<PAGE>   1

                                                        EXHIBIT 10.10.AD; PAGE 1

                            STOCK PURCHASE AGREEMENT

        THIS STOCK PURCHASE AGREEMENT (the "Agreement") is made and entered into
as of the 17th day of January, 2001, by and between ANTHONY M. FRANK KEOGH PLAN
UTA CHARLES SCHWAB & CO., INC. (hereinafter referred to as "Buyer") and
ELECTROPURE, INC., a California corporation (hereinafter referred to as
"Electropure" or the "Company").

        1. PURCHASE AND SALE OF SHARES

        (a) Effective on the date hereof, the Company hereby sells to Buyer and
Buyer hereby purchases up to Two Hundred Fifty Thousand (250,000) Shares of
Electropure, Inc. Series D Convertible Preferred Stock (the "Shares") with an
aggregate value equal to Five Hundred Thousand Dollars ($500,000).

        (b) Buyer shall have the right, but not the obligation, to purchase the
Shares in 50,000 increments each month commencing on the date of this Agreement.

        (c) The Shares shall have the rights, preferences, privileges,
restrictions and other terms set forth in Exhibit "A" hereto, the Certificate of
Determination for the Series D Convertible Preferred Shares (the "Certificate").

        2. REPRESENTATIONS AND WARRANTIES OF BUYER Buyer represents and warrants
to the Company:

        (a) The Shares are being acquired by Buyer for investment for an
indefinite period, for Buyer's own account, not as a nominee or agent, and not
with a view to the sale or distribution of any part thereof, and the Buyer has
no present intention of selling, granting participations in, or otherwise
distributing the same except as may be permitted by the Securities Act of 1933,
as amended (the "Act").

        (b) Buyer does not have any contract, undertaking, agreement or
arrangement with any person to sell, transfer, or grant participation to such
person or to any third person, with respect to the Shares.

        (c) That Buyer understands that the Shares have not been registered
under the Securities Act of 1933, as amended (the "Act"), in reliance upon the
exemptions from the registration provisions of the Act contained in Section 4
(2) thereof, and any continued reliance on such exemption is predicated on the
representations of the Buyer set forth herein.

        (d) Buyer understands that the Shares must be held indefinitely unless
the sale or other transfer thereof is subsequently registered under the Act, as
amended, or an exemption from such registration is available. Buyer further
understands that the Company is under no obligation to register the Securities
on its behalf or to assist him in complying with any exemption from registration
except as otherwise provided herein.

        (e) Buyer (i) has adequate means of providing for his current needs and
possible contingencies, (ii) has no need for liquidity in this investment, (iii)
is able to bear the substantial economic risks of an investment in the Shares
for an indefinite period, (iv) at the present time, can afford a complete loss
of such investment, and (v) does not have an overall commitment to investments
which are not readily marketable that is disproportionate to Buyer's net worth,
and Buyer's investment in the Shares will not cause such overall commitment to
become excessive.

        (f) Buyer is an "accredited investor" (as defined in Regulation D
promulgated under the Act) and the undersigned's total investment in the Shares
does not exceed 10% of the Buyer's net worth.


<PAGE>   2

                                                        EXHIBIT 10.10.AD; PAGE 2


        (g) Buyer recognizes that the Company has had only limited revenues to
date and that the Shares as an investment involve significant risks.

        (h) Buyer will not transfer the Shares without registering them under
applicable federal and state securities laws unless the transfer is exempt from
registration. Buyer realizes that the Company may not allow a transfer of Shares
unless the transferee is also an "accredited investor". Buyer understands that
legends will be placed on certificates representing the Shares, with respect to
the above restrictions on resale or other disposition of the Shares and that
stop transfer instructions have or will be placed with respect to the Shares so
as to restrict the assignment, resale or other disposition thereof.

        (i) The Company will direct its transfer agent to, or will itself, place
such a stop transfer order in its books respecting transfer of the Shares, and
the certificate or certificates representing the Shares will bear the following
legend or a legend substantially similar thereto:

            "THESE SHARES HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF
            1933. THEY MAY NOT BE SOLD OR OFFERED FOR SALE IN THE ABSENCE OF:
            (1) AN EFFECTIVE REGISTRATION STATEMENT AS TO THE SECURITIES UNDER
            THE ACT, OR (2) AN OPINION OF COUNSEL SATISFACTORY TO THE COMPANY
            THAT SUCH REGISTRATION IS NOT REQUIRED."

        (j) That Buyer understands that Rule 144, promulgated by the Securities
and Exchange Commission under the Act, may not be currently available for sale
of the Shares, and there is no assurance that it will be available at any
particular time in the future. If and when Rule 144 is available for sale of the
Common Stock underlying the Shares, such sales in reliance upon Rule 144 may
only be (i) in limited quantities after the Shares have been held for one (1)
year after being sold by the Company, or (ii) in unlimited quantities by
non-affiliates after the Shares have been held for two (2) years after being
sold by the Company, in each case in accordance with the conditions of the Rule,
all of which must be met (including the requirement, if applicable, that
adequate information concerning the Company is then available to the public).
The Company and Buyer acknowledges that the Company has no obligation to supply
the information required for sales under Rule 144.

        (k) The Purchase Price to be paid by Buyer to Company for the Shares has
been determined by Buyer as fair and appropriate based solely upon Buyer's
independent investigation and due diligence of the Company, and neither Buyer
nor the Company nor any of their agents, including, without limitation, any of
their officers, directors, employees, accountants and attorneys, has made any
representations or warranties whatsoever in connection with the sale of the
Shares by the Company to Buyer. Buyer has had sufficient opportunity in
connection with the sale of the Shares to review the Company's business and
affairs (including, without limitation, the Company's financial statements and
other information). The Buyer has had answered to his satisfaction any questions
with respect to the Company's business and affairs. Buyer further has had the
opportunity to obtain independent financial, legal, accounting, business, tax
and other appropriate advice with respect to the transactions contemplated by
this Agreement, and is not relying upon the Company or any of its agents in any
manner in connection with same.


<PAGE>   3

                                                        EXHIBIT 10.10.AD; PAGE 3


        3. REGISTRATION RIGHTS The Company agrees to register under the Act all
of the Common Stock issuable upon conversion of the Shares by the Buyer in
connection with the next Registration Statement filed by the Company with the
Securities and Exchange Commission.

        4. REPRESENTATIONS AND WARRANTIES OF ELECTROPURE

        (a) Electropure is a corporation duly organized and validly existing
under the laws of the State of California without limit as to duration of its
existence, and is authorized and in good standing to do business in no other
state; Electropure has the corporate power and adequate authority, rights and
franchise to own its property and to carry on its business as now conducted;
and, subject to ratification by its Board of Directors, Electropure has the
corporate power and adequate authority to enter into this Agreement.

        (b) The execution and delivery of this Agreement and subject to (1)
ratification by the Board of Directors of the Company and (2) filing the
Certificate with the California Secretary of State, the performance of the
provisions of this Agreement are not in contravention of or in conflict with any
law or regulation or any term or provision of Electropure's Articles of
Incorporation or By-Laws and are duly authorized and do not require the consent
or approval of any governmental body or other regulatory authority; and this
Agreement is a valid, binding and legal obligation of Electropure, enforceable
in accordance with the terms herein.

        5. ENTIRE AGREEMENT This Agreement embodies the entire agreement and
understanding between the parties hereto with respect to the subject matter
hereof and supersedes all prior and contemporaneous agreements and
understandings relating to such subject matter.

        6. AMENDMENT This Agreement may not be amended except by written
document executed by the parties.

        7. SUBJECT HEADINGS Subject headings are included for convenience only
and shall not be deemed part of this Agreement.

        8. SEVERABILITY If any provision of this Agreement shall be held
unenforceable as applied to any circumstance, the remainder of this Agreement
and the application of such provision to other circumstances shall be
interpreted so as best to effect the intent of the parties. The parties further
agree to replace any such unenforceable provision with an enforceable provision
(and to take such other action) which will achieve, to the extent possible, the
purposes of the unenforceable provision.

        9. GOVERNING LAW This Agreement shall be governed by and construed under
the laws of the State of California in force from time to time.


<PAGE>   4


                                                        EXHIBIT 10.10.AD; PAGE 4

        10. PARTIES BOUND This Agreement is binding on and shall inure to the
benefit of the parties and their respective successors, assign, heirs, and legal
representatives.

        11. SURVIVAL The representations, warranties, covenants, and agreements
contained in this Agreement shall survive the consummation of the transactions
contemplated hereby.

        12. COUNTERPARTS This Agreement may be executed in one or more
counterparts, each of which shall be deemed an original but all of which
together shall constitute one and the same instrument.

        IN WITNESS WHEREOF, the parties have executed this Agreement as of the
date first above written.

COMPANY:                                   ELECTROPURE, INC.


                                           By: /s/ FLOYD H. PANNING
                                               ---------------------------------
                                                   Floyd H. Panning, President
                                                   23456 South Pointe Drive
                                                   Laguna Hills, CA 92653-1512


BUYER:                                     By: /s/ ANTHONY M. FRANK
                                               ---------------------------------
                                                   ANTHONY M. FRANK
                                                   320 Meadowood Court
                                                   Pleasant Hill, CA  94523-3176


<PAGE>   5

                                                        EXHIBIT 10.10.AD; PAGE 5


                          CERTIFICATE OF DETERMINATION
                                       OF
                      SERIES D CONVERTIBLE PREFERRED SHARES
                                       OF
                                ELECTROPURE, INC.

                              --------------------

        The undersigned, Floyd H. Panning and Catherine Patterson, do hereby
certify:

            1. That said Floyd H. Panning is, and at all times herein mentioned
was, the duly elected and acting President of Electropure, Inc., a California
corporation, and that said Catherine Patterson is, and at all times herein
mentioned was, the duly elected and acting Secretary of said Corporation;

            2. That at a meeting of the Board of Directors of said Corporation
the 11th day of January, 2001, at which meeting there were at all times present
and acting a quorum of the members of the Board of Directors of said
Corporation, the following resolutions were duly adopted:

        WHEREAS, the Articles of Incorporation of this Corporation provide for a
class of its authorized shares known as Preferred Stock, comprising one million
(1,000,000) shares of $1.00 par value each, issuable from time to time in one or
more series; and

        WHEREAS, the Board of Directors of this Corporation is authorized to fix
or alter the dividend rights, dividend rate, conversion rights, voting rights,
rights and terms of redemption (including sinking fund provisions), the
redemption price or prices, and the liquidation preferences of any wholly
unissued series of preferred shares, and the number of shares constituting any
such series and the designation thereof, or any of them; and

        WHEREAS, it is the desire of the Board of Directors of this Corporation,
pursuant to its authority as aforesaid, to fix the rights, preferences,
privileges, restrictions and other matters relating to a series of said
preferred shares;



<PAGE>   6

                                                        EXHIBIT 10.10.AD; PAGE 6


        NOW, THEREFORE, BE IT RESOLVED, that the Board of Directors does hereby
provide for the issue of a series of preferred stock of the Corporation and does
hereby fix and determine the rights, preferences, privileges, restrictions and
other matters relating to said series of preferred shares as follows:

        (a) The preferred shares provided for herein shall be designated and
            known as "Series D Convertible Preferred Shares" (hereinafter
            referred to as the "Series D Preferred Shares").

        (b) The number of shares constituting the Series D Preferred Shares,
            shall be two hundred fifty thousand (250,000) shares.

        (c) Upon the voluntary or involuntary liquidation, winding up or
            dissolution of the Corporation, out of the assets available for
            distribution to shareholders the Series D Preferred Shares shall be
            entitled to receive, in preference to any payment on the Common
            Stock, Class B Common Stock and Convertible Preferred Stock, only,
            an amount equal to Two Dollars ($2.00) per share and no more. After
            the full preferential liquidation amount has been paid to, or
            determined and set apart for, the Series D Preferred Shares, the
            remaining assets shall be payable to the Common Stock, Class B
            Common Stock and Convertible Preferred Stock. In the event the
            assets of the Corporation are insufficient to pay the full
            preferential liquidation amount required to be paid to the Series D
            Preferred Shares, the Common Stock, Class B Common Stock and
            Convertible Preferred Stock shall receive nothing. A reorganization
            shall not be considered to be a liquidation, winding up or
            dissolution within the meaning of this subdivision (c) and the
            Series D Preferred Shares shall be entitled only to the rights
            provided in the plan of reorganization and Chapters 12 and 13 of the
            California General Corporation Law and elsewhere herein.

        (d) The Series D Preferred Shares shall have no voting rights.

        (e) At the option of the respective holders of Series D Preferred
            Shares, each one (1) of the Series D Preferred Shares shall be
            convertible into three and one-third (3-1/3) shares of Common Stock
            (the "conversion ratio") at any time. Upon any such conversion,
            fractional shares shall be rounded up to a whole share of Common
            Stock.

        (f) The Corporation shall reserve and keep reserved out of its
            authorized but unissued shares of Common Stock sufficient shares to
            effect the conversion of all shares of Series D Preferred Shares
            outstanding from time to time. A holder of Series D Preferred Shares
            subject to conversion shall deliver the share certificate to the
            Corporation at its principal executive office accompanied, if
            appropriate, by a written request to convert, specifying the number
            of shares to be converted. The endorsement of the share certificate
            and the request to convert shall be in form satisfactory to the
            Corporation. Upon the date of such delivery, the conversion is
            deemed to have occurred and the person entitled to receive share
            certificates for Common Stock shall be regarded for all corporate
            purposes from and after such date as the holder of the number of
            shares of Common Stock to which he is entitled upon the conversion.
            In the event of a stock split, reverse stock split, stock dividend,
            reorganization or recapitalization affecting the number of shares of
            Common Stock outstanding, the conversion ratio set forth in this
            subdivision (e) shall be proportionately revised so as to fairly and
            equitably preserve the conversion rights of the Series D Preferred
            Shares.



<PAGE>   7

                                                        EXHIBIT 10.10.AD; PAGE 7


        (g) Except as provided in subdivision (f) hereof, no holder of the
            Series D Preferred Shares shall be entitled as of right to subscribe
            for, purchase, or receive any part of any new or additional shares
            of any class, whether now or hereafter authorized, or of bonds,
            debentures, or other evidences of indebtedness convertible into or
            exchangeable for shares of any class, but all such new or additional
            shares of any class, or bond, debentures, or other evidences of
            indebtedness convertible into or exchangeable for shares, may be
            issued and disposed of by the Board of Directors on such terms and
            for such consideration (to the extent permitted by law), and to such
            person or persons as the Board of Directors in their absolute
            discretion may deem advisable.

RESOLVED, FURTHER, that the President and the Secretary of this Corporation be,
and they hereby are, authorized and directed to prepare and file a certificate
of determination of preferences in accordance with the foregoing resolution and
the provisions of California law.

        3. That the authorized number of preferred shares of said Corporation is
one million (1,000,000), and that the number of preferred shares constituting
Series D Preferred Shares, none of which has been issued, is two hundred fifty
thousand (250,000).

        IN WITNESS WHEREOF, the undersigned have executed this certificate this
16th day of January, 2001.

                                              /s/ FLOYD H. PANNING
                                              ----------------------------------
                                                  FLOYD H. PANNING, President
                                                  of ELECTROPURE, INC.


                                              /s/ CATHERINE PATTERSON
                                              ----------------------------------
                                                  CATHERINE PATTERSON, Secretary
                                                  of ELECTROPURE, INC.


        The undersigned, Floyd H. Panning, President, and Catherine Patterson,
Secretary, of Electropure, Inc., a California corporation, each certifies under
penalty of perjury that the matters set out in the foregoing Certificate of
Determination are true and correct.

        Executed at Laguna Hills, California, on January 16, 2001.


                                              /s/ FLOYD H. PANNING
                                              ----------------------------------
                                                  FLOYD H. PANNING


                                              /s/ CATHERINE PATTERSON
                                              ----------------------------------
                                                  CATHERINE PATTERSON