Dura®, Durachrome®, are trade names of Plating Resources, Inc. Copyright and all other World Rights Reserved, 1990, 1995, 2014.

 

 

 

ANALYTICAL PROCEDURE

Chrome Bath Analysis
The following procedures are used to analyze hexavalent chromium plating solutions. These procedures were developed by Plating Resources, Inc. and are an improvement over other methods used in the industry. These improvements include accuracy of the results obtained, clarity of titration endpoints and speed in obtaining the results.

Standardization Of FAS Solution
FAS is a solution that is used to titrate the chromic acid in order to determine its concentration. The exact strength of the FAS must first be determined by titration against a known chromium standard that is traceable to the NBS. The FAS solution will gradually loose strength over time, hence the need for this procedure.

1. Pipette a 20 ml. aliquot of 0.100 Normal Potassium Dichromate Standard into a 250 ml. Erlenmeyer flask. Place a Teflon coated stir bar into the flask.
2. Add approx. 155 mls. of Cr Titration Diluter.
3. Place on a lighted magnetic stirrer and adjust the speed for thorough agitation without splashing.
4. Add 3 drops of Ferroin Indicator (do not add more than this).
5. Titrate with the FAS solution to a clear bright orange endpoint.
6. Calculate as follows; Normality of FAS Equals:               2.0      
                                                                               Mls. of FAS
7. Enter the normality in the FAS normality log.

Notes:
A. The FAS solution should be standardized weekly, as a minimum.
B. During titration the solution will gradually turn cloudy green, then clear blue, then a hazy gray and, finally, to a bright orange endpoint. This endpoint is very distinct and just 1 - 2 drops past the hazy grey color.
C. The orange endpoint should last for five (5) seconds minimum. If not, then add 1 - 2 drops more until the color stays fast.
D. The same brightness of the orange endpoint should be used for the chromic acid Titration.

FAS Normality Log

Date Normality Eppindorf Factor
(= N x 9.21886)
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     

Chromic Acid Titration
1. Pipette a 0.5 ml. (500 ul) aliquot into a 250 ml. Erlenmeyer flask using an Eppendorf micropipette. Be certain that there are no air bubbles in the tip.
2. Add approx. 150 mls. of Cr Titration Diluter, and place a Teflon stir bar into the flask.
3. Place on a lighted magnetic stirrer and adjust the speed for thorough agitation without splashing.
4. Add 3 drops of Ferroin Indicator.
5. Titrate with the FAS solution to a clear bright orange endpoint. Use the same technique and endpoint color as with the FAS standardization.
6. Calculate as follows:           oz./gal. Chromic Acid Equals:
                                 mls. of FAS titrant x Eppendorf Factor
                        (or: mls. of titration x Normality of FAS x 9.21886)

Note:
The use of the Eppendorf micropipette in step # 1 above saves considerable time. An option is to pipette a 10 ml. aliquot of the bath into a 250 ml. volumetric flask and dilute to 250 mls. with DI water. Using a second pipette, take a 10 ml. sample of this dilution and transfer it into the 250 ml. Erlenmeyer flask. In this case, the calculation is: mls. of titration X normality of FAS X 11.27.

Density Testing
Density tests can be used as a rough field measurement of the chromic acid concentration, in lieu of titration, for operations that wish to minimize their laboratory size. The density of the bath, measured in either Baume’ or specific gravity with a hydrometer, shows a ballpark concentration of the chromic acid level. The exact concentration of chrome is dependent upon the amount of bath contamination present, typically trivalent, iron and copper. These contaminants are also measured with the hydrometer and if not accounted for will end up showing a higher reading of chromic acid than is actually in the bath. Older baths typically will show a significant difference between the density reading and the actual amount of chromic acid present.

Procedure
Pour a sample of the bath into a hydrometer jar and allow it to cool to 60 deg. F. Place a hydrometer into the jar and allow it to float to a steady point. Read the density in either Baume’ or specific gravity. Compare the reading to the chart below.

A bath sample should still be sent to Plating Resources, Inc. on a monthly basis so the actual chromic acid reading is known. Determine the difference between the actual analysis and the density reading, then deduct this amount from the density reading in order to obtain a more accurate level of chromic acid present in the bath.

Baume’
Specific Gravity
oz/gal Chromic Acid
10
1.074
13.6
11
1.082
15.2
12
1.090
16.7
13
1.099
18.3
14
1.107
19.9
15
1.115
21.6
16
1.124
23.3
17
1.133
25.0
18
1.142
26.7
19
1.151
28.5
20
1.160
30.3
21
1.169
32.2
22
1.179
34.1
23
1.189
36.0
24
1.198
37.9
25
1.208
39.8
26
1.219
41.8
27
1.229
43.9
28
1.239
46.0
29
1.250
48.2
30
1.261
50.3
31
1.272
52.5
32
1.283
54.8
33
1.295
57.1
34
1.306
59.5
35
1.318
62.0

Extrapolate for numbers between the values shown. Cooling the solution to 60 deg. F. is important. Subtract the contaminant equivalent from the chromic acid level found to obtain a more accurate reading.

Sulfate Analysis
This procedure uses a special four (4) place electric centrifuge, with aluminum tube retainers and poly. cushions. The centrifuge tubes used are also special in that they have a narrow tip end that is calibrated to read directly in oz/gal. sulfate concentration. Each head position as well as the individual tubes should be marked with numbers #1 - #4.

Important:
For accurate results it is critical that the tubes be calibrated against a bath standard. It is common for factory supplied tubes to have variances in the tube diameter and the scale placement. Such tubes will produce erratic results. Plating Resources, Inc. provides special tubes that have been calibrated for accuracy.

Procedure:

1. Allow any bath solids to settle in the sample before proceeding. Then, Transfer 5 mls. of 5N Hydrochloric Acid into each tube.
2. Pipette a 20 ml. aliquot of the bath into a numbered tube. When pipetting, be sure that the tip does not reach the bottom of the sample so as to avoid picking up any solids. Place a #0 rubber stopper into the centrifuge tube. Remove the tube and shake vigorously for 30 seconds. Return the tube to the appropriate numbered holder.
3. Centrifuge for 5 minutes at 1,000 rpm. Remove the tube and check for solids at the bottom. Record this reading if any are found.
4. Transfer 5 mls. of 30% Barium Chloride into each tube. Stopper and shake vigorously for 60 seconds. Return the tube to the appropriate numbered holder. Allow to stand for 5 minutes, minimum.
5. Centrifuge again for 5 minutes at 1,000 rpm.
6.

Remove the tubes and note the reading. The tubes read directly in oz/gal of sulfate with a scale of 1 - 6 and five subdivisions. Each number represents 0.1 oz/gal and each subdivision represents 0.02 oz/gal. If desired, a third decimal can be extrapolated.

Gently tap the tube end should the precipitate be at an angle. If this does not level the reading, then rotate the tube in the holder 180 degrees.

7. Repeat step # 5 centrifuging two (2) more times as a minimum, noting the reading each time. It is common for this reading to decrease, but sometimes it will increase if the sulfate precipitation was not complete in step # 4. Continue until the readings are the same for a minimum of two (2) times.
8. Subtract any solids found in step # 3 from the final reading in step # 7. The result is the sulfate concentration in oz/gal.

Notes:
It is best to run dual samples with the tubes opposite each other for balancing. The results should be averaged. When running only a single sample the opposite tube must be filled with an equal volume (or slightly more) of DI water for balancing. It is possible to run four (4) individual samples, or two (2) dual samples at the same time.

Should the sulfate reading be above the 0.60 oz/gal level at the top of the tube, then start over using only a 10 ml. sample. In this event, add 10 mls. of DI water in step # 2 and mix well. The reading obtained would then be multiplied by 2 to obtain the sulfate level.

The tubes are quite delicate. It is suggested to have several spares on hand.

This centrifuge method is actually more accurate, when properly performed, than the precipitation and weigh method is. This is due to the uncertainty of complete precipitation, complete washing and complete drying, which causes significant errors in the old method.

Ratio Determination
Chromium baths require a specific ratio be maintained of the chromic acid to the sulfate levels in the bath. The exact ratio is dependent upon the bath type and to a lesser extent the operating conditions. Refer to Plating Resources, Inc. technical bulletins for the optimum ratios. Maintaining the optimum ratio is needed for proper bath operation and high quality deposits.

A bath with 30.0 oz/gal of chromic acid and 0.25 oz/gal of sulfate would have a ratio of 120:1, which is calculated as follows:

Chromic acid concentration
Sulfate concentration

The ratio can be adjusted by either increasing or decreasing the chromic acid or the sulfate levels. There are advantages and disadvantages to both; Plating Resources, Inc. can provide assistance should questions arise as to the best approach.

Trivalent Chrome Analysis
This method uses a simplified colorimetric test that is both fast and fairly accurate. It uses a diluted bath sample that is compared to a color standard with a known trivalent level. Hexavalent chrome solutions become darker in color with increasing trivalent levels, hence the simplicity of this procedure.

Procedure:

1) Analyze the bath for chromic acid concentration. Then determine the dilution amount by dividing 133.5 by the chromic acid oz/gal. The result is the amount of mls. to be used in step # 2. Example: a chrome level of 30 oz/gal would use 4.5 mls. of bath sample.
2) Transfer this amount of the bath sample into a 10 ml. test tube, using a graduated pipette.
3) Fill the tube to the 10 ml. mark with DI water, stopper and agitate thoroughly.
4) View the diluted sample with known color standards in a comparator test block in front of a strong light source. When the color is matched, the percentage shown on the standard equals the bath trivalent level in relation to the chromic acid. Half percentage points can be extrapolated.

Notes:
This test is accurate down to 13 oz/gal of chromic acid. A more accurate method , which uses a similar dilution involves the use of a spectrograph. Another method, which is much more complicated and involved, is by direct titration using an excess of ceric sulfate and back titrating with sodium nitrate while using potentiometric titration techniques.

Dura Catalyst - Additive Analysis
The Dura Catalyst and Additive levels are determined using specially modified specific ion meters with combination electrode attachments. This analytical unit is calibrated against 1 % and 5% standards each time the test is performed. This procedure is reserved for sophisticated laboratories due to the high cost of the instrumentation and the technical skills required for operation. As such, it is not usually practical for field applications. In addition, the procedures used are proprietary, but can be provided on request.

Routine Catalyst/Additive bath additions should be made based on the process technical bulletin. This is usually based on the amount of ampere hours of operation, but can also be based upon the amount of chromic acid consumed.

Plating Resources, Inc. can make recommendations on optimum Catalyst/Additive addition rates. It is also recommended that a bath sample be sent to Plating Resources, Inc. laboratory, on a monthly basis, for catalyst and a complete bath analysis.

Chloride Analysis
Chloride is a very common contaminant in chromium solutions, and rather insignificant amounts have a very negative impact on the quality of the deposit. The chloride level must be kept below 50 ppm, preferably below 20 ppm.

Similar to the Dura Catalyst - Additive analysis, chloride is tested using a specific ion meter with a combination electrode attachment. Plating Resources, Inc. tests for chloride as a routine on all bath samples submitted for analysis.

Iron And Copper Analysis
Iron and copper, as well as other heavy metal contaminants, are analyzed using an Atomic Absorption / Emission Spectrophotometer operated in the absorption mode. This rather sophisticated instrument is calibrated against known standards using a chromic acid background.

As with the specific ion meters, this expensive instrument is not usually suited for field use. Plating Resources, Inc., Inc. tests for iron and copper as a normal routine on all bath samples submitted for analysis.