Titration
A common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of a known reactant. Because volume measurements play a key role in titration, it is also known as volumetric analysis.

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January 21, 2009

SOLUTION FILTRATION 

Definition: Filtration is defined as the separation of solids from liquids by bypassing the liquid through a permeable medium.  Normally the amount of solids present is relatively small as compared to the volume of liquid. 

Filtration is carried out as either a batch or a continuous operation.  In a batch operation the contaminated solution is pumped or dumped from the plating vessel, to a temporary storage tank or sump.  Here the solution can be filtered.  Depending on the solution, the storage tank and/or sump may require a means for heating the solution as it circulates through the filter housing(s). 

In many liquid based process systems, in-line filtration is applied continuously for removal of suspended solids which can be generated by the process itself and can cause undesirable results. In electroplating for example, the normal corrosion of anodes can cause particulate to circulate in the electrolyte. These particles will co-deposit and result in rough deposits or nodules on the backside of the electroplated part. 

Filtration can also remove contamination from the introduction of airborne particles such as dust, lint or paper. 

Solution turnover is very critical for effective filtration.  Solution turnover is the number of times the entire volume of solution is filtered every hour.  A typical turnover in a 600 liter bath might be 5 times per hour. This equals a flow rate through the filtration unit of 50 liters per minute. The actual flow rates needed for effective filtration will vary depending on the process.  

Types of Filters 

In the electroplating industry there are two basic types of filters – Depth and Pleated. 

Depth Filter A depth filter is a filter medium that retains contaminants primarily within tortuous (convoluted: numerous overlapping folds) passages, and is made out of same material, outside to inside.  There are two types of depth filters:

Spun (string) wound – The material (normally polypropylene spun fibers), is wound around a center post.  As the filter is wound, the spaces between the fibers get larger.  Hence the filters will pickup particles of larger size at the OD, and smaller particles at the ID. 

Molded or Sponge wound – This filter is made from a “Styrofoam-like” material that is poured into a mold.  As the filter material sets-up or dries, air is released.  The pockets of air are smaller at the inside, and subsequently larger at the outside, much like the Spun wound filters.  The usage of either the spun wound or the molded variety depends on application. 

Features 

·         Able to holdup under conditions of high heat. 

·         Pore size density is graded so that the larger particles get trapped near the surface of the filter, and the smaller particles get trapped near the center. 

·         Normally always rated as a nominal filter. 

Pleated Filter 

A pleated filter has a center core, made of polypropylene.  Surrounding this core is a length of material that has been folded into a corrugated shape.  Pleated is defined as: A pleat is a type of fold formed by doubling fabric back upon itself and securing it in place.  Because the fabric (polypropylene) is folded, and creased, the surface area is greater.  Because the surface area is the same throughout the filter, the retention of particulates is also greater, and is uniform over the entire surface of the pleated material. 

Features 

·         Better overall filtration.

·         Capable to holding the smallest size particle, <1 micron in diameter.

·         More surface area, to absorb a greater amount of particulates.

·         Higher packing density.

·         Normally always rated as an absolute filter. 

Filter Configurations 

Double Open End – DOE:  

Filter cartridge is open at both ends, to allow more solution flow to pass through. Due to the design, this filter type does not pick up 100% of the contaminants with each pass through.Some plating processes require higher flow rates across the cathode, and are not concerned with absolute filtered material.Sealing at both ends, through a knife edge in the base of the filter housing, as well as one in the filter housing cap, is critical.  

Single Open End – SOE or 222:  

Filter cartridge is closed at one end, and o-ring sealed at the other end.Design is critical for certain applications, like DI water, where 100% pickup of all suspended particulates is absolutely necessary. Flow rates are reduced, as specific process requires better filtration, vs. higher flow. Sealing is at one end only, and life of filter is usually shorter then with DOE filters. 

Filter Ratings

Nominal:  

A nominal filter rating specifies the minimum particle size at which the filter will retain 60 to 98% of those particles. Process parameters such as pressure and temperature will effect the exact efficiency of the filter. Nominal rated filters are less expensive than absolute rated filters. 

Absolute:  

An absolute rating specifies the minimum size particle that will be retained with 100% efficiency under test conditions. Absolute rated filters are used in critical applications where contamination below the specified particle size can not be tolerated. Absolute rated filters are more expensive than the nominal rated type.