electroplating design

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Design and construction of electroplating apparatus for cyanide less silver on copper plating NSF Extreme Ultraviolet Engineering Research Center, Colorado State University Yu-ping Shao, Advisor: Jorge Rocca

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Table of Contents

Abstract …………......................... 4

Introduction

Electroplating background …………......................... 4

Target device description …………......................... 5

Device challenges …………......................... 5

Summary of previous work …………......................... 6

Project management …………......................... 6

Implementation ………….........................

Experimental setup …………......................... 6

Solution level …………......................... 7

Apparatus features …………......................... 9

Tank construction …………......................... 9

Deposition rate …………......................... 10

Procedure ………….........................

Precleaning …………......................... 12

Surface preparation …………......................... 13

Plating …………......................... 13

Operating steps …………......................... 13

Soak clean …………......................... 14

Electrolytic Clean …………......................... 16

Deoxidizing phase …………......................... 18

Plating …………......................... 20

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Conclusion …………......................... 24

Future work recommendation …………......................... 24

Appendix

List of figures and tables …………......................... 25

Percentage volume information …………......................... 25

Chemical formula and symbol …………......................... 26

Original vendor summary …………......................... 27

Supplier tank design …………......................... 29

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Abstract The electroplating of silver on copper is currently performed by an external shop;

therefore, little control is exerted over properties pertaining to electroplating such as thickness,

surface quality, and purity. Therefore, by building a device with controlled electroplating

capability. We can assure that deposition thick goes up to 15µm, has a uniform surface finish and

controlled deposition rate. The device should be capable of depositing 10 to 15µm of pure silver

on the experiment copper target device.

Introduction

Deposition of silver through electrochemistry

Figure 1: Electroplating tank diagram

Electrodeposition of silver is done through the use of an external electron supply. The

cathode of the setup, in this case the copper target device, and the anode of the setup, in this case

the silver are connected to their perspective ends. The external power supply drives the process

when silver surface is ionized to form cations that are then deposited upon the copper surface.

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Target Device:

Figure 2: a) Target device. b) Target device dimensions.

The actual target device is made of copper and has parts of the surface removed so that

strips of approximately 2mm in width can be seen spiraling along the surface from top to bottom.

10 strips are found on the surface with each about 1mm spaced from another.

The target device undergoes electroplating through which the silver deposited on the

strips is from 10 to 15 µm in thickness. In a high vacuum environment, a EUV laser source

causes the ablation of silver.

Device challenge

Generally, electroplating is performed within a chemically and electrically non-reactive

tank with electrolytic solutions to cause the transfer of ions from anode to cathode. While

electroplating copper unto nickel is almost spontaneous and can be performed with immersion

electroplating, deposition of silver on copper is not. This is due to the ranking of materials in

their electron affinity. The ranking: gold, silver, copper, nickel, shows that copper requires a

striking agent in addition to the electrolytic solution. Industrial electroplating of silver unto

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copper in 95% of the electroplating shops requires the use of potassium ferrocyanide. Since

ferrocyanide is a highly hazardous material due to high toxicity, an alternative of non-cyanide

plating is adapted. The solution instead contains organosilver complexed with thiosulphate ions

Project management

This is a single effort. The student is responsible for all the phases of the project except

fundraising, which is provided by the CSU Extreme Ultraviolet Engineering Research Center.

Summary of previous work

With 95% of all plating shops adopt a cyanide containing process, effort to conduct

electroplating in a small scale is not well attempted. The primary enabler for this project is based

upon recent work by EPI Chemicals, from which a electronic class, cyanide less chemicals have

been formulated to be used in industrial application.

Main work will be to synthesize the information given by the vendor technical data, and

plating tank norms used by the plating shops. For both the consideration of space and

application, barrel plating and equipment pertaining to barrel plating is investigated.

Implementation Experimental setup

Due to corrosive vapor produced during the pre-cleaning stage, the entire device must be

kept within a fume hood. In addition, ACS certified respirator must be worn during the plating

procedure to reduce the risk of breathing in corrosive material.

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Figure 3: Layout of the setup.

The motor is running at 6RPM when supplied with 12V. The electrodes involved in the

plating and electrolytic cleaning require another power source. In this case, Agilent 6632A

power supply is chosen for its high stable voltage capability while running on constant current

mode. In addition, its programmable capability offers a degree of control with proper software

interface.

Solution level

A 4 liter beaker has the height of approximately 9.34’’. In designing the apparatus, the

positioning of components must fit within component constraints. When filled up to 3L, the

solution level is approximately 7’’. The chemicals ordered from the supplier are enough to make

3L in the volume of chemicals. Therefore, the shaft that is attached to the cathode and the

extenders that hold the silver anode must extend deep enough to be fully submerged within the

solution. In addition, to eliminate fringe effects on the coating surface, 1.5’’ is left for extra space

between the surfaces of the cathode to the surface of the anode bars.

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Figure 4: Illustrating solution level.

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The Apparatus Features 1. Temperature and chemical resistant tank

2. Rotating target mounting shaft

3. Height adjustable cover

Tank construction:

Due to the rational discussed above in the precleaning procedure, uniform electric fields

distribution is achieved by using a cylindrical tank. Pyrex glass is chosen as the tank material for

reasons pertaining to being chemically and electrically unreactive, and able to withstand high

temperature.

Corrosion

resistant Electrically unreactive

Size (8’’ DIA)

High temp.

Machinability Cost

Stainless steel Yes Won’t affect plating process

Yes Yes (might get hot)

Difficult High

Glass – Pyrex (borosilicate)

Yes Yes N/A Yes N/A (specialized glassblowing)

Med.

Plastic – Acetal

Yes Yes Yes No Good Med.

Plastic – Polycarbonate

Yes Yes Yes Yes Good High

Plastic - Polyethylene

Yes Yes Yes No (melts at 180°F

Good Low

Table 1: Materials considered for tank

Since Pyrex glass is not machinable, it is essential to find a glass apparatus in laboratory

ware catalog with the desired dimensions. A beaker with the capacity of 2000mL has 130 x

190mm for height and outer diameter respectively.

A motorized rotator is mounted on the armature and a threaded shaft attaches the motor

output shaft to the cathode sharing the same axial line. Rotating the work piece at the rate of 6

RPM distributes the deposition evenly on the cathode target device. Given 9’’ in the shaft length,

the height of the tank provides enough head space from the top of the solution to the bottom of

the lid with 2.5 in. in between.

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Adjustable back for cover height becomes necessary to operate the device within the

fume hood. Since beakers are of different sizes, especially between the ones that kept the plating

solution, and the ones for cold water rinsing. Adjustable cover makes it easier to operate.

Deposition rate:

The rate of deposition is a function dependent upon the metal-solution interface, the kinetics of the deposition, the nucleation and growth process of the metal lattice and the structure and properties of the deposits. A good estimation of the deposition rate is as followed

qNanmQ ***= 1.1 Q – total deposition cathodic charge m – number of gram moles of metal deposited n – number of e- in the reaction Na – Avogadro’s number q – fundamental charge of electron F - Faraday’s constant By definition

qNaF *= 1.2 Given that

12310022.6 −×= molNA

Cq 1910602.1 −×=

mol

C.F 4496473=

Fn

Qm

*= 1.3

dtIQ ∫= 1.4

dtFn

Im ∫=

* 1.5

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V

wD = 1.6

D – density

mMww *= 1.7 Mw – molecular weight A – area T - thickness

TA

wD

*= 1.8

dtDAFn

IMw

DA

wT ∫==

**** 1.9

Now assume that current is held constant.

DAFn

tIMwT

***

**= 1.10

Mw(silver) = 107.8682 amu

D(silver) = 10490 3m

kg

Oxidation state(silver) = +1 Ag+1 + e- → Ag so n = 1

)1049.10(**)44.96473(*)1(

**)8682.107(3×

=A

tIT

A

tIT

**1007.1 7−×= 1.11

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From the supplier specs, the optimal cathode current density (A

I) is 1 to 3

2ft

A or 1.07 x 10-7 to

3.22 x 10-7 2m

A

µ

For thickness of 16 µm and running at 1 2ft

A

)1003.1(*)1007.1(

)1016(77

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−−

−

×××=t

t = 1.30*10^8 – 4.36*10^7sec -> 1.5 to 0.5day

Procedure The entire plating flow can be summarized in four steps.

1. Precleaning

2. Surface preparation

3. Plating

4. Post-cleaning/ polishing

Precleaning

The precleaning process combines both soak cleaning and electrolytic cleaning. For soak

cleaning, the device to be cleaned is immersed under a solution diluted with a mixture of surface

agents, detergents, and suspension agents. While in electrolytic cleaning, the process removes

smut, soil deposits, and prepares the surface for plating. The apparatus used in electrolytic

cleaning should bear similarity to the one used in plating. The process of cleaning achieves the

best result based on the uniform distribution of electric field lines from the anode to the cathode

surface. The quality of the cleaning predicts the success of good electroplating.

Electrolytic cleaning is performed after soak cleaning. The process may cause mild

oxidation on the copper surface visible as darkening spots. These are removed using acidic salts

before the actual plating.

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Surface preparation

Before plating, the work surface must be activated by immersion in a solution created

from a dry, acidic salt that etches and temporality deoxidizes the copper surface allowing for

greater time for the plating process

Plating

The conventional method of electroplating involves the use of potassium cyanide as a

striking agent, and potassium carbonate as electrolytic solution. The plating solution provided by

the supplier electroplates silver directly onto copper without a striking agent. The elimination of

cyanide reduced cost of waste management. The solution also works in room temperature and is

specified by the supplier to have exceptional covering and throwing power, and produces fine-

grained, smooth, dense, hard deposition layer with low porosity and high luster.

The concentrated liquid must be diluted with deionized water. The solution must operates

within the pH scale of 8.8 to 9.5 and should be adjusted accordingly using KOH or nitric acid.

Keeping the pH below 10 and temperature between 120°F is essential in keeping the

effectiveness of the bath.

A ratio of 2 to 1 should be kept between the surface area of pure silver anode to the

copper cathode. While a filtration device using carbon filter is recommended, air agitation alone

provided by aquarium air tank is sufficient due to the small-scale nature.

The best way to run the electroplating apparatus is to divide the operation into several

stages. Before running the machine, titration chemicals for each of the solution tank involved

must be prepared in advance to ensure the flow of the operation without interruption in between.

Running each stage is now organized into lab format to convey the clearest understanding from

the supplier’s specifications.

Operating steps

For the actual plating procedure, the procedural steps have been further subdivided into

various stages. Organized as a collection of labs, the manual is meant to convey operational

knowledge as well as calculation as quickly as possible.

Included with each step is also the method of testing involved, as well as pre-quantified

amount of solution chemical mixture used for testing

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1. Soak cleaning

2. Electrolytic cleaning

3. Cold water rinse

4. Deoxidizing phase

5. Cold water rinse

6. Plating

7. D.I water rinse

8. Dry

Soak Clean Vendor Operating parameters: Electrolytic cleaning Soak cleaning Concentration: 4 to 10 oz/gal. of water 8-16 oz/gal of water Temperature: 140° to 200°F 140 - 200˚F

Immersion time: 15 seconds to 2 minutes (

zinc and copper) 1 to 5 minutes (steel)

5 – 10 minutes

Polarity: Anodic (reverse current) N/A Current Density: 10 to 30 amps. (zinc and

copper) 50 to 100 amps. (steel

N/A

Table 2: Soak clean vendor specs. Recommended parameters Bath concentration 8 oz/gal Bath charge-up Equipment

125 mL Erlenmeyer flask

25 mL pipet

Clean 2.5 L beaker

Hot plate

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WARNING: Operate within a fume hood and wear a respirator! Material is caustic soda

and the vapor produced by heating the solution can cause burn. DO NOT MIX E-Kleen

with acidic material or other chemicals.

Material

E-Kleen pellets

Procedure

1. Fill beaker with water.

2. Heat up the temperature of the beaker content to hot enough (~120 F) to melt the E-Kleen

pellets.

3. Measure out E-Kleen pellets in weight (~180 g).

4. Add the pellets slowly to the warm water to avoid sputtering.

5. Stir the solution with pellets until completely dissolved.

Calculation

1 L = 0.264 gal

1 oz = 28.349 g

Note: Need to fill up to 3 L of E-Kleen solution. This is to make sure that the electrodes will be

completely immersed in a 4 L beaker.

3 (L) x 0.264 (gal/L) x 8 (oz/gal) x 28.349 (gram/oz) = 179.62 g

Testing the bath Equipment

Clean 1 L beaker

25 mL pipet

Material

-Methyl orange indicator

-1 N HCl

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Procedure

In an overview, the method described takes a sample of the bath solution, and titrate it with 1 N

HCl. Based on the volume of the HCl (in mL), concentration can be calculated.

1. Allow the solution to cool to room temperature.

2. Take a sample of the solution from the bath using a 25 mL pipet. Transfer to a clean 125

mL Erlenmeyer flask.

3. Add 25 mL of water.

4. Use Methyl orange indicator to produce a yellow colored solution. Add 5 drops. For

strips, hold the strip with a tong.

5. Titrate with 1 N HCl until the indicator turns orange-pink.

Calculation

Concentration of E-Kleen (oz/gal) = (mL of HCL) x (0.3125) x (Normality of HCL)

In this case given that normality is 1 N.

Concentration of E-Kleen (oz/gal) = (ml of HCL) x (0.3125)

Concentration should be 8oz/gal

Running soak cleaning Equipment

Hot plate

Thermometer

1. Heat up the solution to 160 F on the heat plate.

2. Soak the cathode into the solution for 10 minutes

Electrolytic clean Vendor Operating parameters: Electrolytic cleaning Soak cleaning Concentration: 4 to 10 oz/gal. of water 8-16 oz/gal of water Temperature: 140° to 200°F 140 - 200˚F

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Immersion time: 15 seconds to 2 minutes ( zinc and copper) 1 to 5 minutes (steel)

5 – 10 minutes

Polarity: Anodic (reverse current) N/A Current Density: 10 to 30 amps. (zinc and

copper) 50 to 100 amps. (steel

N/A

Table 3: Electrolytic clean vendor specs. Bath charge-up

If steps for charging the soak clean bath have been followed, the used bath can also be used for

electrolytic cleaning

Running the cleaning step

Equipment

DC power supply

Hot plate

Thermometer

Procedure

10 ASF has been selected for copper cleaning

1. Attach the steel anode bars onto the lid of the device.

2. Heat the solution to 160 F.

3. Immerse both the cathode and the steel anodes.

4. Connect the positive terminal to cathode, and negative terminal to anode. This effectively

reverses the current flow for anodic cleaning.

5. Turn on the supply, select current mode and limit it to 0.86 amps. (For detail of the

calculation, refer to running the plating step)

6. Let it run for 2 minutes.

Cold water rinse Equipment

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1 L beaker

Procedure

1. Fill the 1 L beaker up to 0.8 L.

2. Submerge the cathode within the cold water for 5 minutes.

3. While rinsing the cathode. Take off the steel bar electrodes.

4. Wash the other electrodes.

Deoxidizing phase Vendor Operating parameters: Range Optimum Temperature Time 8-24 oz/gal 16 oz/gallon 65-90°F 2-5 minutes

immersion Table 4: Surface activation vendor specs. Recommended parameters

Concentration: 16 oz/gal ratio

Temperature: 90 F

Time: 5 minutes immersion

Bath charge-up Equipment

1 L beaker

WARNING: MUST wear respirator and wear gloves. Material is corrosive.

Material

E-Pik pellets

D.I. water

Procedure

1. Fill the 1 L beaker up to 1 L of D.I. water.

2. Measure out E-Pik pellets. ~ 120 gram.

3. Add salt slowly to the D.I. water

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4. Stir until the pellets completely dissolve.

Calculation

Fill up the beaker completely to ensure full immersion of the target copper electrode.

1 L x 16 (oz/gal) x 0.264 (gal/L) x 28.349 (g/oz) = 119.7 g


1 L beaker

Pipet

Material

50% Sulfuric acid (H2SO4)

KI/EDTA solution

D.I. water

0.1 N Sodium thiosulfate

Starch indicator

Procedure

1. Take 2 mL of the bath sample.

2. Add 3 mL of 50% sulfuric acid.

3. Add 50 mL of D.I. water.

4. Add 10 mL KI/EDTA until the solution turns rusty red.

5. Titrate with 0.1 N sodium thiosulfate to a pale yellow color.

6. Add 2 mL of starch indicator to turn the solution deep blue. If using strips. Hold the

indicator with a tong.

7. Continue to titrate with sodium thiosulfate until the solution becomes colorless for 30

seconds.

Calculation

Concentration of E-Pik (oz/gal) = (mL of 0.1 N Sodium thiosulfate) x 1.18

This should be around 16 oz/gal

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Running the deoxidizing phase Procedure

This is done in room temperature. While the solution does not need to be heated up, it is a

moderately acidic solution.

1. Soak the cathode into the acidic solution for 5 minutes.


1 L beaker

4 silver anodes

Procedure

1. Fill the 1 L beaker up to 0.8 L.


Plating Vendor operating parameters: Optimum Range Silver Metal 2.4 oz/gal 2-2.5 oz/gal pH 8.8 8.5-9.5 Temperature 68°F 60-75°F Cathode current density 1 – 3 0.5 – 5 ASF Anode current density -- 2-10 ASF Agitation air agitation on the anodes,

plus cathode rod agitation or air agitation on the cathodes.

Table 5: Plating vendor specs. Recommended parameter:

68°F = 20 C

Room temp

Bath charge-up

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Equipment

4 L beaker

Material

45 % KOH

40% nitric acid (HNO3)

E-Brite

D.I. water

pH indicator

Procedure

To make a 3 L solution

1. Pour out 1.8 L of E-Brite solution into the beaker.

2. Add in 1.2 L of D.I. water

3. Use the pH indicator to find the pH of the solution

4. Adjust the pH of the solution to 9.0 with KOH solution or nitric acid.

Calculation E-Brite contains 4 (oz/gal) of silver, the concentrate must be diluted with D.I. water

Use a percentage of 70% E-Brite, 30% D.I. water

0.264 gal = L

X L x 4 (oz/gal) x 0.264 (gal/L) => (Y oz/3 L) x (L/0.264 gal) => (Z oz/gal)

X = 1.8 L, so Y = (2.4 oz/gal)


25 mL pipet

125 mL Erlenmeyer flask

Material

0.1 N KSCN

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98% sulfuric acid (H2SO4)

Nitric acid (HNO3)

2% Ferric ammonium

D.I. water

Procedure

1. Transfer 10 mL of solution sample into a 125 mL Erlenmeyer flask

2. Add 50 mL of D.I. water

3. Add 2 mL of nitric acid, 2 mL of sulfuric acid

4. Heat up the solution to boil, and let it boil for 5 minutes.

5. Let cool the solution. Add 5 mL of ferric ammonium sulfate.

6. Titrate with 0.1 N KSCN until the solution turns light orange

Calculation Concentration of silver = (mL KSCN) x 0.144 Running electroplating

The plating can be done without fume hood. Nevertheless, since the whole setup for pre-

cleaning and deoxidizing should be performed within a fume hood. It is best to plate within the

fume hood as well. Prolonged exposure to E-Brite silver solution can cause the absorption of

silver into the blood stream. Please wear gloves accordingly.

Equipment

DC power supply (up to 1 Amp)

Copper cathode

4 Silver anodes

4 L beaker

Material

E-Brite

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Procedure

Recommend using 3 ASF for both anode and cathode

Use the bath from the bath charging step. Refer the bath charging-up.

1. Take the target copper cathode from the cold water rinse.

2. Fit the 4 silver anodes onto the plating lid. Submerge all electrodes within the solution.

3. Turn the DC power supply on. Select current mode and switch up to 0.256 A.

4. Wait for the plating to take place. The amount of time required for 15 micron is.

Calculation Current

ASF – ampere per square foot (amp/ft^2)

Going from mm2 to ft2

Y ft2 = (X mm2) x (cm2/10 mm2) x (in2/2.54 cm2) x (ft2/12 in2)

When X = 16000 mm2, Y = 0.172 ft2

Note: The surface area of the anode must be 4 times greater than the surface area of the copper.

3 (amp/ft2) x 0.172 ft2 = 0.258 A


1 L beaker

Procedure

1. Fill the 1 L beaker up to 0.8 L of cold water.



D.I. water rinse Equipment

1 L beaker

Material

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D.I. water

Procedure

1. Fill the 1 L beaker up to 0.8 L. of D.I. water.



Dry Let it sit in the fume hood and let dry.

Conclusion

The plating machine currently produces silver plating albeit at less uniformity than had

expected. When examined under an optical microscope, the thickness of 15µm has been achieved

using comparison to a 20 µm gold thread. However, the distribution of silver upon the copper

surface has visible bumps. Further testing is required to isolate the cause of uniformity

Through this project, I’ve gained the experience of researching, designing,

manufacturing, and operating the device from the start to finish. I’ve gained skills in mechanical

machining, Solidworks CADDing, and mixing chemicals. The project has stretched my ability

beyond the bounds of engineering education, and has provided an overview of a challenging

engineering work.

Future work recommendation

The project could be expanded and in fact should incorporate these following elements in

the future to fix the problems of non-uniformity on the plating surface as recently observed on

the optical microscope. The first suggestion is a pH indicator, which could be implemented by

using the current calculation, derive from which, the transfer of ions from the silver anodes to the

cathodes. This will demonstrate the rate of the loss of ions within the solution. In order for this to

be implemented, a calculation needs to be done first to characterize the rate of ion loss, then,

repeated plating to test out the change of pH within the solution as compared to the calculation.

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The second recommendation of further work is to monitor the plating machine using the

serial communication capability of the Agilent voltage supply. This will involved the use of

LabView and a graphical interface program. It will monitor the loss of ions from the solution, as

well as the thickness of the deposition as predicted by the deposition equation. This is to be

implemented by a repeated trial of depositions and then characterize the deposition by thickness

measurement under the optical microscope, while varying the deposition time, the deposition

current, and the deoxidizing time as variables.

Appendix List of Figures and Tables Figure 1: Electroplating tank diagram

Figure 2: a) Target device. b) Target device dimensions

Figure 3: Layout of the setup

Figure 4: Illustrating solution level.

Table 1: Materials considered for tank

Table 2: Soak clean vendor specs.

Table 3: Electrolytic clean vendor specs

Table 4: Surface activation vendor specs.

Table 5: Plating vendor specs.

Table 6: Table of symbols, name, molar mass, and risk phrases

Percentage solution

1. % W/W – gram of solute per 100 grams of solution

2. % W/V – gram of solute per 100 mL of solution

3. % V/V – mL of solute per 100 mL of solution

Molar

gmw of solute per 1 L of solution

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Normal

gew of solute per 1 L of solution

or

normal = molar x valence

Chemical symbol and rating

It is strongly suggested to obtain the MSDN sheets for the handling of these materials.

Symbol Name Molar mass (g/mol) Risk phrases HCl hydrochloric acid 36.46 R20 R21 R22 R36 R37 R38 H2SO4 sulfuric acid 98.08 R23 R24 R25 R35 R36 R37

R38 R49 HNO3 nitric acid 63.01 R8 R23 R24 R25 R34 R41 KI potassium iodide 166.00 R36 R38 R42 R43 R61 KOH potassium hydroxide 56.10564 R20 R21 R22 R35 R41 C10H16N2O8 ethylenediaminetetraacetic

acid (EDTA) 292.25 R36

KSCN potassium thiocyanate 97.18 R22 R36 R37 R38. Na2S2O3 sodium thiosulfate 158.09774 R36 R37 R38 FeNH4(SO4)2 12H2O

ferric ammonium sulfate R36 R37 R38

Table 6: Table of symbols, name, molar mass, and risk phrases R20 Harmful by inhalation. R21 Harmful in contact with skin R22 Harmful if swallowed. R23 Toxic by inhalation. R24 Toxic in contact with skin. R25 Toxic if swallowed. R34 Causes burns. R35 Causes severe burns. R36 Irritating to eyes. R37 Irritating to respiratory system. R38 Irritating to skin. R41 Risk of serious damage to the eyes. R42 May cause sensitization by inhalation. R43 May cause sensitization by skin contact. R49 May cause cancer by inhalation.

27

Original vendor summary

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Supplier tank design

electroplating design

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