Determination of Fluoride in Chromium Plating Solutions JOSEPH P. BRANCIAROLI and JUNE G. COLEMAN Chromium Chemicals Division, Diamond A l k a l i Co., Painesville, O h i o
Fluoride is usually determined by distillation as fluosilicic acid, followed by titration of the distillate with thorium nitrate solution at a controlled pH, a method which requires more time and skill than is desirable for control purposes. The method described here separates fluoride by precipitating the interfering metal ions. Chromium is precipitated as silver chromate and the other metals as hydroxides. After the precipitates are removed the filtrate containing the fluoride is titrated with thorium nitrate. A buffer is used to control the pH. Although the method was devised for chromium plating solutions containing fluorides, on which it has been carried out easily and successfully, it may be adaptable to other analytical problems.
thorium nitrate titration. The point a t which no yellow remained in the solution was the easiest for the authors to duplicate. The thorium nitrate solution is standardized against a known weight of sodium fluoride in a chromic acid solution. The same procedure is employed in the standardization as in the analysis of a sample. This helps to reduce the error in the method. EXPERIMENTAL
Reagents. Sodium hydroxide, 1.ON (40.0 grams per liter). Silver nitrate, 0.5N (85.0 grams per liter). Sodium alizarin sulfonate indicator, 0.1% ' aqueous solution. Buffer solution. Dissolve 7.56 grams of monochloroacetic acid in 160 ml. of distilled water and add 40 ml. of 1N sodium hvdroxide. Mix well. Thorium nitrate. 0.04N (5.52 Der liter). . prams Nitric acid, 1 to'50. Procedure. Pipet a 4-ml. sample of the bath into a 250-ml. beaker. From the graph (discussed later) determine the volume of 1.ON sodium hydroxide and 0.5N silver nitrate necessary for the chromic acid content of the sample. Add these quantities to the sample in the order just mentioned. Agitate the sample while adding silver nitrate, then allow the precipitate to settle. Filter through a KO.40 Whatman paper into a 250-ml. Erlenmeyer flask. Rinse the beaker three times with water and wash the precipitate five times with approximately 10 ml. of water each time. Add 10 drops of indicator to the filtrate. The indicator should be pink a t this point. Add 1 to 50 nitric acid dropwise until the color changes to yellow, then add 2.5 ml. of buffer solution. Titrate with thorium nitrate solution to a pink end point which shows no tinge of yellow. Standardization of Thorium Nitrate Solution. Standard chromic acid solution, containing 250 grams per liter, and sodium fluoride solution, containing 2.215 grams per liter of reagent grade sodium fluoride (0.001 gram of fluoride ion per milliliter), are needed. Pipet 4 ml. of standard chromic acid solution and 10 ml. of the sodium fluoride solution into a 250-ml. beaker. Follow the same procedure as for the sample. Calculations.
T
HE chromium plating bath lvhich is used most extensively
for industrial applications contains chromic acid and a sulfate catalyst, usually added as sulfuric acid. Other chromium plating baths are catalyzed by compounds containing the fluoride or fluosilicate ions, which may be used separately or with the sulfate catalyst. These other chromium plating baths have had little industrial use, with the exception of a self-regulating type bath that contains the fluosilicate or sulfate ions (5, 6). One reason for their limited use is the problem of maintenance of these baths. This is caused by the difficult and often inaccurate procedures n-hich are available for the determination of the fluoride or fluosilicate content of the bath. A commonly used procedure for the separation of fluoride, originally suggested by Willard and Winter (Y), is distillation from a perchloric or sulfuric acid solution. A high degree of accuracy is obtained by this method, but considerable time and skill are required. The method of Berzelius ( 2 , 3 ) recommends the precipitation of fluoride as calcium fluoride. This procedure is not only time-consuming but also inaccurate. Low results are usually obtained due to the solubility of the calcium fluoride. The method reported herein is considered very good for control purposes. I t is easily performed and gives greater accuracy than is usually necessary for controlling chromium plating baths.
N of thorium nitrate =
Grams of fluoride ion per liter = iml. of thorium nitrate)(.?' of thorium nitrate).~i0.019)(1000~ 4
THEORY
RESULTS
Sodium hydroxide is added to the bath sample in excess of the amount required to convert the chromic acid to sodium chromate. At this point, the metal contaminants precipitate as hydroxides. Silver nitrate is added in excess to precipitate the chromate as silver chromate. After filtration, a colorless solution is obtained which can be titrated Lvith thorium nitrate. Fluoride reacts with thorium in an acid solution according to the folloaing equation. Th+'
0.010 (ml. of thorium nitrate) (0.019)
Table I s h o w the accuracy and reproducibility of this method. Standard solutions were prepared containing 250 grams of chromic acid per liter, 2 grams of sulfate as sulfuric acid per liter, and varying amounts of fluoride added as sodium fluoride. Samples of 4-ml. volume were taken in all cases. The best accuracy was obtained where a titration of 10 ml. or more of thorium nitrate was required. Table I1 shows the results obtained from chromic acid solutions which contained various concentrations of chromic acid
+ 4HF +ThFd 4 + 4 H 4
A buffer must be added to the solution to eliminate interference from the hydrogen ions liberated by the reaction. Hoskins and Ferris ( 4 ) recommended the use of monochloroacetic acid plus sodium hydroxide. An aqueous sodium alizarin sulfonate solution was recommended by Armstrong ( 1 ) as an indicator for the titration. After the fluoride has been precipitated as thorium fluoride, excess thorium reacts with the indicator to give red thorium alizarin sulfonate. The greatest difficulty in performing this determination lies in the recognition of the pink end point of the
Table I.
Effect of Fluoride Concentration on Accuracy Fluoride. Grams/Liter
Added 1.00 2.00 3.00 4.00
803
Found 1.09, 1.11, 1.07, 1.08 2 . 0 0 2 . 0 0 2 . 0 0 2 . 0 2 2 06 2 06 2 . 9 4 : 2 . 9 1 : 2 . 9 4 : 2 . 9 4 : 3:02: 3 : 0 2 3 . 9 2 , 3 , 9 0 , 3 . 9 2 ,3 . 9 2 , 3 . 9 5 , 3 . 9 6
Av. Error.
%
$8.75 4-1.17 -1.28 -1.79
ANALYTICAL CHEMISTRY
804 Table 11.
1' " "
Effect of Chromic Acid Concentration on .4ccuracy
(Each solution contains 2.50 grains of fluoride per liter.) Av. Chromic Fluoride Acid Concn., Found, Error, F Grams/Liter Grams/Liter /O 200 2.49 -G.4 280 2.50 0.0 300 2.50 0.0 3 50 2 50 0.0 400 2 48 -0 8 I
I
80
100
and a constant quantity of fluoride. There was a slight error in the amount of fluoride found in the solutions a t the extremes of the cliromic acid concentration range. Standards Ivere prepared which contained constant amounts of chromic acid and fluoride ion per liter, and 2.5 grams of contaminating metals per liter. The addition of these metals had little effect on the accuracy of the method, as shown in Table 111. Insufficient washing of the heavy hydroxide and silver rhromate precipitates might explain the slightly lower result obtained on the one standard containing all the contaminants.
Tahle 111. Effect of 3Ietallic Contaminants on Accuracy l l p t e l l i c Contanrinants, Grams/Liter' Cr-+-, Fe++-, c r - - -, i ' r - -, S i *, Cu + -, S i I, Cu +, 2.5 2.3 2,5 2.5 2 . 5 of each
_____._ ~
~~
200
IS0 GrOI
IN
280
300
350
400
450
500
SAMPLE, G R A M S I L I T E R
Figure 1. Amount of reagents required for 4-ml. samples of varying chromic acid contents PI ecipitate the interfering metals as hydroxides because chromium constitutes such a large portion of the interfering ions. CONC LUSION s The results indicate that this method for determining fluoride provides more than sufficient accuracy for control of chromium plating baths. The method is being used very successfully to rontrol industrial chromium plating baths. .4fter the analyst has become familiar with the method, the det,erminat,ion can be completed in approximately 20 minutes. The usual quantities of contaniinating metals do not interfere.
+
Fluoride added, grams/liter 2 50 Fluoride found, 2.46 grams/liter 2.50 BY.error, % -0.8 1 Each bath contained 250
2 50 2 .jO 2.50 2.50 2.50 2.50 2,50 2.50 2.49 0.0 0.0 -0.2 &/liter chromic acid.
2.50 2.44 2.49 -1.4
LITERATURE CITED
(1) .Ilrmstrong, W.D., J . Am. Chern. SOC.55, 1741 (1933). (2) Berzelius, J. J., Schweigg J . 16, 426 (1816); Rose, H., Ann. 72,
343 (1849).
(3) Hillebrand, W. F., Lundell, G. E. F., Bright, YI.
(4)
-1graph was prepared showing the volumes of 1 . 0 s sodium hydroxide and 0 . 5 5 silver nitrate which are required to remove interfering metals from samples of varying chromic acid content (Figure 1). An excess of 2 ml. of sodium hydroxide and 5 ml. of silver nitrate over the stoichiometric amount required by the chromic acid content was added. This excess is sufficient t o
(5)
(6) (7)
S.,Hoffman,
J. I., "Applied Inorganic .Ilnalysis," 2nd ed., 742, Wiley, Yew York, 1953. Hoskins, 15'. M., Ferris, C. A, TND. ENG.CHEM.,ANAL.ED. 8, 6 (1936). Passel, F. (to United Chromium, Inc.), U. S. Patent 2,640,021 (May 26, 1953). Starech, J. E. (to Dnited Chromium, Inc.), Ibid., 2,640,022 (May 26. 1953). Willard. H. H., Winter, 0. B., IXD. ESG. CHEM.,AKAL.ED.5, 7
(1933).
RECEIVED for
review .iugust 20, 1955.
Accepted February 23, 1956.
Equipment for High Pressure Optical and Spectroscopic Studies ERWIN FISHMAN and H. G . DRICKAMER University o f Illinois, Urbana,
//I.
Equipment has been developed for making optical and spectroscopic studies in liquids at pressures up to 12,000 atm. A combination intensifier and bomb for spectroscopic measurements is described, as well as a source assembly particularly suited for use with a PerkinElmer single beam spectrometer. The use of synthetic sapphire windows permits measurements to be made in the wave-length range from 0.2 to 5.0 microns. Experiments are now being undertaken to develop windows which will permit extension of the wave-length range. With this equipment it is possible to study the effect of pressure in all phases of molecular spectroscopy. By suitable modification, it is possihle to measure the effects of pressure on the refractive index and on light scattering from polymer solutions, as well as related optical effects.
T
0 USDEKSTASD, correlate, and predict the physical
and chemical effects of pressure, its effect on intermolecular and intramolecular forces must be determined. Optical and spectroscopic investigations provide important evidence as t o the nature of these effects. This n-ork was confined to the study of equipment which was used successfully for optical and spectroscopic work in liquids at pressures up to 12,000 atm. When infrared spectroscopy is applied to liquids, the vibrational frequencies between atoms in a polyatomic molecule are observed. I n the pressure range mentioned, the effect most generally observed is that of crowding solvent molecules around a given bond. Because of the mutual polarizability of bonds, there is an attractive interaction which depends strongly on intermolecular distance. Solution properties, transport phenomena, and other macroscopic properties depend on these interactions,
