Applying the Photometric Method to Determine Presence of Trivalent Chromium in Hex Chrome Solutions

Chromate ions is precipitated and separated as PbCrO₄, after adding the solution of lead nitrate. Excess of ions Pb (II ) is separated as PbSO₄, after the adding 2.5N sulfuric acid. After the filtration of the solution and the removal of interfering elements, ions of Cr (III) are oxidized to hexavalent state with (NH₄)₂ S₂O₈ in the presence of AgNO₃ (catalyst) in acid solution. The determination of Cr (Vl) is completed the photometric method with diphenylcarbazide.

The conventional chromium plating bath contains chromic acid and sulfuric acid in the ratio of 100:1. Trivalent chromium, iron, zinc and aluminum are the main metallic contaminants in hard chromium plating. Maximum contents of these contaminants in hard chromium plating solution is < 15g/l. [1]

The slow but persistent buildup of cationic impurities in a chromium bath has historically been the life-limiting step for a solution to poorer quality deposits. The solution conductivity will also be reduced, eventually limiting the current that can be passed by a giving rectifier voltage.

Trivalent chromium in hexavalent chromium baths is the most vicious of all metallic impurities. It has about twice the effect on voltage requirements as other metals. A rise in the trivalent chromium content can be seen by a darkening of the color of the solution. Pure chromic acid solutions are a bright orange/ brown. An increase in the green trivalent content turns the solution much darker brown and far less bright. Excess trivalent should be reduced by electrolysis. [2].

However, it is not recommended that a solution be entirely free of trivalent chromium, since studies have shown that a small amount (0.1-3 g/l) increases the throwing power of the electrolyte and leads to a wider range of bright plating regimes. Trivalent chromium is formed as an intermediary product of the partial reduction of chromium acid on the cathode; therefore, freshly, prepared electrolytes require break-in at low cathodic current densities. It is also recommended to add organic compounds (oxalic, tartaric or citric acids or saccharin), which are oxidized by the chromic acid and equivalent amounts of hexavalent chromium are thus reduced into trivalent. Part of trivalent chromium ions passing into the solution are oxidized again into hexavalent on the anode. By proper selection of the ratio between the cathodic and anodic areas, it is possible to maintain the trivalent chromium concentration within the required range. The trivalent chromium content of the bath should remain constant when it operated correctly. [3,4]

The most frequently used method, stated in the literature is Thiosulfate method (Routine Method). The hexavalent chromium content is determined and then trivalent chromium is oxidized, using ammonium peroxydisulfate into hexavalent, and the overall chromium content is determined. The quality of trivalent chromium is then evaluated by the difference. There is, however, an objection to this procedure in that the result depends upon a small difference in the titrations of two high concentrations of chromium so that a relatively small error in either determination might lead to serious error in the reported concentration of trivalent chromium [5], and increase the requirements to the quality of chromium plating require a more reliable method for the accurate determination of trivalent chromium content. Difficulty of the problem is the necessity to determine small amounts of trivalent chromium in a medium containing a high concentration of hexavalent chromium. (0.1g/l-3g/l Cr⁺³ in a medium containing up to 263g/l Cr⁺⁶).

The determination of small amounts of Cr⁺³ in a medium of Cr⁺⁶ may be received only after the separation Cr⁺³ from Cr^+6.

The various methods commonly are employed for the separation of Cr⁺³ from Cr⁺⁶ [6]. In the work [4], the authors used method suggested by Griffin [7]. Nikolova P, Dobrov T., Monev M. were determinated trivalent chromium, using photometric measurement by bonding Cr (III) into a complex with EDTA (lowercase Lambda -545nm) and potentiometric titration with hexacyanoferrate, after the separation of chromate ions with lead nitrate (Method Griffin). As the authors noted: “It must be noted, that both methods can be applied as described only for the study of pure solutions. The analysis of electrolytes used as routine in plating shops probably will be difficult due to the presence of other metal ions. When facing a similar case, a concrete approach must be found, depending on the particular features.”

The scope of the work was to elaborate and to determine of the optimum conditions for the determination of amount of chromium (III) 0.1g/l–3g/l in chromium plating solution, containing 231g/l–263g/l of hexavalent chromium and other elements, usually presenting in hard chromium electrolytes. In routine method for the determination of chromium (III), this element is oxidized to the hexavalent state by ammonium peroxydisulfate in presence AgNO₃ (catalyst) in acid solution of and then Cr (VI) may be determine using photometric method with diphenylcarbazide or titrimetric method with sodium thiosulfate (iodometric titration).[6]

The solution of the task was divided into two stages:

Stage 1: Separation ions Cr (III) from ions Cr (Vl). As an initial step in this study, approach used by Griffin, for the separation of chromate ions with lead nitrate were used. Precipitate PbCrO₄ was separated from solution containing ions Cr (III). However, the determination of Cr (III) as Cr (Vl), after the reaction oxidation with (NH₄) ₂S₂O₈ in this solution is not possible through containing in the solution ions lead and others.

Stage 2. Removal of interference ions from the solution and determination Cr (III) as Cr (Vl), using routine photometric or volumetric methods.

EXPERIMENTAL

Reagents and apparatus. All the materials were reagent grade and were used without further purification. Distilled and deionized water were used in the preparation of all solutions. Solutions were stored in glass and polyethylene bottles. Used deionized water (µ-1–2microSiemens).

Standard solution chromium (III) (2mg/ml). Dissolve1g of chromium (met.) into 30ml H2SO4 (1:1). Transfer solution into 500ml volumetric flask and dilute with distilled water to mark.

Lead(II) nitrate. Dilute 2,5g of Pb(NO₃)₂ into 10ml of hot distilledwater.

Ammonium peroxydisulfate solution 0.4%. Dissolve 0.4g of (NH₄)2 S₂O₈ in water and dilute to 100ml . Do not use a solution that has stood more than 12h.

Diphenylcarbazide solution 0.1%.(C₁₃H₁₄ON₄). Dissolve 0.1g of diphenylcarbazide in 90ml of alcohol (C₂H₅OH) and 10ml of acetic acid. Do not use a solution that has stood for more than 8h.

Sulfuric acid (1:4). Mix carefully and with stirring 1 volume of concentrated sulfuric acid (d-1.84g/cm³) into 4 volume of water.

Standard solution silver nitrate 0.1N. 16.988 g of AgNO₃ transfer into 1liter of volumetric flask and dilute to mark with deionized water.

All absorbance measurements were made with spectrophotometer Unicam Helios ”Alpha &Beta.”

Titration procedures and pH measurements were performed with DMS Titrino 716 and Contiburette m10 (Cat.M.Zipper GmbH).

Procedure. Transfer 2.5g Pb(NO₃)₂ to 50 ml beaker, add 10 ml of hot distilled water and mix. Add 2 ml of sample into the solution Pb(NO₃)₂ and mix.

Allow the precipitate PbCrO₄ to settle and filter through Filter Cup GFD–filter paper. Wash precipitate 5-6 times with water. Dilute the filtrate to 100 ml in a volumetric flask. Transfer aliquot 20 ml of the solution to 50 ml beaker and add 10 ml 2.5 N H₂SO₄. Allow precipitate PbSO₄ (white) to settle. Cool the solution with precipitate by ice-cool water and filter through # 41 Whatman filter paper (double filters). Wash the precipitate on the filter with cool distilled water. Boil the filtrate in the beaker (V-250 ml) to about 30 ml, cool and transfer the solution into the 50 ml volumetric flask, dilute to the mark and use the solution for the determination of Cr (III). Take aliquot 10 ml in beaker. Add 2 ml H₂SO₄ (1:4), 1 ml 0.1N AgNO₃ (catalyst), 10ml 0.4% (NH₄)₂S₂O₈, and 20 ml H2O. Heat to boiling. (Boil for about 10–15 min.) The excess of ammonium peroxydisulfate is destroyed by boiling. Cool the solution to room temperature. Chromium (III) is oxidized to hexavalent state. [6]

Transfer the solution to the 100 ml volumetric flask. Add 2 ml H₂SO₄ (1:4) and 2 ml 0.1% diphenylcarbazide solution [8]. Hexavalent chromium forms soluble red-violet complex with diphenylcarbazide in acid solution (pH 1-2). Dilute to 100 ml and mix. Measure the absorbance of the sample, blank and calibration solutions at lowercase Lambda -546 nm.

The recommended concentration range is from 2µg to 50 µg of chromium (VI) per 100 ml. The color of chromium complex develops almost immediately.

Photometric measurements should be made within 10 min after developing the color. Reference solution–distilled water. Plot the photometric readings of the calibration solutions against µg of chromium per 100 ml of solution. Convent the photometric reading of the test solution to mg of chromium by means the calibration curve.

Calculate of chromium (III) g/l, as follows:

Cr (III)^g/_l = ___A___                   B x 1000           

A - mg of Cr(VI), found in 100ml of final solutionB - ml (aliquot) of sample, represented in 100 ml of final solution.

Results and Discussion

The investigation of the possibility the determination Cr (III) as Cr (VI) by the diphenilcarbazide (photometric) method, after the pre-separation Cr(VI), sulfates of Pb and other elements, use the method “added-determined” and the check up the control accuracy (use of independent method of analysis) by the titrimetric method, after the peroxydisulfate oxidation Cr (III) to Cr (VI), show that the proposed methods can be applied successfully without a significant error and the elements, ordinarily present in the chromium baths, do not interfere.

The samples were analyzed using the methods–calibration curve and normal solution (the solution of the comparison is the concentration of the determine element of near to the concentration of element in the sample). A number of samples were analyzed using the method of standard additions.

The precision results for the determination of small amounts of Cr⁺³ in medium Cr⁺⁶ may be received only after the separation Cr⁺³ from Cr⁺⁶. In proposed method: chromate ions is precipitated and separated as PbCrO₄ after adding the solution of lead nitrate. Excess of ions Pb (II) is separated, as PbSO₄ after the adding 2.5N sulfuric acid. After the filtration of the solution and the removal of interfering elements, ions of Cr (III) is oxidized to hexavalent state with (NH₄) ₂S₂O₈ in presence of AgNO₃ (catalyst) in acid solution. The determination of Cr(VI) is completed the photometric method with diphenylcarbazide.

Proposed method for the determination of Cr⁺³ is rapid, simple and precision, can be applied successfully without a significant systematic error and the elements ordinarily present in the chromium baths do not interfere.

REFERENCES

1. M&T HEEF-25 “Chromium Plating Process”, p.3.
2. Weiner R., Walmsley A., “Chromium Plating”, Finishing Publication Ltd., 1980.
3. Kadaner L.I., “Reference book of galvanostegia”, “Technica”, Kiev, 1976.
4. Nikolova S., Dobrev T., Monev M., “Determination of trivalent chromium in chromium plating electrolytes”, Metal Finishing , May, 1995, p.12-18.
5. Langford K.E., Parker J.E., “Analysis of Electroplating and Related Solutions”, Fourth Edition, 1971.
6. Lurie Y.Y., Ribnicova A.I., “Chemical analysis of Wastewater,” “Chemistry”, 1974.
7. Griffin J.L., Plating, 53:196, 1966.
8. Larina O., Timoshenko N., “Quantitative analysis of oxide and nitride phases in steels and alloys.”, 1978, p.94.