<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2007-08-03</periodOfReport>

    <issuer>
        <issuerCik>0001096560</issuerCik>
        <issuerName>SULPHCO INC</issuerName>
        <issuerTradingSymbol>SUF</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001207572</rptOwnerCik>
            <rptOwnerName>SCHAFRAN LAWRENCE G</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O SULPHCO, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>4333 W SAM HOUSTON PARKWAY N, STE. 190</rptOwnerStreet2>
            <rptOwnerCity>HOUSTON</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>77043</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Option to Buy</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>3.53</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2007-08-03</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>100000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2007-08-03</value>
            </exerciseDate>
            <expirationDate>
                <value>2017-08-03</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>100000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>150000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Mr. Schafran was originally granted 50,000 shares of common stock upon joining the Board of Directors of SulphCo, Inc.  These shares were never issued to Mr. Schafran, and on August 3, 2007, the Board of Directors rescinded such grant and authorized the grant to Mr. Schafran of options to purchase 100,000 shares of common stock in lieu thereof.  These options are fully vested upon grant and are otherwise subject to the terms, definitions and provisions of the SulphCo, Inc. 2006 Stock Option Plan, and the accompanying stock option agreement under which they are granted.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Lawrence G. Schafran</signatureName>
        <signatureDate>2007-08-15</signatureDate>
    </ownerSignature>
</ownershipDocument>
