure l ) , is an example of the fact that no other serious interferences were present. Therefore, in this particular raw cane sugar the flame emissions for potassium, sodium, calcium, and magnesium are the same a8 those obtained in the mixture of the same concentrations of the respective chlorides (or of some sulfates) in the presence of an equal amount of sucrose. Anions such as phosphate which are known to repress emissions ( 1 5 )are not present in sufficient quantity to have any effect.
30
1
Sucrose,
Figure 2.
a
I
I
1 r
Ex
Enhancement of Flame Emission by Sucrose, Acetylene Flame
The results from the determinations by flame photometry are presented as part of the summary compilation in Table IV. Solutions 1 and 2 were prepared from separate portions of the original reference sugar. Solution 2 was compared with a synthetic mixture which gave identical emissions for the four metals, and the concentrations listed in Table I V are the same as those in the mixture. The results for solution 1 are read from calibration curves. There is no significant difference between the two sets of results, and, except for magnesium, the values are satisfactory for routine determinations.
It is very inconvenient to prepare a synthetic PROCEDURE. mixture for each determination. Because potassium is the major constituent in raw sugars, calibration curves for the other three metals should be made for several potassium concentrations. The suggested procedure for sodium using a Beckman DU spectrophotometer might be as follows: Set the instrument to read 100% transmittance with standard
1489
V O L U M E 26, NO. 9, S E P T E M B E R 1 9 5 4 1mM sodium chloride in water and about 0.05-mm. slit width. Calibrate with solutions containing varying amounts of sodium chloride up to 1mJT in 5" Brix sucrose a t 0,2, and 10mM potassium chloride. For actual determinations, set the instrument with the same standard and take three readings in rapid succession for each sugar sample made to 5" Brix. From the average of the readings, obtain the sodium content from the calibration curve corresponding to the proper potassium concentration which had been determined in a similar manner. The limitations of the procedure will be revealed only after more tests have been completed on a variety of sugars. Calcium and Magnesium by Titration. Titration with ethylenediaminetetraacetic acid has been successfully applied to measure hardness of sugar liquors (8). Ethylenediaminetetraacetic acid chelates nearly all polyvalent ions, and in commercial sugar liquors, these are confined largely to calcium and magnesium. Titration in an ammonia-ammonium chloride buffer a t pH 10 n i t h eriochrome black T as indicator gives the total of calcium and magnesium, and titration above p H 12 with murexide as indicator yields the results for calcium (9).
60
2 2
I w 40 w
t = eo j U w
I
I
1
2
I
3
4
MAGNESIUM, m H
F i g u r e 3. Emission of Magnesium, Hydrogen Flame, 371 mp The results, obtained by the titration procedures, agree well with the flame photometry measurements. When the color of the liquor is not too high to obscure the end point, the titration for calcium plus magnesium is very precise. When the calcium content is knom-n, this method for estimating magnesium is more rapid and precise than flame photometry. The titration for calcium with murexide as indicator entails end point difficulties and is precise only when the calcium content is a t least as high as that in washed sugars. Because no calibration curve is required, both titration methods are more convenient than flame photometry where conditions permit their application. PROCEDURES. For the sum of calcium plus magnesium, dilute an aliquot of sugar liquor ( 5 ml. of 25" Brix reference sugar was used) to about 50 ml , and add 10 drops of ammonia-ammonium chloride buffer and 3 drops of eriochrome black T indicator. Titrate with ethylenediaminetetraacetic acid until the red magnesium complex disappears Standardize the ethylenediaminetetraacetir acid solution against the 8.00mM calcium solution by this procedure. The end point is very sharp and the use of hydroxylamine or cyanide ( 5 ) to repress the attack of trace elements on the indicator is usually not necessary. For the determination of calcium, dilute a similar aliquot of sugar liquor to 50 ml., and dissolve in the solution 1 pellet (about 0.15 gram) of sodium or potassium hydroxide. Add about 0.2 gram of murexide mixture and, after dissolving, immediately titrate with the ethylenediaminetetraacetic acid solution to the violet end point a t which stage another drop does not change the color further. If the end point does not appear sharp, spectrophotometric titration ( a t about 650 mHj can be applied. Standardize the ethylenediaminetetraacetic acid solution according to this procedure. Chloride Determination. Chloride was determined by a conventional conductometric titration with 0.1M silver nitrate. Aliquots of 5 to 10 ml. of 25' Brix reference sugar solutions were diluted to 200 ml. with water and sufficient ethyl alcohol was added so that the initial specific conductivity was not over 200 micromhos. The average of the results is given in Table IV.
Phosphate and Silicate Determinations. Raw sugars produced from clarified juices are low in both inorganic orthophosphate and soluble silicate. Therefore, the sensitive molybdenum blue method is most suited for the analysis of either constituent. Because the respective heteropoly acid forms under different conditions, it is possible to make an independent estimate of each. The phosphate-molybdic acid complex forms rapidly in fairly acid solution. Immediately after formation, the complex is reduced with stannous chloride, and the intensity of the resulting color is almost proportional t o the phosphate concentration (10,11,17). Organicphosphates, which may account for the major portion of the phosphorus in raw sugar (IO), are not determined by this method. Molybdisilicic acid is slowly formed ( 1 ) along with the molybdiphosphoric acid at p H 1.9. After the complexes are formed, concentrated acid is introduced to decompose the phosphate complex ( 1 3 ) before the solution is reduced with stannous chloride. Thus, the amount of molybdenum blue is a measure of the silicate content. The absorbances of both blue solutions were measured a t 720 mp because this Tvave length has also been used to estimate turbidity in sugar liquors. The absorption maximum for the silicate-molybdenum blue lies somewhat above 750 mp.
PROCEDURE FOR PHOSPHLTE. Introduce2.00 ml. of 25' Brix test solution into a 50-ml. volumetric flask. A4ddsufficient water to Pipet into the flask 2.0 ml. of acid molybdate well. Quiclily add 1.0 ml. of the diluted chlorostannous acid, mix thoroughly, and dilute to volume. Measure absorbance, a t 720 mH, 5 minutes after adding the reducing agent. When the phosphate concentration is very low, record the maximum absorbance if this occurs after 5 minutes. Prepare a water blank by omitting the sucrose. Prepare a sucrose blank by adding all reagents except the chlorostannous acid. When the absorbance of the water blank is not above 0.060, no color will form in 1' Brix sucrose in the absence of phosphate. The phosphomolybdenum blue absorbance minus the sucrose blank is nearly proportional to the phosphate concentration for werose contents of 1" Brix and above. Make the calibration curve in 1O Brix sucrose periodically. This procedure was applied to the reference sugar, in which the final concentration n-as varied from 0.1 ' to 1' Brix. Addition of known amounts of phosphate showed that the incremental increase in absorbance was essentially independent of the sucrose concentration. For example, the absorbances a t 720 mp corresponding to 0, 0.01, and 0.02mLW added phosphate were, respectively, 0.30, 0.55, 0.80 in 0.5 o Brix reference sugar and 0.08,
T a b l e 11. Absorbance of Silicomolybdenum Blue Solutions Sucrose, Brix
1.1
5.6
s103,m M Added Total
0 005 0 005 0 020 0 020 0.020 0.020
Added Pod, m.M
Deionized Sucrose 0 005 0 005 0 030 0 020 0 020 0.030 0,020 0.020 0.030
Reference Sugar 0.010 0.010 0.030 0.010 0.56 0,005 0.010 0.015 0.33 0 0.003 0.11 0 0.001 0.010 0.010 5 Blank, no reduction with chlorostannous acid. b Corrected for blank. 1.1
0 0 0 0 0.005 0.010
Absorbance, 720 .%fr
0 082 0 080 0 297 0 277 0.135 0.087 0 . 072a
0.313 0.171b 0.084b 0.149b 0.212b 0.052b 0.019b 0.160b
1490
ANALYTICAL CHEMISTRY Table 111. Turbidimetric Estimation of Sulfate
Sucrose, Brix Reference Total None 1
1
1
3 1 1
1 3 5 2
1 1 1
1 1
1
10 2 2 1 1
1
1
1
Sulfate Added, mM 0.078
Time, Sec., BaClz Soln. 10
0.267 0.267 0.267 0.40 0.40 0.40 0.40 0.40 0.40 0 0 0 0 0 0.067 0.133
15 50 15
15 35 70 33 35 30 25
40 25
35 70 29 15
Readings Coleman Beckman 13.2 0.174
55.3 55.4 56.3 88.8 87 0 83.0 90.8 85.6 76.6 106.0 50.2 48.9 47.0 51.3 62.8 76.3
0.670 0.663 0,657 0.995 1.085 1 037 0.905
stir a t a controlled rate. Add about 0.25 gram of barium chloride in one portion and stir until completely dissolved. The rate of stirring had previously been adjusted so that the solution time was between 30 and 45 seconds. The presence of sucrose requires higher stirring rates. The remaining 50 ml. of solution is used for the blank. To measure scattered light set the Coleman nephocolorimeter a t maximum sensitivity with the red 655 mp filter inserted. With the instrument set a t zero with the blank, read the scattered light for the barium sulfate suspension directly from the galvanometer scale. For transmittance measurements, the Beckman DU spectrophotometer set a t 720 mp was employed with 10-cm. cells and the readings were matched against the blank. The maximum readings were taken in each case. The times required to read the maximum with the Coleman instrument were approximately as follows for different concentrations of sucrose and sulfate.
' Brix
1.052 0.500 0 480 0.495 0.570 0.680 0.830
0.33, 0.56 in water only. Because the large amount of invert sugar present caused a slow formation of molybdenum blue, the sugar blank is high-almost 0.10 a t 1 Brix. A final concentration of O.OlmM phosphate gives an absorbance of about 0.25. Silicate in amounts equal to the phosphate concentration reduces the absorbance a negligible amount, about 2%.
PROCEDURE FOR SILICATE. Place a 2-ml. aliquot of 25' Brix liquor in a 50-ml. volumetric flask and dilute to 28 to 30 ml. Add 2.0 ml. of acid molybdate solution B, mix, and let stand for 7 minutes. Add 10 ml. of 6-47 perchloric acid or 12N sulfuric acid, swirl the flask, and introduce by pipet 1.0 ml. of 0.25% chlorostannous acid. Dilute to volume and measure absorbance a t 720 mp within 20 minutes. Prepare a sugar blank with the same amount of sucroqe but omit the reduction with chlorostannous acid. The absorbance of the blank, caused by the formation of the blue color in the 7-minute period, must be determined for each sugar and subtracted. Raw sugar solutions with large buffering capacities should be initially adjusted to p H 1.9. The results xith deionized sucrose are presented in Table 11. The absorbance of the color formed was practically proportional to the concentration of silicate. Sugar present at a solids concentration of 1 " Brix had only a small effect, but a t 5 " Brix the formation of molybdisilicic acid was appreciably retarded. The color was much more stable than that of the corresponding phosphate solutions. Phosphate gave a positive error in the absence of sucrose, but a negative error in its presence. Phosphate, even in the absence of reducing sugars, caused the absorbance to become somewhat time dependent. The combination of phosphate with the invert sugar present in the reference sugar produred an appreciable formation of blue color during the 7 minutes in which molybdisilicic acid is formed. From the value of the single blank, a correction proportional to the phosphate concentration is made. Subtracting this correction gives absorbance readings very nearly equal that for the pure sucrose solutions (Table 11). The precision is of the same order as that for phosphate. Sulfate Determination. The best rapid method for determining sulfate appears to be that of Rudy ( 1 6 ) in which a controlled precipitation of barium sulfate is obtained by using solid barium chloride, dissolved at a governed rate. The turbidity of the fairly stable suspension formed can be measured by either transmitted or scattered light. Two titration methods ( 7 , 1 4 ) were not studied because of end point difficulties. PROCEDTRE.Mix test liquor, 7 ml. of 6N hydrochloric acid, and sufficient water to make 200.0 ml. Depending upon the amount of sulfate present, the final solution should be in the range of 1' to 5" Briu. Sulfate should be in the range 0.1 to 0.4mM. Transfer a 150-ml. portion into a 250-ml. beaker and
m M SOa 0 08
0 13
0 4
The Beckman measurements a t 720 mp (at about 0.06-mm. slit) usually attained a maximum 5 to 15 minutes later than the Coleman readings. For some solutions the readings continue to increase after an hour. Calibration was made with granulated sugar (tested for absence of sulfate) from 1' to 10' Brix and potassium sulfate from 0.1 to 0.4mhl. Below O.lmM sulfate, the results became erratic until at 0.04 mM sulfate when no precipitate formed. The calibration was linear within experimental errors for both the nephelometer and transmittance measurements. Both types of readings rise somewhat as the sugar content n-as increased to about 3 Brix, beyond which point the values fell rather quickly. There is some correlation between the nephelometer readings and the time of solution of the barium chloride. Compared with a given amount of sulfate at the 30- to 35-second dissolution time, the 15-second solutions give readings about 2y0 higher and the 70-second solutions, 4% lower. Ambient temperature variations between 28" and 32" C. have little effect.
Table IV. Inorganic Constituents in Reference Sugar
K
Na Ca Ca Ca
+ Zlg
hlg
CI SO8 1/z
P10a
Si02
Method Flame Flame Flame Titration Titration Difference Titration Beckman Coleman Gravimetric Color Color
Millimoles per Gram Soh l a Soln 2"
0.143 0.0073 0 0258 0.0371 0.0258 0.0113
0,0018 0,0009
Average Values Zl1111ZIg equivalents oxide per gram per gram 0,143 6.74 0.0071 0.22 0.051 1.42
0.143 0,0009 0 0248 0.0362 0,0250 0.0112 Total 0.095 0.0183 0.0214 0.0208
0.023 0.224
0 46
0.095
3.36
0.0416 0.0036 0.0018 Total 0 . 1 4 2
1.66 0.13
__ 0 05
~
14.04
Sum of eight constituents (adjusted for oxygen) 1 3 . 2 8 Total ash by ignition a t 550' C. 14.5 a Solutions 1 and 2 are prepared from two riffled samples of the same material.
The behavior was appreciably different with the solution of reference sugar. The nephelometer measurements acted normally with the results about 4% higher than the gravimetric analysis. The transmittance measurements gave rising values with time even beyond an hour Kith the final results still about 10% low. Attempts were made to find the cause of this discrepancy. The presence of original color, if it remains in solution, will cause the nephelometer reading to be low instead of high and cause little effect on the transmittance measurement. This was verified with pure sucrose and carmel color. However, if the
V O L U M E 26, N O , 9, S E P T E M B E R 1 9 5 4 color of the raw sugar is adsorbed by the barium sulfate, the indicated discrepancy would follow. Calculation shows that all the color must be adsorbed before the two measurements give the same value for sulfate. Filtering out the precipitate left the nephelometer reading the same as the original and the absorbance still 0.8 as high. Therefore, the discrepancy is due mostly to the difference in crystal formation rather than a simple correction for color. Results are presented in Table 111. The nephelometer measurements are more easily made and give results in greater concordance with the calibration with granulated sugar. Transmittance measurements are useful only when the calibration is made with the type of sugar being analyzed. DISCUS SIOIV
The eight substances reported in Table IV constitute nearly all of the inorganic ash in raw sugars. The remainder includes aluminum and ferric oxides which amount to the same order of magnitude as soluble silicate. The total milliequivalent of inorganic cations is 1.6 times that of the inorganic anions. The difference, made up by organic anions, is implicitly taken into account by the two-conductance method of determining ash ( 3 ) .
Table
V. Summary of Rapid Analytical Procedures
hl e t hod Flame Flame Flame Flame Titration Titration Titration Coleman Color Color Based on total solids
Detection Limit, P.P.I\I .a
Attainable Precision,
70
Measurement Time, Min.
1 1 3 30 10
1 30 100 3 3
The total weight of the eight substances is somewhat less than the weight of gravimetric ash. The comparison is only qualitative, because the values represent different quantities Depending on the conditions of ignition, chloride and some sulfate are expelled and replaced to a certain extent by carbonate. If the chloride \\ere driven off and all the metal oxides were carbonates, the total ash would be 14.6 mg. per gram of sugar solids. Table V gives the performance of the procedures studied in terms of the relative precision under optimum conditions and the lowest detectable concentration, Except for chloride and sulfate, the detection limit does not exceed 3 p.p.ni. based on the total sugar solids with the proper choice of method. The sulfate limit can be improved somewhat by adding a measured amount of sulfate to bring the barium sulfate turbidity into measuring range. Nevertheless, the determination of chloride and sulfate is hardly possible in granulated sugars where the total ash is usually below 300 p.p.m. For calcium, for potassium below 0 . 5 m X j or for sodium below 0 . 2 n d f , the flame emission is practically linear with concentration. The calibration for granulated sugars is, therefore, relatively simple. Magnesium in refined sugars must be determined as the sum with calcium by titration in 25 O Brix liquor. Phosphate and silicate are negligible in the granulated sugars tested thus far. For r a x and ~ a s h e dsugars, the calcium can be determined with the same ease and precision by either flame photometry or titration. The magnesium value obtained by difference using the results of the titration procedures is more accurate. Because radiation interferences are high for magnesium, the relatively simple titration is preferred. Although fuel cost is higher, the hydrogen flame is recommended rather than the acetylene because of the lower background luminosity and smaller radiation interference, as well as the ability
1491 to detect magnesium. The reduced sensitivity for potassium, sodium, and calcium is a minor disadvantage when a sensitive instrument is used. Actual measuring time to determine all eight constituents is 2 t o 3 hours. Khen properly set up, determinations are possible on two sugars per man-day. Simplifications may be developed, of course, after the procedures are tested on many other sugars of commerce.
coac LU SIOh s Rapid procedures for determining inorganic constituents in the presence of sugar were studied, which, for the major components, have a t least the precision of the gravimetric ash determination. The usefulness of the Beckman DU spectrophotometer with flame attachment for potassium, sodium, calcium, and magnesium in the presence of sucrose has been explored. The hydrogen flame is recommended for this instrument. The emission of each of these four elements in raw sugar has been found to be the same as the emission from a synthetic mixture of their chlorides in the same concentration of pure sucrose. Alternative methods of determining calcium and calcium plus magnesium by titration with ethylenediaminetetraacetic arid give results in good agreement. The titrations are rapid and the results for calcium plus magnesium can be combined with the flame photometry value for calcium to get a better estimation of magnesium. The determination of chloride by conductometric titration is satisfactory. The rapid procedures for sulfate, phosphate, and silicate depend upon measurements on unstable systems. For the range encountered with sugars, linearity in calibration can be assumed without contributing to experimental errors. When temperature and timing are controlled, these measurements yield satisfactory results. This procedure has been tested on only one raw sugar, and other undiscovered factors may be important. The methods will be tested with a variety of commercial sugars before a general procedure can be recommended. LITERATURE CITED
Alexander, G. B., J . Am. Chem. SOC.,75, 5655 (1953). Bauserman, H. M., and Cerney, R. R., Jr., ANAL. CHEM.,25, 1821-4 (19531. Browne, C. A.; and Zerban, F. IT., “Physical and Chemical llethodd of Sugar Analysis,” 3rd ed., pp. 1353, Xew York, John Tliiley & Sons, 1941. Close, P., Smith, W.E., and Watson, bl. T., ,Ir., Ari.4~.CHEM., 25, 1022-5 (1953). Diehl, H., Goets, C. A, and Hach, C. C., J . Am. Watw W o r k s ASSOC., 42,40-8 (1950). Diskant, E. hl., ASAL. CHEM.,24, 1856-7 (1952). Edwards, A. H., Proc. Ana. SOC.Sugar Bed Techno!., 3, 541-6 (1942). Emmerich, -L, Zucker-Beih., No. 2, 13-22 (1953). Fivian, W., and Moser, M.,Zucker, 4, 317-20 (1951). Honig, P., “Principles of Sugar Technology,” pp. 340-9, New York, Elsevier Publishing Co., 1353. Jaffe, E., Ann. chinb. apjd., 38, 456-9 (1948). Knight, S. B., Mathias, W.C., and Graham, J. R., AXAL.CHEM., 23, 1704-6 (1951). LIilton, R. F., Analyst, 76, 431 (1351). Alunger, J. R., Nippler, R. W., and Ingols, R. S., ANAL.CHEM., 22, 1455-i (1950). Parks, T. D., Johnson, H. O., Lykken, L., Ibid., 20, 822-6 (1948). Rudy, R. B., J . Research Satl. Bur. Standards, 16, 555-61 (1936); RP893. Woods, J. T., and Alellon, &I. G . , IXD.ESG.CHEM.,ASAL. ED., 13, 760-5 (1941).
RECEIVED for review February 18, 1954. Accepted b l a y 26, 1954. Presented before the Division of Carbohydrate a n d Analytical Chemistry, Symposium on hnalytical Methods and Instrumentation Applied to Sugars a n d Other Carbohydrates, a t the 121th Meeting of the AMERICAN CHEMICAL SOCIETY, Chicago, Ill., September 1953. This investigation was sponsored a8 a joint research project undertaken b y the Bone Char Research Project, Inc., a n d t h e Sational Bureau of Standards.
