Colorimetric Determination of Small L4mounts of Metaphosphate and Pyrophosphate Determination of Iron with Thiocyanate in the Presence of Metaphosphate and Pyrophosphate H E l H l E. WIHTH. The Ohio S t a t e Uni\ersity, Columbus, Ohio
S
drogen phosphate \+ere used with similar results. Sodium pyrophosphate was prepared by fusing c. P. disodium hydrogen phosphate. The 20 per cent ammonium thiocyanate solution used mas freed of iron by three extractions with 5 to 2 isoamyl alcoholethyl ether solution. The standard iron solution was prepared from ferrous ammonium sulfate.
IKCE small arnouuts of metaphosphate and pyrophos-
phate are added to water in order to control scale formation and corrosion it is desirable to have some method for the determination of 0.01 to 5 p. 11. m. of these substances if suitable concentrations are to be maintained in all parts of treated systems. The method employed by the Hall Laboratories (3) depends on the determination of the increase in orthophosphate content on reversion of metaphosphate after boiling for 4 hours. This method is inaccurate in those cases where the initial orthophosphate content is abnormally high. Pyrophosphate has been shoT1-n by TVoods and Mellon (5) and others to interfere seriously with the determination of iron by the thiocyanate method, owing to the formation of complexes with the iron. At low p H metaphosphate has a greater tendency than pyrophosphate to form such coniplexes. These facts are made the basis of a colorimetric method for the determination of small amounts of pyrophosphate and metaphosphate in water. The decrease in the ferric thiocyanate color when a fixed amount of iron i5 present is a measure of the amount of metaphosphate and pyrophosphate present. This is the principle employed by Foster ( 2 ) for the colorimetric estimation of fluoride. It was also found that in the presence of a large excess of aluminum ion the iron is released from its complexes with metaphosphate and pyrophosphate. Iron can therefore be accurately determined with thiocyanate in the presence of relatively high concentrations of metaphosphate and pyrophosphate if an aluminum nitrate-nitric acid reagent is employed.
PH FIGURE2. EFFECT OF METAPHOSPHATE, PYROPHOSPHATE, OS FERRICTHIOCYAKATE ORTHOPHOSPHATE, A S D FLUORIDE COLORIS 50 PER CENTACETONESOLUTIOX AS .4 FCSCTIOS OF ‘rHE pH Iron concentration 1.0 p. p. m.
Materials Commercial glassy sodium metaphosphate (“unadjusted Calgon flakes”) and samples prepared by fu4ng c. P. sodium dihy-
Aluminum Nitrate Reagent. To 200 grams of aluminum nitrate nonahydrate dissolved in 200 ml. of water were added 15 ml. of 20 per cent ammonium thiocyanate. The solution was extracted three times with 75-ml. portions of 5 to 2 isoamyl alcohol-ether. Concentrated nitric acid (200 ml.) was added and the solution warmed until the ammonium thiocyanate was completely decomposed. The solution was cooled and diluted to 500 ml.
Apparatus An industrial model glass electrode was used to determine the pH of the solutions. The probable accuracy was *O.l unit. The per cent transmittance was determined using a Coleman regional spectrophotometer with a 30-mp slit. The wave length was set at 470 mp. The values were reproducible to within i 0 . 5 per cent. Procedure Three variations of the thiocyanate method for the determination of iron were employed. I. Iron standard plus a hatever quantities of metaphosphate, pyrophosphate, orthophosphate, or fluoride were to be tested, and nitric acid required to give the desired pH were diluted t o 90 0 I 2 3 4 ml., 5 ml. of 20 per cent ammonium thiocyanate were added, and PH the solution was diluted to exactly 100 ml. After 5 t o 10 minutes the transmittance and pH were determined. FIGURE1. EFFECT OF METAPHOSPHATE, PYROPHO~PHATE,11. Iron standard, metaphosphate (or other added substance), AND ORTHOPHOSPHATE ON FERRIC THIOCYANATE COLOR I S and nitric acid were diluted with 50 ml. of acetone before addition AQUEOUSSOLUTIOS AS A FUNCTIOS OF pH of 5 ml. of ammonium thiocyanate. The solution was diluted to 100 ml. and the pH and transmittance were determined. Iron concentration 2.0 p p. m 122
ANALYTICAL EDITION
September 15, 1942
too W
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PH FIGURE 3. E F F E C T OF METAPHOSPHATE, PYROPHOSPHATL, AXD ORTHOPHOSPHATE ON THE FERRIC THIOCYAXATE COLOR IX T s o m Y L ALCOHOL-ETHER SOLUTIOS AS A FUNCTIOX OF pH OF AQUEOUS LAYER I r o n concentration 0.20 p . p. rn.
111. In this method, the aqueous solution prepared as in Method I was shaken lvith 15 ml. of 5 to 2 isoamyl alcohol-ether solution. Exactly 5 ml. of the isoamyl alcohol-ether layer were removed, 0.5 ml. of acetone was added to clear up any turbidity, and this solution was used for the transmittance measurement. The pH of the aqueous layer was determined. Thr amount of iron chosqn in each method was sufficient to give a transmittance of about 30 per cent in the absence of interfering ions. The amounts of iron, metaphosphate, pyrophosphate, rtc., added were those required to give the concentrations indicatrd in the figures after dilution to 100 ml.
Results I n order to establish the conditions for maximum decrease in the ferric thiocyanate color by metaphosphate and pyrophosphate, the transmittance was determined for given amounts of iron and metaphosphate or pyrophosphate at different pH wlues. For comparison the effect of p H on the fading of the ferric thiocyanate color by orthophosphate and fluoride was also determined. The results are given in Figures 1, 2, and 3 for the three different modifications of the thiocyanate method. The curve for 100 p. p. m. of sodium fluoride was found to be identical n-ithin the experimental error n i t h the curve for 10 p. p. m. of sodium pyrophosphate (Figure 1). The points for 10 p. p. m. of sodium fluoride are close to those for 100 p, p. m. of potassium dihydrogen phosphate in Figure 3. The decrease in the ferric thiocyanate color by metaphosphate is influenced by the p H , but not to the same estent as the decrease due to pyrophosphate, fluoride, and orthophosphate. I n all cases the amount of decrease is greater a t higher p H values. If t h e decrease in color is due to the combination of ferric ion with some substance that is also in equilibrium with t h e hydrogen ion, then the reduction in concentration of t h e effective ion in solutions of low p H will be related to the ionization constant of the corresponding acid. The effect of p H on the decrease in ferric thiocyanate color should increase in the order of decreasing strength of t h e acids. This is clearly shoan in Figure 3 . The steep slopes of the curves emphasize the necessity for close control of the p H in all methods based on the fading of the ferric thiocyanate color. An error of 0.3 p H unit at a p H of 3 can cause a 50 per cent error in the determination of pyrophosphate (Figures 3 and 4). In comparing the results in Figures 1, 2, and 3 i t should be kept in mind that although the Fe/NaPOa, Fe/Na4P20,, etc.,
723
ratios are kept constant, the fading effect of different substances does not change in the same way with concentration; so that the different relative positions of the curves is a combined effect of the method and the concentration employed. [Calculations made on the results reported in Figure 6 indicate t h a t at a p H of 3 there is one ferric ion and one metaphosphate ion (or one dimeric or trimeric metaphosphate ion) in the complex, and two ferric ions to each pyrophosphate ion in its corresponding complex ] Other reagents for the determination of iron which depend on the formation of a comples with the ferric ion are also affected by metaphosphate and pyrophosphate. For example, the color of ferron-iron reagent used by Fahey (I) for the determination of fluoride is also faded by metaphosphate. IKTERFERIXG IONS.It is to be expected that the fading of the ferric thiocyanate color due to metaphosphate would be decreased by the presence of any ion which would form a complex with metaphosphate. Table I gives values for the percentage decrease in trmsniission produced by various concentrations of ions (commonly found in natural waters) in solutions containing 0, 1, and 5 p. p. m. of sodium metaphosphate. The results, calculated by the formula given in Table I and reported in the last two columns of the table, represent the decrease in sensitivity in the method caused by the interfering ion. TABLEI.
E F F E C T OF
FOREIGN IONS
[Itelativr. drwraaae in per cent tsansmittance = ~
2
(II-
)
~where 0
11
13).
,
is t h e transmittance reading i n t h e presence of NaP03, 12 is t h e transmittance reading i n t h e presence of N a p 0 3 a n d t h e foreign ion, a n d 1 3 is the t r a n s m i t t a n c r reading for 0.2 p. p. m. of F e i n t h e absence of KaPOa. Extraction method, p H = 0.51 Effect on F e Determipation Concentration, 0 p. p . m. 1 p. p . m. D p. p . m. P. p . ni. of S a p 0 3 of N a p 0 3 of N a p 0 3
added Substance
7 27 136
C a + (added as CaCl?) +
272 _ _
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17 i0 0 0 0 12
I1
0 0 0 0 0 0 0
1360 12 60 120 600 3000 3000 500
A\Ig+- (added as
%
..
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0.1
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48 1
6
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6
20 69 78 84
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Fe (2P.PM)
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0 16 24 37
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0.25 0.5 1.0
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AQUEOUS SOLUTION
I
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20
10
P.P M.of (Na POJxor Na,?
FIGURE4. EFFECTOF PHATE
COXCENTRATION
30 0,
h ~ E T h P H 0 8 P H h T E .4SD PYROPHOSO S TRbXSNITTbNCE O F AQUEOUS
SOLUTIOKS CONTAINING 2.0 P. P. M. pH
= 0.5
OF
IRON
724
INDUSTRIAL AND ENGINEERING CHEMISTRY
Monovalent positive ions have no effect on the fading of the iron color by metaphosphate, divalent positive ions have appreciable effects, and aluminum ion has a very pronounced effect. This is the order of affinity of the metaphosphate for the different types of ions.
Vol. 14, No. 9
There is no advantage in using the acetone method in the analysis of water samples if the size of the sample is taken into consideration. While the acetone method is twice as sensitive as Method I for the determination of iron and metaphosphate, a smaller original sample must be used (40 ml. as compared to 90 ml.), so there is no over-all gain in sensitivity. Since divalent and trivalent positive ions affect the sensitivity of the method it is not possible to use standards prepared from distilled water in the analysis of samples of appreciable mineral content. Standards should be prepared from metaphosphate- and pyrophosphate-free water of the same composition as that to be tested. Such water is usually available (water previous to treatment with the conditioning agent) or can be prepared by boiling a n acid solution of treated water for several hours. (As shown by Figures 1, 2, and 3, orthophosphate does not seriously interfere.) The aluminum-ion concentration should not be greater than 0.1 p. p. m.
Suggested Method for 0.05 t o 2 P. P. M. of Sodium Metaphosphate or 0.05 to 1 P. P. M. of Sodium Pyrophosphate The iron in portions of the sample and standard is determined by adding 5 ml. of aluminum nitrate reagent and 5 ml. of 20 per cent ammonium thiocyanate solution to 90-ml. samples and comP.P.M of (NaPO,),or NqP2C$ paring the color with that developed in distilled water standards FIGURE5. EFFECTOF METAPHOSPHATE AND PYROPHOS- containing known amounts of iron treated with the same reagents. The amount of dilute (0.5 N ) nitric acid required to PHATE CONCENTRATIOK O N TRAKSMITTAXCE OF AQUEOUS give a pH of 3.0 is determined on separate 90-ml. samples to which SOLUTIONS CONTAINING 2.0 P. P. M. OF IROK 5 ml. of ammonium thiocyanate and the amount of iron standard pH = 2.0 required to give an iron concentration of 0.2 p. p. m. have been added. The glass electrode is best employed for the determination of pH, although suitable indicators may be used. To 90-ml. portions of the metaphosphate and pyrophosphateDETERMINATION OF IRON IN THE PRESENCE OF METAPHOS- free water are added 0,0.2,0.4, 0.6, 1.0, 1.5, and 2.0 ml. of standard sodium metaphosphate (0.100 gram per liter) or sodium pyroPHATE AND PYROPHOSPHATE. Since aluminum ion forms a phosphate (0.100 gram per liter), sufficient iron standard to give stable complex with metaphosphate and pyrophosphate but a total ferric ion concentration of 0.2 . p. m., nitric acid to give does not interfere with the determination of iron i t is possible a pH of 3.0 to the final solution, a n 8 5 ml. of 20 per cent amto determine iron in the presence of these substances. If 5 monium thiocyanate. The solutions are diluted to 100 ml. Ninety-milliliter samples are treated with iron standard to give ml. of aluminum nitrate reagent are used in place of an equal 0.2 p. p. m. of iron, nitric acid to give a pH of 3.0, and 5 ml. of volume of 6 N nitric acid (Method I, p H = 0.5) the presence ammonium thiocyanate. After 5 minutes the colors are comof 100 p. p. m. of metaphosphate does not introduce any error pared with the standards. One hundred-milliliter Nessler tubes in the determination of 2 p. p. m. of iron. An error of but 7 may be used for the comparison. Increased sensitivity is obtained by extracting the solutions with 15.0 ml. of 5 to 2 isoamyl per cent is caused by 500 p. p. m. of metaphosphate. With alcohol-ether, removing 5.0 ml. of the extract, adding 0.5 ml. of the extraction method 10 p. p. m. of metaphosphate do not acetone, and determining the per cent transmittance with a interfere in the determination of 0.2 p. p. m. of iron when the photoelectric colorimeter. aluminum nitrate reagent is used. Similar results are obtained in the presence of pyrophosphate. Vanossi (4) has suggested the use of aluminum chloride or zirconium oxychloride to liberate iron from its complexes Fe(02PPM) with metaphosphate, pyrophosphate, orthophosphate, and DH-30 fluoride. Aluminum nitrate was found to be more convenient AMYL ALCOHOL- ETHER EXTRACT and effective than aluminum chloride. CHOICEOF METHOD.The choice of method for the determination of metaphosphate or pyrophosphate is governed by the amounts present and possible interfering ions. For amounts of metaphosphate between 1 and 30 p. p. m. and in the absence of pyrophosphate, Method I using a p H of 0.5 (5 ml. of 6 N nitric acid per 100 ml.) is preferable. By using the low pH, difficulties of p H regulation are reduced. Figure 4 is a typical standard curve obtained in distilled water. u At a p H of 2 sodium metaphosphate and sodium pyrophosphate have about the same effect on the ferric thiocyanate color in the concentration range 1 to 15 p. p. m. (Figure 5 ) . IO L B y making determinations at p H values of 0.5 and 2 i t is PPM of (Na POJxor Na4%0, possible to estimate metaphosphate and pyrophosphate separately when they are present together in the sample. FIGURE 6. EFFECT OF METAPHOSPHATE AKD PYROPHOSPHATE For metaphosphate contents between 0.05 and 2 p. p. m. CONCENTRATION ON TRANSMITTANCE OF ISOAMYL ALCOHOLand for pyrophosphate contents between 0.05 and 1 p. p. m. ETHER EXTRACTS OF SOLUTIOKS CONTAINIXG 0.20 P. P. M. OF the extraction method is to be preferred. Figure 6 is a typical IRON standard curve. pH = 3.0
September 15, 1942
ANALYTICAL EDITION
For larger amounts of metaphosphate the. iron concentration is adjusted to 2 p. p. m. and 5 ml. of 6 N nitric acid are added to standards and unknown. Extraction is not necessary.
Literature Cited
.~,
ill Fshev. J. J.. IND.ENU.CHEM.. ANAL.Eo... 11.. 362 (1939). . . (2) Fost& M. D., Ibid.. d.. 5, 234 (1933). ~
Acknowledgments
725
~
(3) Partridge. E. P., private rivate communication. (4) Vanossi, R., Amles moo. qulm. argentina. 29, 48 (1941). Certain preliminary experiments were performed by Man~ .AN*=. ANAL. ,. , 10 J. ,T., .n*.\and Mellon, M. G . , IND.ENQ C n Hm ~M ED., , Woods. calgon, II ~~ ~~. ., (5) riel R~~~~workinc workinff on an N, y, A, project, calgOn, 13, 551 (1941). supplied the glassy sodium metaphosphate.
Evaluation of Metal-Cleaning Compounds I
A Quantitative Method 0. M. MORGAN AND J. G. LANKLER National Aniline Division. Allied Chemical and Dye Corporation, New Ycmk, N . Y.
A new q u a n t i t a t i v e method has been dev e l o p e d for the e v a l u a t i o n of metal-cleaning compounds. Since mineral oil fluoresces b r i g h t l y under u l t r a v i o l e t light and this f l u o r e s c e n c e is capable of being photographed, 3 c o n v e n i e n t means is provided for detecting and recording oil residues on metal s u r f a c e s both before and after c l e a n i n g .
T
HE property of removing oil from metals is relatively easy t o recognize and evaluate qualitatively, but somewhat more difficult to evaluate quantitatively. Practically any alkali cleans most of the oil from the metal and unless a quantitative evaluation of the residual oil can be made, the true value of the metal cleaner is left as a matter for,conjecture. A survey of the literature for methods of evaluating the performance of metal-cleaning compounds did not disclose a n accurate method for the detection of traces of oil. A real need existed for a quantitative test which would be both rapid and capable of permanent record for the comparison of cleaning products. The measurement of one or several physical or chemical characteristics of a n a h l i n e cleaning solution is not sufficient t o establish quantitatively its merit aa a cleaning agent. Hence, a performance test involving the cleaning of uniformly soiled metal samples and the observation of the residual traces of oil appeared t o be the best method of studying metal cleaners.
”.
To establish the minimum v.ioihln . ~ . ~omnnnt . y~y u . nf vnairl Luu.Jualoil, gravimetric oil determinations were made on metal test panels representative of the lower limits of observation. It was found that 0.000004 gram of oil per sq. em. is visually detectable. A test of this kind is probably most accurately defined as “visually quantitative”.
Method of Oiling the Metal The metal test strips, 5 X 10 em. (2 X 4 inches) in size are scrubbed by hand with a 0.50 per cent solution of Nacconof NR at 110” F., thoroughly rinsed in warm water, rinsed in alcohol, and dowed to dry. Strips of wool flannel 4.7 X 9.7 em. (1.875 X 3.875 inches) are saturated with a mineral oil composition such as is used in the rolling of brass. These oil-saturated wool strips are alternated with the metal strips t o fonn a stack with protecting metal plates and oil-saturated wool strips above and below the stack of experimental strips. The stack of plates is placed between the jaws of a hydraulic press, the edges of the stack of plates being carefully squared up. A pressure of 35 kg. per sq. om. (500 pounds per sq. inch) is a p plied, and the pressure maintained until no more oil oozes out from the edge of the stack, the edges being constantly wiped with a clean cotton cloth, At the end of this process the presmre is released, and the strips are removed and retained in a perfectly horizontal position until taken out of the stack, one at a, time, for the cleaning experiment.
Principle of Testing Method Mineral oil fluoresces brightly under ultraviolet light. Animal and vegetable oils which do not fluoresce in their own right can be made t o fluoresce by the addition of a n oilsoluble fluorescent dyestuff such as Fluorescent Oil Green H. W., obtainable from Wilmot & Cassidy, 108 Provost St., Brooklyn, N. Y. Since this white fluorescence of the various oils is proportional t o the amount of oil adhering to the metal surface and since clean metal appears black under ultraviolet light, a natural scale of measurement is established for evaluating the e5ciency of a metal-cleaning compound. Extremities of this scale are shown in Figure 1. The amount of oil adhering t o the panel on the right, as determined from a n average of ten such panels, is 0.000113 gram per sq. om. The average oil deviation from the mean in this set of ten panels was 7.3 per cent by weight. The fluorescence is capable of being photowaphed, thereby providing a permanent rkcord of the cleaning ability. By the use of this method fine distinctions can be made between cleaning compounds
FIGURE1. LUMINOGRAM OF STEELWITH MINERAL OIL Left c1ean steel. “0 011 Rlght. Oiled steel, 113 x 14- gram per aq cm
